[Info-ZIP note, 20040528:  this file is based on PKWARE's appnote.txt of
 15 February 1996, taking into account PKWARE's revised appnote.txt
 version 6.2.0 of 26 April 2004.  It has been unofficially corrected
 and extended by Info-ZIP without explicit permission by PKWARE.
 Although Info-ZIP believes the information to be accurate and complete,
 it is provided under a disclaimer similar to the PKWARE disclaimer below,
 differing only in the substitution of "Info-ZIP" for "PKWARE".  In other
 words, use this information at your own risk, but we think it's correct.

 Specification info from PKWARE that was obviously wrong has been corrected
 silently (e.g. missing structure fields, wrong numbers).
 As of PKZIPW 2.50, two new incompatibilities have been introduced by PKWARE;
 they are noted below.  Note that the "NTFS tag" conflict is currently not
 real; PKZIPW 2.50 actually tags NTFS files as having come from a FAT
 file system, too.]

File:    APPNOTE.TXT - .ZIP File Format Specification
Version: 6.2.0 - NOTIFICATION OF CHANGE
Revised: 04/26/2004 [2004-05-28 Info-ZIP]
Copyright (c) 1989 - 2004 PKWARE Inc., All Rights Reserved.

I. Purpose
----------

This specification is intended to define a cross-platform,
interoperable file format.  Since its first publication
in 1989, PKWARE has remained committed to ensuring the
interoperability of the .ZIP file format through this
specification.  We trust that all .ZIP compatible vendors
and application developers that have adopted this format
will share and support this commitment.


II. Disclaimer
--------------

Although PKWARE will attempt to supply current and accurate
information relating to its file formats, algorithms, and the
subject programs, the possibility of error or omission can not
be eliminated. PKWARE therefore expressly disclaims any warranty
that the information contained in the associated materials relating
to the subject programs and/or the format of the files created or
accessed by the subject programs and/or the algorithms used by
the subject programs, or any other matter, is current, correct or
accurate as delivered.  Any risk of damage due to any possible
inaccurate information is assumed by the user of the information.
Furthermore, the information relating to the subject programs
and/or the file formats created or accessed by the subject
programs and/or the algorithms used by the subject programs is
subject to change without notice.

If the version of this file is marked as a NOTIFICATION OF CHANGE,
the content defines an Early Feature Specification (EFS) change
to the .ZIP file format that may be subject to modification prior
to publication of the Final Feature Specification (FFS).  This
document may also contain information on Planned Feature
Specifications (PFS) defining recognized future extensions.


III. Change Log
---------------

Version       Change Description                      Date
-------       ------------------                     ----------
5.2           -Single Password Symmetric Encryption  06/02/2003
               storage

6.1.0         -Smart Card compatibility              01/20/2004
              -Documentation on certificate storage

6.2.0         -Introduction of Central Directory     04/26/2004
               Encryption for encrypting metadata
              -Added OS/X to Version Made By values


IV. General Format of a .ZIP file
---------------------------------

  Files stored in arbitrary order.  Large .ZIP files can span multiple
  diskette media or be split into user-defined segment sizes.  [The
  minimum user-defined segment size for a split .ZIP file is 64K.
  (removed by PKWare 2003-06-01)]

  Overall .ZIP file format:

    [local file header 1]
    [file data 1]
    [data descriptor 1]
    .
    .
    .
    [local file header n]
    [file data n]
    [data descriptor n]
    [archive decryption header] (EFS)
    [archive extra data record] (EFS)
    [central directory]
    [zip64 end of central directory record]
    [zip64 end of central directory locator]
    [end of central directory record]


  A.  Local file header:

        local file header signature     4 bytes  (0x04034b50)
        version needed to extract       2 bytes
        general purpose bit flag        2 bytes
        compression method              2 bytes
        last mod file time              2 bytes
        last mod file date              2 bytes
        crc-32                          4 bytes
        compressed size                 4 bytes
        uncompressed size               4 bytes
        file name length                2 bytes
        extra field length              2 bytes

        file name (variable size)
        extra field (variable size)


  B.  File data

      Immediately following the local header for a file
      is the compressed or stored data for the file.
      The series of [local file header][file data][data
      descriptor] repeats for each file in the .ZIP archive.


  C.  Data descriptor:

      [Info-ZIP discrepancy:
       The Info-ZIP zip program starts the data descriptor with a 4-byte
       PK-style signature.  Despite the specification, none of the PKWARE
       programs supports the data descriptor.  PKZIP 4.0 -fix function
       (and PKZIPFIX 2.04) ignores the data descriptor info even when bit 3
       of the general purpose bit flag is set.
        data descriptor signature       4 bytes  (0x08074b50)
      ]
        crc-32                          4 bytes
        compressed size                 4 bytes
        uncompressed size               4 bytes

      This descriptor exists only if bit 3 of the general
      purpose bit flag is set (see below).  It is byte aligned
      and immediately follows the last byte of compressed data.
      This descriptor is used only when it was not possible to
      seek in the output .ZIP file, e.g., when the output .ZIP file
      was standard output or a non seekable device.  For Zip64 format
      archives, the compressed and uncompressed sizes are 8 bytes each.


  D.  Archive decryption header:  (EFS)

      The Archive Decryption Header is introduced in version 6.2
      of the ZIP format specification.  This record exists in support
      of the Central Directory Encryption Feature implemented as part of
      the Strong Encryption Specification as described in this document.
      When the Central Directory Structure is encrypted, this decryption
      header will precede the encrypted data segment.  The encrypted
      data segment will consist of the Archive extra data record (if
      present) and the encrypted Central Directory Structure data.
      The format of this data record is identical to the Decryption
      header record preceding compressed file data.  If the central
      directory structure is encrypted, the location of the start of
      this data record is determined using the Start of Central Directory
      field in the Zip64 End of Central Directory record.  Refer to the
      section on the Strong Encryption Specification for information
      on the fields used in the Archive Decryption Header record.


  E.  Archive extra data record: (EFS)

        archive extra data signature    4 bytes  (0x08064b50)
        extra field length              4 bytes
        extra field data                (variable size)

      The Archive Extra Data Record is introduced in version 6.2
      of the ZIP format specification.  This record exists in support
      of the Central Directory Encryption Feature implemented as part of
      the Strong Encryption Specification as described in this document.
      When present, this record immediately precedes the central
      directory data structure.  The size of this data record will be
      included in the Size of the Central Directory field in the
      End of Central Directory record.  If the central directory structure
      is compressed, but not encrypted, the location of the start of
      this data record is determined using the Start of Central Directory
      field in the Zip64 End of Central Directory record.


  F.  Central directory structure:

      [file header 1]
      .
      .
      .
      [file header n]
      [digital signature]

      File header:

        central file header signature   4 bytes  (0x02014b50)
        version made by                 2 bytes
        version needed to extract       2 bytes
        general purpose bit flag        2 bytes
        compression method              2 bytes
        last mod file time              2 bytes
        last mod file date              2 bytes
        crc-32                          4 bytes
        compressed size                 4 bytes
        uncompressed size               4 bytes
        file name length                2 bytes
        extra field length              2 bytes
        file comment length             2 bytes
        disk number start               2 bytes
        internal file attributes        2 bytes
        external file attributes        4 bytes
        relative offset of local header 4 bytes

        file name (variable size)
        extra field (variable size)
        file comment (variable size)

      Digital signature:

        header signature                4 bytes  (0x05054b50)
        size of data                    2 bytes
        signature data (variable size)

      With the introduction of the Central Directory Encryption
      feature in version 6.2 of this specification, the Central
      Directory Structure may be stored both compressed and encrypted.
      Although not required, it is assumed when encrypting the
      Central Directory Structure, that it will be compressed
      for greater storage efficiency.  Information on the
      Central Directory Encryption feature can be found in the section
      describing the Strong Encryption Specification. The Digital
      Signature record will be neither compressed nor encrypted.


  G.  Zip64 end of central directory record

        zip64 end of central dir
        signature                       4 bytes  (0x06064b50)
        size of zip64 end of central
        directory record                8 bytes
        version made by                 2 bytes
        version needed to extract       2 bytes
        number of this disk             4 bytes
        number of the disk with the
        start of the central directory  4 bytes
        total number of entries in the
        central directory on this disk  8 bytes
        total number of entries in the
        central directory               8 bytes
        size of the central directory   8 bytes
        offset of start of central
        directory with respect to
        the starting disk number        8 bytes
        zip64 extensible data sector    (variable size)

        The above record structure defines Version 1 of the
        Zip64 end of central directory record. Version 1 was
        implemented in versions of this specification preceding
        6.2 in support of the ZIP64(tm) large file feature. The
        introduction of the Central Directory Encryption feature
        implemented in version 6.2 as part of the Strong Encryption
        Specification defines Version 2 of this record structure.
        Refer to the section describing the Strong Encryption
        Specification for details on the version 2 format for
        this record.


  H.  Zip64 end of central directory locator

        zip64 end of central dir locator
        signature                       4 bytes  (0x07064b50)
        number of the disk with the
        start of the zip64 end of
        central directory               4 bytes
        relative offset of the zip64
        end of central directory record 8 bytes
        total number of disks           4 bytes


  I.  End of central directory record:

        end of central dir signature    4 bytes  (0x06054b50)
        number of this disk             2 bytes
        number of the disk with the
        start of the central directory  2 bytes
        total number of entries in the
        central directory on this disk  2 bytes
        total number of entries in
        the central directory           2 bytes
        size of the central directory   4 bytes
        offset of start of central
        directory with respect to
        the starting disk number        4 bytes
        .ZIP file comment length        2 bytes
        .ZIP file comment       (variable size)


  J.  Explanation of fields:

      version made by (2 bytes)

          [PKWARE describes "OS made by" now (since 1998) as follows:
          The upper byte indicates the compatibility of the file
          attribute information.  If the external file attributes
          are compatible with MS-DOS and can be read by PKZIP for
          DOS version 2.04g then this value will be zero.  If these
          attributes are not compatible, then this value will
          identify the host system on which the attributes are
          compatible.]
          The upper byte indicates the host system (OS) for the
          file.  Software can use this information to determine
          the line record format for text files etc.  The current
          mappings are:

          0 - FAT file system (DOS, OS/2, NT)      + PKWARE 2.50+ VFAT, NTFS
          1 - Amiga
          2 - OpenVMS
          3 - Unix
          4 - VM/CMS
          5 - Atari ST
          6 - HPFS file system (OS/2, NT 3.x)
          7 - Macintosh
          8 - Z-System
          9 - CP/M
          ---------------------------------------------------------------------
               PKWARE assignment             | Info-ZIP assignment
          -----------------------------------|---------------------------------
          10 - Windows NTFS                  | TOPS-20
               (since PKZIPW 2.50, but       | (assigned Oct-1992,
                not used by any PKWARE prog) |  no longer used)
          11 - MVS                           | NTFS file system (WinNT)
                                             | (actively used by Info-ZIP's
                                             |  Zip for NT since Sep-1993)
          12 - VSE                           | SMS/QDOS
          ---------------------------------------------------------------------
          13 - Acorn RISC OS
          14 - VFAT file system (Win95, NT)     [Info-ZIP reservation, unused]
          15 - MVS [PKWARE describes this assignment as "alternate MVS"]
          16 - BeOS (BeBox or PowerMac)
          17 - Tandem
          18 - OS/400 (IBM)                  | THEOS
          19 - OS/X (Darwin)
          20 thru 29 - unused
          30 - AtheOS/Syllable
          31 thru 255 - unused

          The lower byte indicates the ZIP specification version
          (the version of this document) supported by the software
          used to encode the file.  The value/10 indicates the major
          version number, and the value mod 10 is the minor version
          number.

      version needed to extract (2 bytes)

          The minimum supported ZIP specification version needed to
          extract the file, mapped as above.  This value is based on
          the specific format features a ZIP program must support to
          be able to extract the file.  If multiple features are
          applied to a file, the minimum version should be set to the
          feature having the highest value. New features or feature
          changes affecting the published format specification will be
          implemented using higher version numbers than the last
          published value to avoid conflict.

          Current minimum feature versions are as defined below:

          1.0 - Default value
          1.1 - File is a volume label
          2.0 - File is a folder (directory)
          2.0 - File is compressed using Deflate compression
          2.0 - File is encrypted using traditional PKWARE encryption
          2.1 - File is compressed using Deflate64(tm)
          2.5 - File is compressed using PKWARE DCL Implode
          2.7 - File is a patch data set
          4.5 - File uses ZIP64 format extensions
          4.6 - File is compressed using BZIP2 compression*
          5.0 - File is encrypted using DES
          5.0 - File is encrypted using 3DES
          5.0 - File is encrypted using original RC2 encryption
          5.0 - File is encrypted using RC4 encryption
          5.1 - File is encrypted using AES encryption
          5.1 - File is encrypted using corrected RC2 encryption**
          5.2 - File is encrypted using corrected RC2-64 encryption**
          6.1 - File is encrypted using non-OAEP key wrapping***
          6.2 - Central directory encryption


          * Early 7.x (pre-7.2) versions of PKZIP incorrectly set the
          version needed to extract for BZIP2 compression to be 50
          when it should have been 46.

          ** Refer to the section on Strong Encryption Specification
          for additional information regarding RC2 corrections.

          *** Certificate encryption using non-OAEP key wrapping is the
          intended mode of operation for all versions beginning with 6.1.
          Support for OAEP key wrapping should only be used for
          backward compatibility when sending ZIP files to be opened by
          versions of PKZIP older than 6.1 (5.0 or 6.0).

          When using ZIP64 extensions, the corresponding value in the
          Zip64 end of central directory record should also be set.
          This field currently supports only the value 45 to indicate
          ZIP64 extensions are present.

      general purpose bit flag: (2 bytes)

          Bit 0: If set, indicates that the file is encrypted.

          (For Method 6 - Imploding)
          Bit 1: If the compression method used was type 6,
                 Imploding, then this bit, if set, indicates
                 an 8K sliding dictionary was used.  If clear,
                 then a 4K sliding dictionary was used.
          Bit 2: If the compression method used was type 6,
                 Imploding, then this bit, if set, indicates
                 3 Shannon-Fano trees were used to encode the
                 sliding dictionary output.  If clear, then 2
                 Shannon-Fano trees were used.

          (For Methods 8 and 9 - Deflating)
          Bit 2  Bit 1
            0      0    Normal (-en) compression option was used.
            0      1    Maximum (-exx/-ex) compression option was used.
            1      0    Fast (-ef) compression option was used.
            1      1    Super Fast (-es) compression option was used.

          Note:  Bits 1 and 2 are undefined if the compression
                 method is any other.

          Bit 3: If this bit is set, the fields crc-32, compressed
                 size and uncompressed size are set to zero in the
                 local header.  The correct values are put in the
                 data descriptor immediately following the compressed
                 data.  (Note: PKZIP version 2.04g for DOS only
                 recognizes this bit for method 8 compression, newer
                 versions of PKZIP recognize this bit for any
                 compression method.)
                [Info-ZIP note: This bit was introduced by PKZIP 2.04 for
                 DOS. In general, this feature can only be reliably used
                 together with compression methods that allow intrinsic
                 detection of the "end-of-compressed-data" condition. From
                 the set of compression methods described in this Zip archive
                 specification, only "deflate" and "bzip2" fulfill this
                 requirement.
                 Especially, the method STORED does not work!
                 The Info-ZIP tools recognize this bit regardless of the
                 compression method; but, they rely on correctly set
                 "compressed size" information in the central directory entry.]

          Bit 4: Reserved for use with method 8, for enhanced
                 deflating.

          Bit 5: If this bit is set, this indicates that the file is
                 compressed patched data.  (Note: Requires PKZIP
                 version 2.70 or greater)

          Bit 6: Strong encryption.  If this bit is set, you should
                 set the version needed to extract value to at least
                 50 and you must also set bit 0.  If AES encryption
                 is used, the version needed to extract value must
                 be at least 51.

          Bit 7: Currently unused.

          Bit 8: Currently unused.

          Bit 9: Currently unused.

          Bit 10: Currently unused.

          Bit 11: Currently unused.

          Bit 12: Reserved by PKWARE for enhanced compression.

          Bit 13: Used when encrypting the Central Directory to indicate
                  selected data values in the Local Header are masked to
                  hide their actual values.  See the section describing
                  the Strong Encryption Specification for details.

          Bit 14: Reserved by PKWARE.

          Bit 15: Reserved by PKWARE.

      compression method: (2 bytes)

          (see accompanying documentation for algorithm
          descriptions)

          0 - The file is stored (no compression)
          1 - The file is Shrunk
          2 - The file is Reduced with compression factor 1
          3 - The file is Reduced with compression factor 2
          4 - The file is Reduced with compression factor 3
          5 - The file is Reduced with compression factor 4
          6 - The file is Imploded
          7 - Reserved for Tokenizing compression algorithm
          8 - The file is Deflated
          9 - Enhanced Deflating using Deflate64(tm)
         10 - PKWARE Data Compression Library Imploding
         11 - Reserved by PKWARE
         12 - File is compressed using BZIP2 algorithm

      date and time fields: (2 bytes each)

          The date and time are encoded in standard MS-DOS format.
          If input came from standard input, the date and time are
          those at which compression was started for this data.
          If encrypting the central directory and general purpose bit
          flag 13 is set indicating masking, the value stored in the
          Local Header will be zero.

      CRC-32: (4 bytes)

          The CRC-32 algorithm was generously contributed by
          David Schwaderer and can be found in his excellent
          book "C Programmers Guide to NetBIOS" published by
          Howard W. Sams & Co. Inc.  The 'magic number' for
          the CRC is 0xdebb20e3.  The proper CRC pre and post
          conditioning is used, meaning that the CRC register
          is pre-conditioned with all ones (a starting value
          of 0xffffffff) and the value is post-conditioned by
          taking the one's complement of the CRC residual.
          If bit 3 of the general purpose flag is set, this
          field is set to zero in the local header and the correct
          value is put in the data descriptor and in the central
          directory.  If encrypting the central directory and general
          purpose bit flag 13 is set indicating masking, the value
          stored in the Local Header will be zero.

      compressed size: (4 bytes)
      uncompressed size: (4 bytes)

          The size of the file compressed and uncompressed,
          respectively.  If bit 3 of the general purpose bit flag
          is set, these fields are set to zero in the local header
          and the correct values are put in the data descriptor and
          in the central directory.  If an archive is in zip64 format
          and the value in this field is 0xFFFFFFFF, the size will be
          in the corresponding 8 byte zip64 extended information
          extra field.  If encrypting the central directory and general
          purpose bit flag 13 is set indicating masking, the value stored
          for the uncompressed size in the Local Header will be zero.

      file name length: (2 bytes)
      extra field length: (2 bytes)
      file comment length: (2 bytes)

          The length of the file name, extra field, and comment
          fields respectively.  The combined length of any
          directory record and these three fields should not
          generally exceed 65,535 bytes.  If input came from standard
          input, the file name length is set to zero.

         [Info-ZIP note:
          This feature is not yet supported by any PKWARE version of ZIP
          (at least not in PKZIP for DOS and PKZIP for Windows/WinNT).
          The Info-ZIP programs handle standard input differently:
          If input came from standard input, the filename is set to "-"
          (length one).]


      disk number start: (2 bytes)

          The number of the disk on which this file begins.  If an
          archive is in zip64 format and the value in this field is
          0xFFFF, the size will be in the corresponding 4 byte zip64
          extended information extra field.

      internal file attributes: (2 bytes)

          Bits 1 and 2 are reserved for use by PKWARE.

          The lowest bit of this field indicates, if set, that
          the file is apparently an ASCII or text file.  If not
          set, that the file apparently contains binary data.
          The remaining bits are unused in version 1.0.

          The 0x0002 bit of this field indicates, if set, that a
          4 byte variable record length control field precedes each
          logical record indicating the length of the record. This
          flag is independent of text control characters, and if used
          in conjunction with text data, includes any control
          characters in the total length of the record. This value is
          provided for mainframe data transfer support.

      external file attributes: (4 bytes)

          The mapping of the external attributes is
          host-system dependent (see 'version made by').  For
          MS-DOS, the low order byte is the MS-DOS directory
          attribute byte.  If input came from standard input, this
          field is set to zero.

      relative offset of local header: (4 bytes)

          This is the offset from the start of the first disk on
          which this file appears, to where the local header should
          be found.  If an archive is in zip64 format and the value
          in this field is 0xFFFFFFFF, the size will be in the
          corresponding 8 byte zip64 extended information extra field.

      file name: (Variable)

          The name of the file, with optional relative path.
          The path stored should not contain a drive or
          device letter, or a leading slash.  All slashes
          should be forward slashes '/' as opposed to
          backwards slashes '\' for compatibility with Amiga
          and Unix file systems etc.  If input came from standard
          input, there is no file name field.  If encrypting
          the central directory and general purpose bit flag 13 is set
          indicating masking, the file name stored in the Local Header
          will not be the actual file name.  A masking value consisting
          of a unique hexadecimal value will be stored.  This value will
          be sequentially incremented for each file in the archive.  See
          the section on the Strong Encryption Specification for details
          on retrieving the encrypted file name.
         [Info-ZIP discrepancy:
          If input came from standard input, the file name is set
          to "-" (without the quotes).
          As far as we know, the PKWARE specification for "input from
          stdin" is not supported by PKZIP/PKUNZIP for DOS, OS/2, Windows
          Windows NT.]

      extra field: (Variable)

          This is for expansion.  If additional information
          needs to be stored for special needs or for specific
          platforms, it should be stored here.  Earlier versions
          of the software can then safely skip this file, and
          find the next file or header.  This field will be 0
          length in version 1.0.

          In order to allow different programs and different types
          of information to be stored in the 'extra' field in .ZIP
          files, the following structure should be used for all
          programs storing data in this field:

          header1+data1 + header2+data2 . . .

          Each header should consist of:

            Header ID - 2 bytes
            Data Size - 2 bytes

          Note: all fields stored in Intel low-byte/high-byte order.

          The Header ID field indicates the type of data that is in
          the following data block.

          Header ID's of 0 thru 31 are reserved for use by PKWARE.
          The remaining ID's can be used by third party vendors for
          proprietary usage.

          The current Header ID mappings defined by PKWARE are:

          0x0001        ZIP64 extended information extra field
          0x0007        AV Info
          0x0008        Reserved for future Unicode file name data (PFS)
          0x0009        OS/2 extended attributes      (also Info-ZIP)
          0x000a        NTFS (Win9x/WinNT FileTimes)
          0x000c        OpenVMS                       (also Info-ZIP)
          0x000d        Unix
          0x000e        Reserved for file stream and fork descriptors
          0x000f        Patch Descriptor
          0x0014        PKCS#7 Store for X.509 Certificates
          0x0015        X.509 Certificate ID and Signature for
                        individual file
          0x0016        X.509 Certificate ID for Central Directory
          0x0017        Strong Encryption Header
          0x0018        Record Management Controls
          0x0019        PKCS#7 Encryption Recipient Certificate List
          0x0065        IBM S/390 (Z390), AS/400 (I400) attributes
                        - uncompressed
          0x0066        Reserved for IBM S/390 (Z390), AS/400 (I400)
                        attributes - compressed

          The Header ID mappings defined by Info-ZIP and third parties are:

          0x07c8        Info-ZIP Macintosh (old, J. Lee)
          0x2605        ZipIt Macintosh (first version)
          0x2705        ZipIt Macintosh v 1.3.5 and newer (w/o full filename)
          0x2805        ZipIt Macintosh 1.3.5+
          0x334d        Info-ZIP Macintosh (new, D. Haase's 'Mac3' field)
          0x4154        Tandem NSK
          0x4341        Acorn/SparkFS (David Pilling)
          0x4453        Windows NT security descriptor (binary ACL)
          0x4704        VM/CMS
          0x470f        MVS
          0x4854        Theos, old inofficial port
          0x4b46        FWKCS MD5 (see below)
          0x4c41        OS/2 access control list (text ACL)
          0x4d49        Info-ZIP OpenVMS (obsolete)
          0x4d63        Macintosh SmartZIP, by Macro Bambini
          0x4f4c        Xceed original location extra field
          0x5356        AOS/VS (binary ACL)
          0x5455        extended timestamp
          0x554e        Xceed unicode extra field
          0x5855        Info-ZIP Unix (original; also OS/2, NT, etc.)
          0x6542        BeOS (BeBox, PowerMac, etc.)
          0x6854        Theos
          0x7441        AtheOS (AtheOS/Syllable attributes)
          0x756e        ASi Unix
          0x7855        Info-ZIP Unix (new)
          0xfb4a        SMS/QDOS

          Detailed descriptions of Extra Fields defined by third
          party mappings will be documented as information on
          these data structures is made available to PKWARE.
          PKWARE does not guarantee the accuracy of any published
          third party data.

          The Data Size field indicates the size of the following
          data block. Programs can use this value to skip to the
          next header block, passing over any data blocks that are
          not of interest.

          Note: As stated above, the size of the entire .ZIP file
                header, including the file name, comment, and extra
                field should not exceed 64K in size.

          In case two different programs should appropriate the same
          Header ID value, it is strongly recommended that each
          program place a unique signature of at least two bytes in
          size (and preferably 4 bytes or bigger) at the start of
          each data area.  Every program should verify that its
          unique signature is present, in addition to the Header ID
          value being correct, before assuming that it is a block of
          known type.

          In the following descriptions, note that "Short" means two bytes,
          "Long" means four bytes, and "Long-Long" means eight bytes,
          regardless of their native sizes.  Unless specifically noted, all
          integer fields should be interpreted as unsigned (non-negative)
          numbers.


         -ZIP64 Extended Information Extra Field (0x0001):
          ===============================================

          The following is the layout of the ZIP64 extended
          information "extra" block. If one of the size or
          offset fields in the Local or Central directory
          record is too small to hold the required data,
          a ZIP64 extended information record is created.
          The order of the fields in the ZIP64 extended
          information record is fixed, but the fields will
          only appear if the corresponding Local or Central
          directory record field is set to 0xFFFF or 0xFFFFFFFF.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value      Size       Description
          -----      ----       -----------
  (ZIP64) 0x0001     2 bytes    Tag for this "extra" block type
          Size       2 bytes    Size of this "extra" block
          Original
          Size       8 bytes    Original uncompressed file size
          Compressed
          Size       8 bytes    Size of compressed data
          Relative Header
          Offset     8 bytes    Offset of local header record
          Disk Start
          Number     4 bytes    Number of the disk on which
                                this file starts

          This entry in the Local header must include BOTH original
          and compressed file sizes.


         -OS/2 Extended Attributes Extra Field (0x0009):
          =============================================

          The following is the layout of the OS/2 extended attributes "extra"
          block.  (Last Revision 19960922)

          Note: all fields stored in Intel low-byte/high-byte order.

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (OS/2)  0x0009        Short       tag for this extra block type
          TSize         Short       total data size for this block
          BSize         Long        uncompressed EA data size
          CType         Short       compression type
          EACRC         Long        CRC value for uncompressed EA data
          (var.)        variable    compressed EA data

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (OS/2)  0x0009        Short       tag for this extra block type
          TSize         Short       total data size for this block (4)
          BSize         Long        size of uncompressed local EA data

          The value of CType is interpreted according to the "compression
          method" section above; i.e., 0 for stored, 8 for deflated, etc.

          The OS/2 extended attribute structure (FEA2LIST) is
          compressed and then stored in its entirety within this
          structure.  There will only ever be one "block" of data in
          the variable-length field.


         -OS/2 Access Control List Extra Field:
          ====================================

          The following is the layout of the OS/2 ACL extra block.
          (Last Revision 19960922)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (ACL)   0x4c41        Short       tag for this extra block type ("AL")
          TSize         Short       total data size for this block
          BSize         Long        uncompressed ACL data size
          CType         Short       compression type
          EACRC         Long        CRC value for uncompressed ACL data
          (var.)        variable    compressed ACL data

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (ACL)   0x4c41        Short       tag for this extra block type ("AL")
          TSize         Short       total data size for this block (4)
          BSize         Long        size of uncompressed local ACL data

          The value of CType is interpreted according to the "compression
          method" section above; i.e., 0 for stored, 8 for deflated, etc.

          The uncompressed ACL data consist of a text header of the form
          "ACL1:%hX,%hd\n", where the first field is the OS/2 ACCINFO acc_attr
          member and the second is acc_count, followed by acc_count strings
          of the form "%s,%hx\n", where the first field is acl_ugname (user
          group name) and the second acl_access.  This block type will be
          extended for other operating systems as needed.


         -Windows NT Security Descriptor Extra Field (0x4453):
          ===================================================

          The following is the layout of the NT Security Descriptor (another
          type of ACL) extra block.  (Last Revision 19960922)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (SD)    0x4453        Short       tag for this extra block type ("SD")
          TSize         Short       total data size for this block
          BSize         Long        uncompressed SD data size
          Version       Byte        version of uncompressed SD data format
          CType         Short       compression type
          EACRC         Long        CRC value for uncompressed SD data
          (var.)        variable    compressed SD data

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (SD)    0x4453        Short       tag for this extra block type ("SD")
          TSize         Short       total data size for this block (4)
          BSize         Long        size of uncompressed local SD data

          The value of CType is interpreted according to the "compression
          method" section above; i.e., 0 for stored, 8 for deflated, etc.
          Version specifies how the compressed data are to be interpreted
          and allows for future expansion of this extra field type.  Currently
          only version 0 is defined.

          For version 0, the compressed data are to be interpreted as a single
          valid Windows NT SECURITY_DESCRIPTOR data structure, in self-relative
          format.


         -PKWARE Win95/WinNT Extra Field (0x000a):
          =======================================

          The following description covers PKWARE's "NTFS" attributes
          "extra" block, introduced with the release of PKZIP 2.50 for
          Windows. (Last Revision 20001118)

          (Note: At this time the Mtime, Atime and Ctime values may
          be used on any WIN32 system.)
         [Info-ZIP note: In the current implementations, this field has
          a fixed total data size of 32 bytes and is only stored as local
          extra field.]

          Value         Size        Description
          -----         ----        -----------
  (NTFS)  0x000a        Short       Tag for this "extra" block type
          TSize         Short       Total Data Size for this block
          Reserved      Long        for future use
          Tag1          Short       NTFS attribute tag value #1
          Size1         Short       Size of attribute #1, in bytes
          (var.)        SubSize1    Attribute #1 data
          .
          .
          .
          TagN          Short       NTFS attribute tag value #N
          SizeN         Short       Size of attribute #N, in bytes
          (var.)        SubSizeN    Attribute #N data

          For NTFS, values for Tag1 through TagN are as follows:
          (currently only one set of attributes is defined for NTFS)

          Tag        Size       Description
          -----      ----       -----------
          0x0001     2 bytes    Tag for attribute #1
          Size1      2 bytes    Size of attribute #1, in bytes (24)
          Mtime      8 bytes    64-bit NTFS file last modification time
          Atime      8 bytes    64-bit NTFS file last access time
          Ctime      8 bytes    64-bit NTFS file creation time

          The total length for this block is 28 bytes, resulting in a
          fixed size value of 32 for the TSize field of the NTFS block.

          The NTFS filetimes are 64-bit unsigned integers, stored in Intel
          (least significant byte first) byte order. They determine the
          number of 1.0E-07 seconds (1/10th microseconds!) past WinNT "epoch",
          which is "01-Jan-1601 00:00:00 UTC".


         -PKWARE OpenVMS Extra Field (0x000c):
          ===================================

          The following is the layout of PKWARE's OpenVMS attributes
          "extra" block.  (Last Revision 12/17/91)

          Note: all fields stored in Intel low-byte/high-byte order.

          Value         Size        Description
          -----         ----        -----------
  (VMS)   0x000c        Short       Tag for this "extra" block type
          TSize         Short       Total Data Size for this block
          CRC           Long        32-bit CRC for remainder of the block
          Tag1          Short       OpenVMS attribute tag value #1
          Size1         Short       Size of attribute #1, in bytes
          (var.)        Size1       Attribute #1 data
          .
          .
          .
          TagN          Short       OpenVMS attribute tage value #N
          SizeN         Short       Size of attribute #N, in bytes
          (var.)        SizeN       Attribute #N data

          Rules:

          1. There will be one or more of attributes present, which
             will each be preceded by the above TagX & SizeX values.
             These values are identical to the ATR$C_XXXX and
             ATR$S_XXXX constants which are defined in ATR.H under
             OpenVMS C.  Neither of these values will ever be zero.

          2. No word alignment or padding is performed.

          3. A well-behaved PKZIP/OpenVMS program should never produce
             more than one sub-block with the same TagX value.  Also,
             there will never be more than one "extra" block of type
             0x000c in a particular directory record.


         -Info-ZIP VMS Extra Field:
          ========================

          The following is the layout of Info-ZIP's VMS attributes extra
          block for VAX or Alpha AXP.  The local-header and central-header
          versions are identical.  (Last Revision 19960922)

          Value         Size        Description
          -----         ----        -----------
  (VMS2)  0x4d49        Short       tag for this extra block type ("JM")
          TSize         Short       total data size for this block
          ID            Long        block ID
          Flags         Short       info bytes
          BSize         Short       uncompressed block size
          Reserved      Long        (reserved)
          (var.)        variable    compressed VMS file-attributes block

          The block ID is one of the following unterminated strings:

                "VFAB"          struct FAB
                "VALL"          struct XABALL
                "VFHC"          struct XABFHC
                "VDAT"          struct XABDAT
                "VRDT"          struct XABRDT
                "VPRO"          struct XABPRO
                "VKEY"          struct XABKEY
                "VMSV"          version (e.g., "V6.1"; truncated at hyphen)
                "VNAM"          reserved

          The lower three bits of Flags indicate the compression method.  The
          currently defined methods are:

                0       stored (not compressed)
                1       simple "RLE"
                2       deflated

          The "RLE" method simply replaces zero-valued bytes with zero-valued
          bits and non-zero-valued bytes with a "1" bit followed by the byte
          value.

          The variable-length compressed data contains only the data corre-
          sponding to the indicated structure or string.  Typically multiple
          VMS2 extra fields are present (each with a unique block type).


         -Info-ZIP Macintosh Extra Field:
          ==============================

          The following is the layout of the (old) Info-ZIP resource-fork extra
          block for Macintosh.  The local-header and central-header versions
          are identical.  (Last Revision 19960922)

          Value         Size        Description
          -----         ----        -----------
  (Mac)   0x07c8        Short       tag for this extra block type
          TSize         Short       total data size for this block
          "JLEE"        beLong      extra-field signature
          FInfo         16 bytes    Macintosh FInfo structure
          CrDat         beLong      HParamBlockRec fileParam.ioFlCrDat
          MdDat         beLong      HParamBlockRec fileParam.ioFlMdDat
          Flags         beLong      info bits
          DirID         beLong      HParamBlockRec fileParam.ioDirID
          VolName       28 bytes    volume name (optional)

          All fields but the first two are in native Macintosh format
          (big-endian Motorola order, not little-endian Intel).  The least
          significant bit of Flags is 1 if the file is a data fork, 0 other-
          wise.  In addition, if this extra field is present, the filename
          has an extra 'd' or 'r' appended to indicate data fork or resource
          fork.  The 28-byte VolName field may be omitted.


         -ZipIt Macintosh Extra Field (long):
          ==================================

          The following is the layout of the ZipIt extra block for Macintosh.
          The local-header and central-header versions are identical.
          (Last Revision 19970130)

          Value         Size        Description
          -----         ----        -----------
  (Mac2)  0x2605        Short       tag for this extra block type
          TSize         Short       total data size for this block
          "ZPIT"        beLong      extra-field signature
          FnLen         Byte        length of FileName
          FileName      variable    full Macintosh filename
          FileType      Byte[4]     four-byte Mac file type string
          Creator       Byte[4]     four-byte Mac creator string


         -ZipIt Macintosh Extra Field (short, for files):
          ==============================================

          The following is the layout of a shortened variant of the
          ZipIt extra block for Macintosh (without "full name" entry).
          This variant is used by ZipIt 1.3.5 and newer for entries of
          files (not directories) that do not have a MacBinary encoded
          file.  The local-header and central-header versions are identical.
          (Last Revision 20030602)

          Value         Size        Description
          -----         ----        -----------
  (Mac2b) 0x2705        Short       tag for this extra block type
          TSize         Short       total data size for this block (min. 12)
          "ZPIT"        beLong      extra-field signature
          FileType      Byte[4]     four-byte Mac file type string
          Creator       Byte[4]     four-byte Mac creator string
          fdFlags       beShort     attributes from FInfo.frFlags,
                                    may be omitted
          0x0000        beShort     reserved, may be omitted


         -ZipIt Macintosh Extra Field (short, for directories):
          ====================================================

          The following is the layout of a shortened variant of the
          ZipIt extra block for Macintosh used only for directory
          entries. This variant is used by ZipIt 1.3.5 and newer to
          save some optional Mac-specific information about directories.
          The local-header and central-header versions are identical.

          Value         Size        Description
          -----         ----        -----------
  (Mac2c) 0x2805        Short       tag for this extra block type
          TSize         Short       total data size for this block (12)
          "ZPIT"        beLong      extra-field signature
          frFlags       beShort     attributes from DInfo.frFlags, may
                                    be omitted
          View          beShort     ZipIt view flag, may be omitted


          The View field specifies ZipIt-internal settings as follows:

          Bits of the Flags:
              bit 0           if set, the folder is shown expanded (open)
                              when the archive contents are viewed in ZipIt.
              bits 1-15       reserved, zero;


         -Info-ZIP Macintosh Extra Field (new):
          ====================================

          The following is the layout of the (new) Info-ZIP extra
          block for Macintosh, designed by Dirk Haase.
          All values are in little-endian.
          (Last Revision 19981005)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (Mac3)  0x334d        Short       tag for this extra block type ("M3")
          TSize         Short       total data size for this block
          BSize         Long        uncompressed finder attribute data size
          Flags         Short       info bits
          fdType        Byte[4]     Type of the File (4-byte string)
          fdCreator     Byte[4]     Creator of the File (4-byte string)
          (CType)       Short       compression type
          (CRC)         Long        CRC value for uncompressed MacOS data
          Attribs       variable    finder attribute data (see below)


          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (Mac3)  0x334d        Short       tag for this extra block type ("M3")
          TSize         Short       total data size for this block
          BSize         Long        uncompressed finder attribute data size
          Flags         Short       info bits
          fdType        Byte[4]     Type of the File (4-byte string)
          fdCreator     Byte[4]     Creator of the File (4-byte string)

          The third bit of Flags in both headers indicates whether
          the LOCAL extra field is uncompressed (and therefore whether CType
          and CRC are omitted):

          Bits of the Flags:
              bit 0           if set, file is a data fork; otherwise unset
              bit 1           if set, filename will be not changed
              bit 2           if set, Attribs is uncompressed (no CType, CRC)
              bit 3           if set, date and times are in 64 bit
                              if zero date and times are in 32 bit.
              bit 4           if set, timezone offsets fields for the native
                              Mac times are omitted (UTC support deactivated)
              bits 5-15       reserved;


          Attributes:

          Attribs is a Mac-specific block of data in little-endian format with
          the following structure (if compressed, uncompress it first):

          Value         Size        Description
          -----         ----        -----------
          fdFlags       Short       Finder Flags
          fdLocation.v  Short       Finder Icon Location
          fdLocation.h  Short       Finder Icon Location
          fdFldr        Short       Folder containing file

          FXInfo        16 bytes    Macintosh FXInfo structure
            FXInfo-Structure:
                fdIconID        Short
                fdUnused[3]     Short       unused but reserved 6 bytes
                fdScript        Byte        Script flag and number
                fdXFlags        Byte        More flag bits
                fdComment       Short       Comment ID
                fdPutAway       Long        Home Dir ID

          FVersNum      Byte        file version number
                                    may be not used by MacOS
          ACUser        Byte        directory access rights

          FlCrDat       ULong       date and time of creation
          FlMdDat       ULong       date and time of last modification
          FlBkDat       ULong       date and time of last backup
            These time numbers are original Mac FileTime values (local time!).
            Currently, date-time width is 32-bit, but future version may
            support be 64-bit times (see flags)

          CrGMTOffs     Long(signed!)   difference "local Creat. time - UTC"
          MdGMTOffs     Long(signed!)   difference "local Modif. time - UTC"
          BkGMTOffs     Long(signed!)   difference "local Backup time - UTC"
            These "local time - UTC" differences (stored in seconds) may be
            used to support timestamp adjustment after inter-timezone transfer.
            These fields are optional; bit 4 of the flags word controls their
            presence.

          Charset       Short       TextEncodingBase (Charset)
                                    valid for the following two fields

          FullPath      variable    Path of the current file.
                                    Zero terminated string (C-String)
                                    Currently coded in the native Charset.

          Comment       variable    Finder Comment of the current file.
                                    Zero terminated string (C-String)
                                    Currently coded in the native Charset.


         -SmartZIP Macintosh Extra Field:
          ====================================

          The following is the layout of the SmartZIP extra
          block for Macintosh, designed by Marco Bambini.

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
          0x4d63        Short       tag for this extra block type ("cM")
          TSize         Short       total data size for this block (64)
          "dZip"        beLong      extra-field signature
          fdType        Byte[4]     Type of the File (4-byte string)
          fdCreator     Byte[4]     Creator of the File (4-byte string)
          fdFlags       beShort     Finder Flags
          fdLocation.v  beShort     Finder Icon Location
          fdLocation.h  beShort     Finder Icon Location
          fdFldr        beShort     Folder containing file
          CrDat         beLong      HParamBlockRec fileParam.ioFlCrDat
          MdDat         beLong      HParamBlockRec fileParam.ioFlMdDat
          frScroll.v    Byte        vertical pos. of folder's scroll bar
          fdScript      Byte        Script flag and number
          frScroll.h    Byte        horizontal pos. of folder's scroll bar
          fdXFlags      Byte        More flag bits
          FileName      Byte[32]    full Macintosh filename (pascal string)

          All fields but the first two are in native Macintosh format
          (big-endian Motorola order, not little-endian Intel).
          The extra field size is fixed to 64 bytes.
          The local-header and central-header versions are identical.


         -Acorn SparkFS Extra Field:
          =========================

          The following is the layout of David Pilling's SparkFS extra block
          for Acorn RISC OS.  The local-header and central-header versions are
          identical.  (Last Revision 19960922)

          Value         Size        Description
          -----         ----        -----------
  (Acorn) 0x4341        Short       tag for this extra block type ("AC")
          TSize         Short       total data size for this block (20)
          "ARC0"        Long        extra-field signature
          LoadAddr      Long        load address or file type
          ExecAddr      Long        exec address
          Attr          Long        file permissions
          Zero          Long        reserved; always zero

          The following bits of Attr are associated with the given file
          permissions:

                bit 0           user-writable ('W')
                bit 1           user-readable ('R')
                bit 2           reserved
                bit 3           locked ('L')
                bit 4           publicly writable ('w')
                bit 5           publicly readable ('r')
                bit 6           reserved
                bit 7           reserved


         -VM/CMS Extra Field:
          ==================

          The following is the layout of the file-attributes extra block for
          VM/CMS.  The local-header and central-header versions are
          identical.  (Last Revision 19960922)

          Value         Size        Description
          -----         ----        -----------
 (VM/CMS) 0x4704        Short       tag for this extra block type
          TSize         Short       total data size for this block
          flData        variable    file attributes data

          flData is an uncompressed fldata_t struct.


         -MVS Extra Field:
          ===============

          The following is the layout of the file-attributes extra block for
          MVS.  The local-header and central-header versions are identical.
          (Last Revision 19960922)

          Value         Size        Description
          -----         ----        -----------
  (MVS)   0x470f        Short       tag for this extra block type
          TSize         Short       total data size for this block
          flData        variable    file attributes data

          flData is an uncompressed fldata_t struct.


         -PKWARE Unix Extra Field (0x000d):
          ================================

          The following is the layout of PKWARE's Unix "extra" block.
          It was introduced with the release of PKZIP for Unix 2.50.
          Note: all fields are stored in Intel low-byte/high-byte order.
          (Last Revision 19980901)

          This field has a minimum data size of 12 bytes and is only stored
          as local extra field.

          Value         Size        Description
          -----         ----        -----------
 (Unix0)  0x000d        Short       Tag for this "extra" block type
          TSize         Short       Total Data Size for this block
          AcTime        Long        time of last access (UTC/GMT)
          ModTime       Long        time of last modification (UTC/GMT)
          UID           Short       Unix user ID
          GID           Short       Unix group ID
          (var)         variable    Variable length data field

          The variable length data field will contain file type
          specific data.  Currently the only values allowed are
          the original "linked to" file names for hard or symbolic
          links, and the major and minor device node numbers for
          character and block device nodes.  Since device nodes
          cannot be either symbolic or hard links, only one set of
          variable length data is stored.  Link files will have the
          name of the original file stored.  This name is NOT NULL
          terminated.  Its size can be determined by checking TSize -
          12.  Device entries will have eight bytes stored as two 4
          byte entries (in little-endian format).  The first entry
          will be the major device number, and the second the minor
          device number.

         [Info-ZIP note: The fixed part of this field has the same layout as
          Info-ZIP's abandoned "Unix1 timestamps & owner ID info" extra field;
          only the two tag bytes are different.]


         -PATCH Descriptor Extra Field (0x000f):
          =====================================

          The following is the layout of the Patch Descriptor "extra"
          block.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value         Size        Description
          -----         ----        -----------
  (Patch) 0x000f        Short       Tag for this "extra" block type
          TSize         Short       Size of the total "extra" block
          Version       Short       Version of the descriptor
          Flags         Long        Actions and reactions (see below)
          OldSize       Long        Size of the file about to be patched
          OldCRC        Long        32-bit CRC of the file about to be patched
          NewSize       Long        Size of the resulting file
          NewCRC        Long        32-bit CRC of the resulting file


          Actions and reactions

          Bits          Description
          ----          ----------------
          0             Use for auto detection
          1             Treat as a self-patch
          2-3           RESERVED
          4-5           Action (see below)
          6-7           RESERVED
          8-9           Reaction (see below) to absent file
          10-11         Reaction (see below) to newer file
          12-13         Reaction (see below) to unknown file
          14-15         RESERVED
          16-31         RESERVED

          Actions

          Action       Value
          ------       -----
          none         0
          add          1
          delete       2
          patch        3

          Reactions

          Reaction     Value
          --------     -----
          ask          0
          skip         1
          ignore       2
          fail         3

          Patch support is provided by PKPatchMaker(tm) technology and is
          covered under U.S. Patents and Patents Pending.


         -PKCS#7 Store for X.509 Certificates (0x0014):
          ============================================

          This field contains information about each of the certificates
          files may be signed with.  When the Central Directory Encryption
          feature is enabled for a ZIP file, this record will appear in
          the Archive Extra Data Record, otherwise it will appear in the
          first central directory record and will be ignored in any
          other record.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value     Size     Description
          -----     ----     -----------
  (Store) 0x0014    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of the store data
          SData     TSize    Data about the store

          SData
          Value     Size        Description
          -----     ----        -----------
          Version   2 bytes     Version number, 0x0001 for now
          StoreD    (variable)  Actual store data

          The StoreD member is suitable for passing as the pbData
          member of a CRYPT_DATA_BLOB to the CertOpenStore() function
          in Microsoft's CryptoAPI.  The SSize member above will be
          cbData + 6, where cbData is the cbData member of the same
          CRYPT_DATA_BLOB.  The encoding type to pass to
          CertOpenStore() should be
          PKCS_7_ANS_ENCODING | X509_ASN_ENCODING.


         -X.509 Certificate ID and Signature for individual file (0x0015):
          ===============================================================

          This field contains the information about which certificate in
          the PKCS#7 store was used to sign a particular file.  It also
          contains the signature data.  This field can appear multiple
          times, but can only appear once per certificate.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value         Size        Description
          -----         ----        -----------
  (CID)   0x0015        2 bytes     Tag for this "extra" block type
          CSize         2 bytes     Size of Method
          Method        (variable)

          Method
          Value         Size        Description
          -----         ----        -----------
          Version       2 bytes     Version number, for now 0x0001
          AlgID         2 bytes     Algorithm ID used for signing
          IDSize        2 bytes     Size of Certificate ID data
          CertID        (variable)  Certificate ID data
          SigSize       2 bytes     Size of Signature data
          Sig           (variable)  Signature data

          CertID
          Value         Size        Description
          -----         ----        -----------
          Size1         4 bytes     Size of CertID, should be (IDSize - 4)
          Size1         4 bytes     A bug in version one causes this value
                                    to appear twice.
          IssSize       4 bytes     Issuer data size
          Issuer        (variable)  Issuer data
          SerSize       4 bytes     Serial Number size
          Serial        (variable)  Serial Number data

          The Issuer and IssSize members are suitable for creating a
          CRYPT_DATA_BLOB to be the Issuer member of a CERT_INFO
          struct. The Serial and SerSize members would be the
          SerialNumber member of the same CERT_INFO struct.  This
          struct would be used to find the certificate in the store
          the file was signed with.  Those structures are from the MS
          CryptoAPI.

          Sig and SigSize are the actual signature data and size
          generated by signing the file with the MS CryptoAPI using a
          hash created with the given AlgID.


         -X.509 Certificate ID and Signature for central directory (0x0016):
          =================================================================

          This field contains the information about which certificate in
          the PKCS#7 store was used to sign the central directory structure.
          When the Central Directory Encryption feature is enabled for a
          ZIP file, this record will appear in the Archive Extra Data Record,
          otherwise it will appear in the first central directory record,
          along with the store.  The data structure is the
          same as the CID, except that SigSize will be 0, and there
          will be no Sig member.

          This field is also kept after the last central directory
          record, as the signature data (ID 0x05054b50, it looks like
          a central directory record of a different type).  This
          second copy of the data is the Signature Data member of the
          record, and will have a SigSize that is non-zero, and will
          have Sig data.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value     Size     Description
          -----     ----     -----------
  (CDID)  0x0016    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of data that follows
          TData     TSize    Data


         -Strong Encryption Header (0x0017) (EFS):
          ===============================

          Value     Size     Description
          -----     ----     -----------
          0x0017    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of data that follows
          Format    2 bytes  Format definition for this record
          AlgID     2 bytes  Encryption algorithm identifier
          Bitlen    2 bytes  Bit length of encryption key
          Flags     2 bytes  Processing flags
          CertData  TSize-8  Certificate decryption extra field data
                             (refer to the explanation for CertData
                              in the section describing the
                              Certificate Processing Method under
                              the Strong Encryption Specification)


         -Record Management Controls (0x0018):
          ===================================

          Value     Size     Description
          -----     ----     -----------
(Rec-CTL) 0x0018    2 bytes  Tag for this "extra" block type
          CSize     2 bytes  Size of total extra block data
          Tag1      2 bytes  Record control attribute 1
          Size1     2 bytes  Size of attribute 1, in bytes
          Data1     Size1    Attribute 1 data
            .
            .
            .
          TagN      2 bytes  Record control attribute N
          SizeN     2 bytes  Size of attribute N, in bytes
          DataN     SizeN    Attribute N data


         -PKCS#7 Encryption Recipient Certificate List (0x0019): (EFS)
          =====================================================

          This field contains the information about each of the certificates
          that files may be encrypted with. This field should only appear
          in the archive extra data record. This field is not required and
          serves only to aide archive modifications by preserving public
          encryption data. Individual security requirements may dictate
          that this data be omitted to deter information exposure.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value     Size     Description
          -----     ----     -----------
 (CStore) 0x0019    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of the store data
          TData     TSize    Data about the store

          TData:

          Value     Size     Description
          -----     ----     -----------
          Version   2 bytes  Format version number - must 0x0001 at this time
          CStore    (var)    PKCS#7 data blob


         -MVS Extra Field (PKWARE, 0x0065):
          ================================

          The following is the layout of the MVS "extra" block.
          Note: Some fields are stored in Big Endian format.
          All text is in EBCDIC format unless otherwise specified.

          Value       Size          Description
          -----       ----          -----------
  (MVS)   0x0065      2 bytes       Tag for this "extra" block type
          TSize       2 bytes       Size for the following data block
          ID          4 bytes       EBCDIC "Z390" 0xE9F3F9F0 or
                                    "T4MV" for TargetFour
          (var)       TSize-4       Attribute data


         -OS/400 Extra Field (0x0065):
          ===========================

          The following is the layout of the OS/400 "extra" block.
          Note: Some fields are stored in Big Endian format.
          All text is in EBCDIC format unless otherwise specified.

          Value       Size          Description
          -----       ----          -----------
  (OS400) 0x0065      2 bytes       Tag for this "extra" block type
          TSize       2 bytes       Size for the following data block
          ID          4 bytes       EBCDIC "I400" 0xC9F4F0F0 or
                                    "T4MV" for TargetFour
          (var)       TSize-4       Attribute data


         -Extended Timestamp Extra Field:
          ==============================

          The following is the layout of the extended-timestamp extra block.
          (Last Revision 19970118)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (time)  0x5455        Short       tag for this extra block type ("UT")
          TSize         Short       total data size for this block
          Flags         Byte        info bits
          (ModTime)     Long        time of last modification (UTC/GMT)
          (AcTime)      Long        time of last access (UTC/GMT)
          (CrTime)      Long        time of original creation (UTC/GMT)

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (time)  0x5455        Short       tag for this extra block type ("UT")
          TSize         Short       total data size for this block
          Flags         Byte        info bits (refers to local header!)
          (ModTime)     Long        time of last modification (UTC/GMT)

          The central-header extra field contains the modification time only,
          or no timestamp at all.  TSize is used to flag its presence or
          absence.  But note:

              If "Flags" indicates that Modtime is present in the local header
              field, it MUST be present in the central header field, too!
              This correspondence is required because the modification time
              value may be used to support trans-timezone freshening and
              updating operations with zip archives.

          The time values are in standard Unix signed-long format, indicating
          the number of seconds since 1 January 1970 00:00:00.  The times
          are relative to Coordinated Universal Time (UTC), also sometimes
          referred to as Greenwich Mean Time (GMT).  To convert to local time,
          the software must know the local timezone offset from UTC/GMT.

          The lower three bits of Flags in both headers indicate which time-
          stamps are present in the LOCAL extra field:

                bit 0           if set, modification time is present
                bit 1           if set, access time is present
                bit 2           if set, creation time is present
                bits 3-7        reserved for additional timestamps; not set

          Those times that are present will appear in the order indicated, but
          any combination of times may be omitted.  (Creation time may be
          present without access time, for example.)  TSize should equal
          (1 + 4*(number of set bits in Flags)), as the block is currently
          defined.  Other timestamps may be added in the future.


         -Info-ZIP Unix Extra Field (type 1):
          ==================================

          The following is the layout of the old Info-ZIP extra block for
          Unix.  It has been replaced by the extended-timestamp extra block
          (0x5455) and the Unix type 2 extra block (0x7855).
          (Last Revision 19970118)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (Unix1) 0x5855        Short       tag for this extra block type ("UX")
          TSize         Short       total data size for this block
          AcTime        Long        time of last access (UTC/GMT)
          ModTime       Long        time of last modification (UTC/GMT)
          UID           Short       Unix user ID (optional)
          GID           Short       Unix group ID (optional)

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (Unix1) 0x5855        Short       tag for this extra block type ("UX")
          TSize         Short       total data size for this block
          AcTime        Long        time of last access (GMT/UTC)
          ModTime       Long        time of last modification (GMT/UTC)

          The file access and modification times are in standard Unix signed-
          long format, indicating the number of seconds since 1 January 1970
          00:00:00.  The times are relative to Coordinated Universal Time
          (UTC), also sometimes referred to as Greenwich Mean Time (GMT).  To
          convert to local time, the software must know the local timezone
          offset from UTC/GMT.  The modification time may be used by non-Unix
          systems to support inter-timezone freshening and updating of zip
          archives.

          The local-header extra block may optionally contain UID and GID
          info for the file.  The local-header TSize value is the only
          indication of this.  Note that Unix UIDs and GIDs are usually
          specific to a particular machine, and they generally require root
          access to restore.

          This extra field type is obsolete, but it has been in use since
          mid-1994.  Therefore future archiving software should continue to
          support it.  Some guidelines:

              An archive member should either contain the old "Unix1"
              extra field block or the new extra field types "time" and/or
              "Unix2".

              If both the old "Unix1" block type and one or both of the new
              block types "time" and "Unix2" are found, the "Unix1" block
              should be considered invalid and ignored.

              Unarchiving software should recognize both old and new extra
              field block types, but the info from new types overrides the
              old "Unix1" field.

              Archiving software should recognize "Unix1" extra fields for
              timestamp comparison but never create it for updated, freshened
              or new archive members.  When copying existing members to a new
              archive, any "Unix1" extra field blocks should be converted to
              the new "time" and/or "Unix2" types.


         -Info-ZIP Unix Extra Field (type 2):
          ==================================

          The following is the layout of the new Info-ZIP extra block for
          Unix.  (Last Revision 19960922)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (Unix2) 0x7855        Short       tag for this extra block type ("Ux")
          TSize         Short       total data size for this block (4)
          UID           Short       Unix user ID
          GID           Short       Unix group ID

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (Unix2) 0x7855        Short       tag for this extra block type ("Ux")
          TSize         Short       total data size for this block (0)

          The data size of the central-header version is zero; it is used
          solely as a flag that UID/GID info is present in the local-header
          extra field.  If additional fields are ever added to the local
          version, the central version may be extended to indicate this.

          Note that Unix UIDs and GIDs are usually specific to a particular
          machine, and they generally require root access to restore.


         -ASi Unix Extra Field:
          ====================

          The following is the layout of the ASi extra block for Unix.  The
          local-header and central-header versions are identical.
          (Last Revision 19960916)

          Value         Size        Description
          -----         ----        -----------
  (Unix3) 0x756e        Short       tag for this extra block type ("nu")
          TSize         Short       total data size for this block
          CRC           Long        CRC-32 of the remaining data
          Mode          Short       file permissions
          SizDev        Long        symlink'd size OR major/minor dev num
          UID           Short       user ID
          GID           Short       group ID
          (var.)        variable    symbolic link filename

          Mode is the standard Unix st_mode field from struct stat, containing
          user/group/other permissions, setuid/setgid and symlink info, etc.

          If Mode indicates that this file is a symbolic link, SizDev is the
          size of the file to which the link points.  Otherwise, if the file
          is a device, SizDev contains the standard Unix st_rdev field from
          struct stat (includes the major and minor numbers of the device).
          SizDev is undefined in other cases.

          If Mode indicates that the file is a symbolic link, the final field
          will be the name of the file to which the link points.  The file-
          name length can be inferred from TSize.

          [Note that TSize may incorrectly refer to the data size not counting
           the CRC; i.e., it may be four bytes too small.]


         -BeOS Extra Field:
          ================

          The following is the layout of the file-attributes extra block for
          BeOS.  (Last Revision 19970531)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
  (BeOS)  0x6542        Short       tag for this extra block type ("Be")
          TSize         Short       total data size for this block
          BSize         Long        uncompressed file attribute data size
          Flags         Byte        info bits
          (CType)       Short       compression type
          (CRC)         Long        CRC value for uncompressed file attribs
          Attribs       variable    file attribute data

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (BeOS)  0x6542        Short       tag for this extra block type ("Be")
          TSize         Short       total data size for this block (5)
          BSize         Long        size of uncompr. local EF block data
          Flags         Byte        info bits

          The least significant bit of Flags in both headers indicates whether
          the LOCAL extra field is uncompressed (and therefore whether CType
          and CRC are omitted):

                bit 0           if set, Attribs is uncompressed (no CType, CRC)
                bits 1-7        reserved; if set, assume error or unknown data

          Currently the only supported compression types are deflated (type 8)
          and stored (type 0); the latter is not used by Info-ZIP's Zip but is
          supported by UnZip.

          Attribs is a BeOS-specific block of data in big-endian format with
          the following structure (if compressed, uncompress it first):

              Value     Size        Description
              -----     ----        -----------
              Name      variable    attribute name (null-terminated string)
              Type      Long        attribute type (32-bit unsigned integer)
              Size      Long Long   data size for this sub-block (64 bits)
              Data      variable    attribute data

          The attribute structure is repeated for every attribute.  The Data
          field may contain anything--text, flags, bitmaps, etc.


         -AtheOS Extra Field:
          ==================

          The following is the layout of the file-attributes extra block for
          AtheOS.  This field is a very close spin-off from the BeOS e.f.
          The only differences are:
           - a new extra field signature
           - numeric field in the attributes data are stored in little-endian
             format ("i386" was initial hardware for AtheOS)
          (Last Revision 20040908)

          Local-header version:

          Value         Size        Description
          -----         ----        -----------
 (AtheOS) 0x7441        Short       tag for this extra block type ("At")
          TSize         Short       total data size for this block
          BSize         Long        uncompressed file attribute data size
          Flags         Byte        info bits
          (CType)       Short       compression type
          (CRC)         Long        CRC value for uncompressed file attribs
          Attribs       variable    file attribute data

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
 (AtheOS) 0x7441        Short       tag for this extra block type ("At")
          TSize         Short       total data size for this block (5)
          BSize         Long        size of uncompr. local EF block data
          Flags         Byte        info bits

          The least significant bit of Flags in both headers indicates whether
          the LOCAL extra field is uncompressed (and therefore whether CType
          and CRC are omitted):

                bit 0           if set, Attribs is uncompressed (no CType, CRC)
                bits 1-7        reserved; if set, assume error or unknown data

          Currently the only supported compression types are deflated (type 8)
          and stored (type 0); the latter is not used by Info-ZIP's Zip but is
          supported by UnZip.

          Attribs is a AtheOS-specific block of data in little-endian format
          with the following structure (if compressed, uncompress it first):

              Value     Size        Description
              -----     ----        -----------
              Name      variable    attribute name (null-terminated string)
              Type      Long        attribute type (32-bit unsigned integer)
              Size      Long Long   data size for this sub-block (64 bits)
              Data      variable    attribute data

          The attribute structure is repeated for every attribute.  The Data
          field may contain anything--text, flags, bitmaps, etc.


         -SMS/QDOS Extra Field:
          ====================

          The following is the layout of the file-attributes extra block for
          SMS/QDOS.  The local-header and central-header versions are identical.
          (Last Revision 19960929)

          Value         Size        Description
          -----         ----        -----------
  (QDOS)  0xfb4a        Short       tag for this extra block type
          TSize         Short       total data size for this block
          LongID        Long        extra-field signature
          (ExtraID)     Long        additional signature/flag bytes
          QDirect       64 bytes    qdirect structure

          LongID may be "QZHD" or "QDOS".  In the latter case, ExtraID will
          be present.  Its first three bytes are "02\0"; the last byte is
          currently undefined.

          QDirect contains the file's uncompressed directory info (qdirect
          struct).  Its elements are in native (big-endian) format:

          d_length      beLong          file length
          d_access      byte            file access type
          d_type        byte            file type
          d_datalen     beLong          data length
          d_reserved    beLong          unused
          d_szname      beShort         size of filename
          d_name        36 bytes        filename
          d_update      beLong          time of last update
          d_refdate     beLong          file version number
          d_backup      beLong          time of last backup (archive date)


         -AOS/VS Extra Field:
          ==================

          The following is the layout of the extra block for Data General
          AOS/VS.  The local-header and central-header versions are identical.
          (Last Revision 19961125)

          Value         Size        Description
          -----         ----        -----------
  (AOSVS) 0x5356        Short       tag for this extra block type ("VS")
          TSize         Short       total data size for this block
          "FCI\0"       Long        extra-field signature
          Version       Byte        version of AOS/VS extra block (10 = 1.0)
          Fstat         variable    fstat packet
          AclBuf        variable    raw ACL data ($MXACL bytes)

          Fstat contains the file's uncompressed fstat packet, which is one of
          the following:

                normal fstat packet             (P_FSTAT struct)
                DIR/CPD fstat packet            (P_FSTAT_DIR struct)
                unit (device) fstat packet      (P_FSTAT_UNIT struct)
                IPC file fstat packet           (P_FSTAT_IPC struct)

          AclBuf contains the raw ACL data; its length is $MXACL.


         -Tandem NSK Extra Field:
          ======================

          The following is the layout of the file-attributes extra block for
          Tandem NSK.  The local-header and central-header versions are
          identical.  (Last Revision 19981221)

          Value         Size        Description
          -----         ----        -----------
  (TA)    0x4154        Short       tag for this extra block type ("TA")
          TSize         Short       total data size for this block (20)
          NSKattrs      20 Bytes    NSK attributes


         -THEOS Extra Field:
          =================

          The following is the layout of the file-attributes extra block for
          Theos.  The local-header and central-header versions are identical.
          (Last Revision 19990206)

          Value         Size        Description
          -----         ----        -----------
  (Theos) 0x6854        Short       'Th' signature
          size          Short       size of extra block
          flags         Byte        reserved for future use
          filesize      Long        file size
          fileorg       Byte        type of file (see below)
          keylen        Short       key length for indexed and keyed files,
                                    data segment size for 16 bits programs
          reclen        Short       record length for indexed,keyed and direct,
                                    text segment size for 16 bits programs
          filegrow      Byte        growing factor for indexed,keyed and direct
          protect       Byte        protections (see below)
          reserved      Short       reserved for future use

            File types
            ==========

            0x80  library (keyed access list of files)
            0x40  directory
            0x10  stream file
            0x08  direct file
            0x04  keyed file
            0x02  indexed file
            0x0e  reserved
            0x01  16 bits real mode program (obsolete)
            0x21  16 bits protected mode program
            0x41  32 bits protected mode program

            Protection codes
            ================

            User protection
            ---------------
            0x01  non readable
            0x02  non writable
            0x04  non executable
            0x08  non erasable

            Other protection
            ----------------
            0x10  non readable
            0x20  non writable
            0x40  non executable Theos before 4.0
            0x40  modified Theos 4.x
            0x80  not hidden


         -THEOS old inofficial Extra Field:
          ================================

          The following is the layout of an inoffical former version of a
          Theos file-attributes extra blocks.  This layout was never published
          and is no longer created. However, UnZip can optionally support it
          when compiling with the option flag OLD_THEOS_EXTRA defined.
          Both the local-header and central-header versions are identical.
          (Last Revision 19990206)

          Value         Size        Description
          -----         ----        -----------
  (THS0)  0x4854        Short       'TH' signature
          size          Short       size of extra block
          flags         Short       reserved for future use
          filesize      Long        file size
          reclen        Short       record length for indexed,keyed and direct,
                                    text segment size for 16 bits programs
          keylen        Short       key length for indexed and keyed files,
                                    data segment size for 16 bits programs
          filegrow      Byte        growing factor for indexed,keyed and direct
          reserved      3 Bytes     reserved for future use


         -FWKCS MD5 Extra Field (0x4b46):
          ==============================

          The FWKCS Contents_Signature System, used in automatically
          identifying files independent of filename, optionally adds
          and uses an extra field to support the rapid creation of
          an enhanced contents_signature.
          There is no local-header version; the following applies
          only to the central header.  (Last Revision 19961207)

          Central-header version:

          Value         Size        Description
          -----         ----        -----------
  (MD5)   0x4b46        Short       tag for this extra block type ("FK")
          TSize         Short       total data size for this block (19)
          "MD5"         3 bytes     extra-field signature
          MD5hash       16 bytes    128-bit MD5 hash of uncompressed data
                                    (low byte first)

          When FWKCS revises a .ZIP file central directory to add
          this extra field for a file, it also replaces the
          central directory entry for that file's uncompressed
          file length with a measured value.

          FWKCS provides an option to strip this extra field, if
          present, from a .ZIP file central directory. In adding
          this extra field, FWKCS preserves .ZIP file Authenticity
          Verification; if stripping this extra field, FWKCS
          preserves all versions of AV through PKZIP version 2.04g.

          FWKCS, and FWKCS Contents_Signature System, are
          trademarks of Frederick W. Kantor.

          (1) R. Rivest, RFC1321.TXT, MIT Laboratory for Computer
              Science and RSA Data Security, Inc., April 1992.
              ll.76-77: "The MD5 algorithm is being placed in the
              public domain for review and possible adoption as a
              standard."


      file comment: (Variable)

          The comment for this file.

      number of this disk: (2 bytes)

          The number of this disk, which contains central
          directory end record.  If an archive is in zip64 format
          and the value in this field is 0xFFFF, the size will
          be in the corresponding 4 byte zip64 end of central
          directory field.

      number of the disk with the start of the central directory: (2 bytes)

          The number of the disk on which the central
          directory starts.  If an archive is in zip64 format
          and the value in this field is 0xFFFF, the size will
          be in the corresponding 4 byte zip64 end of central
          directory field.

      total number of entries in the central dir on this disk: (2 bytes)

          The number of central directory entries on this disk.
          If an archive is in zip64 format and the value in
          this field is 0xFFFF, the size will be in the
          corresponding 8 byte zip64 end of central
          directory field.

      total number of entries in the central dir: (2 bytes)

          The total number of files in the .ZIP file.  If an
          archive is in zip64 format and the value in this field
          is 0xFFFF, the size will be in the corresponding 8 byte
          zip64 end of central directory field.

      size of the central directory: (4 bytes)

          The size (in bytes) of the entire central directory.
          If an archive is in zip64 format and the value in
          this field is 0xFFFFFFFF, the size will be in the
          corresponding 8 byte zip64 end of central
          directory field.

      offset of start of central directory with respect to
      the starting disk number:  (4 bytes)

          Offset of the start of the central directory on the
          disk on which the central directory starts.  If an
          archive is in zip64 format and the value in this
          field is 0xFFFFFFFF, the size will be in the
          corresponding 8 byte zip64 end of central
          directory field.

      .ZIP file comment length: (2 bytes)

          The length of the comment for this .ZIP file.

      .ZIP file comment: (Variable)

          The comment for this .ZIP file.  ZIP file comment data
          is stored unsecured.  No encryption or data authentication
          is applied to this area at this time.  Confidential information
          should not be stored in this section.

      zip64 extensible data sector    (variable size)

          (currently reserved for use by PKWARE)


  K.  General notes:

      1)  All fields unless otherwise noted are unsigned and stored
          in Intel low-byte:high-byte, low-word:high-word order.

      2)  String fields are not null terminated, since the
          length is given explicitly.

      3)  Local headers should not span disk boundaries.  Also, even
          though the central directory can span disk boundaries, no
          single record in the central directory should be split
          across disks.

      4)  The entries in the central directory may not necessarily
          be in the same order that files appear in the .ZIP file.

      5)  Spanned/Split archives created using PKZIP for Windows
          (V2.50 or greater), PKZIP Command Line (V2.50 or greater),
          or PKZIP Explorer will include a special spanning
          signature as the first 4 bytes of the first segment of
          the archive.  This signature (0x08074b50) will be
          followed immediately by the local header signature for
          the first file in the archive.  A special spanning
          marker may also appear in spanned/split archives if the
          spanning or splitting process starts but only requires
          one segment.  In this case the 0x08074b50 signature
          will be replaced with the temporary spanning marker
          signature of 0x30304b50.  Spanned/split archives
          created with this special signature are compatible with
          all versions of PKZIP from PKWARE.  Split archives can
          only be uncompressed by other versions of PKZIP that
          know how to create a split archive.

      6)  If one of the fields in the end of central directory
          record is too small to hold required data, the field
          should be set to -1 (0xFFFF or 0xFFFFFFFF) and the
          Zip64 format record should be created.

      7)  The end of central directory record and the
          Zip64 end of central directory locator record must
          reside on the same disk when splitting or spanning
          an archive.

V. UnShrinking - Method 1
-------------------------

Shrinking is a Dynamic Ziv-Lempel-Welch compression algorithm
with partial clearing.  The initial code size is 9 bits, and
the maximum code size is 13 bits.  Shrinking differs from
conventional Dynamic Ziv-Lempel-Welch implementations in several
respects:

1)  The code size is controlled by the compressor, and is not
    automatically increased when codes larger than the current
    code size are created (but not necessarily used).  When
    the decompressor encounters the code sequence 256
    (decimal) followed by 1, it should increase the code size
    read from the input stream to the next bit size.  No
    blocking of the codes is performed, so the next code at
    the increased size should be read from the input stream
    immediately after where the previous code at the smaller
    bit size was read.  Again, the decompressor should not
    increase the code size used until the sequence 256,1 is
    encountered.

2)  When the table becomes full, total clearing is not
    performed.  Rather, when the compressor emits the code
    sequence 256,2 (decimal), the decompressor should clear
    all leaf nodes from the Ziv-Lempel tree, and continue to
    use the current code size.  The nodes that are cleared
    from the Ziv-Lempel tree are then re-used, with the lowest
    code value re-used first, and the highest code value
    re-used last.  The compressor can emit the sequence 256,2
    at any time.


VI. Expanding - Methods 2-5
---------------------------

The Reducing algorithm is actually a combination of two
distinct algorithms.  The first algorithm compresses repeated
byte sequences, and the second algorithm takes the compressed
stream from the first algorithm and applies a probabilistic
compression method.

The probabilistic compression stores an array of 'follower
sets' S(j), for j=0 to 255, corresponding to each possible
ASCII character.  Each set contains between 0 and 32
characters, to be denoted as S(j)[0],...,S(j)[m], where m<32.
The sets are stored at the beginning of the data area for a
Reduced file, in reverse order, with S(255) first, and S(0)
last.

The sets are encoded as { N(j), S(j)[0],...,S(j)[N(j)-1] },
where N(j) is the size of set S(j).  N(j) can be 0, in which
case the follower set for S(j) is empty.  Each N(j) value is
encoded in 6 bits, followed by N(j) eight bit character values
corresponding to S(j)[0] to S(j)[N(j)-1] respectively.  If
N(j) is 0, then no values for S(j) are stored, and the value
for N(j-1) immediately follows.

Immediately after the follower sets, is the compressed data
stream.  The compressed data stream can be interpreted for the
probabilistic decompression as follows:


let Last-Character <- 0.
loop until done
    if the follower set S(Last-Character) is empty then
        read 8 bits from the input stream, and copy this
        value to the output stream.
    otherwise if the follower set S(Last-Character) is non-empty then
        read 1 bit from the input stream.
        if this bit is not zero then
            read 8 bits from the input stream, and copy this
            value to the output stream.
        otherwise if this bit is zero then
            read B(N(Last-Character)) bits from the input
            stream, and assign this value to I.
            Copy the value of S(Last-Character)[I] to the
            output stream.

    assign the last value placed on the output stream to
    Last-Character.
end loop


B(N(j)) is defined as the minimal number of bits required to
encode the value N(j)-1.


The decompressed stream from above can then be expanded to
re-create the original file as follows:


let State <- 0.

loop until done
    read 8 bits from the input stream into C.
    case State of
        0:  if C is not equal to DLE (144 decimal) then
                copy C to the output stream.
            otherwise if C is equal to DLE then
                let State <- 1.

        1:  if C is non-zero then
                let V <- C.
                let Len <- L(V)
                let State <- F(Len).
            otherwise if C is zero then
                copy the value 144 (decimal) to the output stream.
                let State <- 0

        2:  let Len <- Len + C
            let State <- 3.

        3:  move backwards D(V,C) bytes in the output stream
            (if this position is before the start of the output
            stream, then assume that all the data before the
            start of the output stream is filled with zeros).
            copy Len+3 bytes from this position to the output stream.
            let State <- 0.
    end case
end loop


The functions F,L, and D are dependent on the 'compression
factor', 1 through 4, and are defined as follows:

For compression factor 1:
    L(X) equals the lower 7 bits of X.
    F(X) equals 2 if X equals 127 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 1 bit of X) * 256 + Y + 1.
For compression factor 2:
    L(X) equals the lower 6 bits of X.
    F(X) equals 2 if X equals 63 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 2 bits of X) * 256 + Y + 1.
For compression factor 3:
    L(X) equals the lower 5 bits of X.
    F(X) equals 2 if X equals 31 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 3 bits of X) * 256 + Y + 1.
For compression factor 4:
    L(X) equals the lower 4 bits of X.
    F(X) equals 2 if X equals 15 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 4 bits of X) * 256 + Y + 1.


VII. Imploding - Method 6
-------------------------

The Imploding algorithm is actually a combination of two distinct
algorithms.  The first algorithm compresses repeated byte
sequences using a sliding dictionary.  The second algorithm is
used to compress the encoding of the sliding dictionary output,
using multiple Shannon-Fano trees.

The Imploding algorithm can use a 4K or 8K sliding dictionary
size. The dictionary size used can be determined by bit 1 in the
general purpose flag word; a 0 bit indicates a 4K dictionary
while a 1 bit indicates an 8K dictionary.

The Shannon-Fano trees are stored at the start of the compressed
file. The number of trees stored is defined by bit 2 in the
general purpose flag word; a 0 bit indicates two trees stored, a
1 bit indicates three trees are stored.  If 3 trees are stored,
the first Shannon-Fano tree represents the encoding of the
Literal characters, the second tree represents the encoding of
the Length information, the third represents the encoding of the
Distance information.  When 2 Shannon-Fano trees are stored, the
Length tree is stored first, followed by the Distance tree.

The Literal Shannon-Fano tree, if present is used to represent
the entire ASCII character set, and contains 256 values.  This
tree is used to compress any data not compressed by the sliding
dictionary algorithm.  When this tree is present, the Minimum
Match Length for the sliding dictionary is 3.  If this tree is
not present, the Minimum Match Length is 2.

The Length Shannon-Fano tree is used to compress the Length part
of the (length,distance) pairs from the sliding dictionary
output.  The Length tree contains 64 values, ranging from the
Minimum Match Length, to 63 plus the Minimum Match Length.

The Distance Shannon-Fano tree is used to compress the Distance
part of the (length,distance) pairs from the sliding dictionary
output. The Distance tree contains 64 values, ranging from 0 to
63, representing the upper 6 bits of the distance value.  The
distance values themselves will be between 0 and the sliding
dictionary size, either 4K or 8K.

The Shannon-Fano trees themselves are stored in a compressed
format. The first byte of the tree data represents the number of
bytes of data representing the (compressed) Shannon-Fano tree
minus 1.  The remaining bytes represent the Shannon-Fano tree
data encoded as:

    High 4 bits: Number of values at this bit length + 1. (1 - 16)
    Low  4 bits: Bit Length needed to represent value + 1. (1 - 16)

The Shannon-Fano codes can be constructed from the bit lengths
using the following algorithm:

1)  Sort the Bit Lengths in ascending order, while retaining the
    order of the original lengths stored in the file.

2)  Generate the Shannon-Fano trees:

    Code <- 0
    CodeIncrement <- 0
    LastBitLength <- 0
    i <- number of Shannon-Fano codes - 1   (either 255 or 63)

    loop while i >= 0
        Code = Code + CodeIncrement
        if BitLength(i) <> LastBitLength then
            LastBitLength=BitLength(i)
            CodeIncrement = 1 shifted left (16 - LastBitLength)
        ShannonCode(i) = Code
        i <- i - 1
    end loop


3)  Reverse the order of all the bits in the above ShannonCode()
    vector, so that the most significant bit becomes the least
    significant bit.  For example, the value 0x1234 (hex) would
    become 0x2C48 (hex).

4)  Restore the order of Shannon-Fano codes as originally stored
    within the file.

Example:

    This example will show the encoding of a Shannon-Fano tree
    of size 8.  Notice that the actual Shannon-Fano trees used
    for Imploding are either 64 or 256 entries in size.

Example:   0x02, 0x42, 0x01, 0x13

    The first byte indicates 3 values in this table.  Decoding the
    bytes:
            0x42 = 5 codes of 3 bits long
            0x01 = 1 code  of 2 bits long
            0x13 = 2 codes of 4 bits long

    This would generate the original bit length array of:
    (3, 3, 3, 3, 3, 2, 4, 4)

    There are 8 codes in this table for the values 0 thru 7.  Using
    the algorithm to obtain the Shannon-Fano codes produces:

                                  Reversed     Order     Original
Val  Sorted   Constructed Code      Value     Restored    Length
---  ------   -----------------   --------    --------    ------
0:     2      1100000000000000        11       101          3
1:     3      1010000000000000       101       001          3
2:     3      1000000000000000       001       110          3
3:     3      0110000000000000       110       010          3
4:     3      0100000000000000       010       100          3
5:     3      0010000000000000       100        11          2
6:     4      0001000000000000      1000      1000          4
7:     4      0000000000000000      0000      0000          4


The values in the Val, Order Restored and Original Length columns
now represent the Shannon-Fano encoding tree that can be used for
decoding the Shannon-Fano encoded data.  How to parse the
variable length Shannon-Fano values from the data stream is beyond
the scope of this document.  (See the references listed at the end of
this document for more information.)  However, traditional decoding
schemes used for Huffman variable length decoding, such as the
Greenlaw algorithm, can be successfully applied.

The compressed data stream begins immediately after the
compressed Shannon-Fano data.  The compressed data stream can be
interpreted as follows:

loop until done
    read 1 bit from input stream.

    if this bit is non-zero then       (encoded data is literal data)
        if Literal Shannon-Fano tree is present
            read and decode character using Literal Shannon-Fano tree.
        otherwise
            read 8 bits from input stream.
        copy character to the output stream.
    otherwise              (encoded data is sliding dictionary match)
        if 8K dictionary size
            read 7 bits for offset Distance (lower 7 bits of offset).
        otherwise
            read 6 bits for offset Distance (lower 6 bits of offset).

        using the Distance Shannon-Fano tree, read and decode the
          upper 6 bits of the Distance value.

        using the Length Shannon-Fano tree, read and decode
          the Length value.

        Length <- Length + Minimum Match Length

        if Length = 63 + Minimum Match Length
            read 8 bits from the input stream,
            add this value to Length.

        move backwards Distance+1 bytes in the output stream, and
        copy Length characters from this position to the output
        stream.  (if this position is before the start of the output
        stream, then assume that all the data before the start of
        the output stream is filled with zeros).
end loop

VIII. Tokenizing - Method 7
---------------------------

This method is not used by PKZIP.

IX. Deflating - Method 8
------------------------

The Deflate algorithm is similar to the Implode algorithm using
a sliding dictionary of up to 32K with secondary compression
from Huffman/Shannon-Fano codes.

The compressed data is stored in blocks with a header describing
the block and the Huffman codes used in the data block.  The header
format is as follows:

   Bit 0: Last Block bit     This bit is set to 1 if this is the last
                             compressed block in the data.
   Bits 1-2: Block type
      00 (0) - Block is stored - All stored data is byte aligned.
               Skip bits until next byte, then next word = block
               length, followed by the ones compliment of the block
               length word. Remaining data in block is the stored
               data.

      01 (1) - Use fixed Huffman codes for literal and distance codes.
               Lit Code    Bits             Dist Code   Bits
               ---------   ----             ---------   ----
                 0 - 143    8                 0 - 31      5
               144 - 255    9
               256 - 279    7
               280 - 287    8

               Literal codes 286-287 and distance codes 30-31 are
               never used but participate in the huffman construction.

      10 (2) - Dynamic Huffman codes.  (See expanding Huffman codes)

      11 (3) - Reserved - Flag a "Error in compressed data" if seen.

Expanding Huffman Codes
-----------------------
If the data block is stored with dynamic Huffman codes, the Huffman
codes are sent in the following compressed format:

   5 Bits: # of Literal codes sent - 257 (257 - 286)
           All other codes are never sent.
   5 Bits: # of Dist codes - 1           (1 - 32)
   4 Bits: # of Bit Length codes - 4     (4 - 19)

The Huffman codes are sent as bit lengths and the codes are built as
described in the implode algorithm.  The bit lengths themselves are
compressed with Huffman codes.  There are 19 bit length codes:

   0 - 15: Represent bit lengths of 0 - 15
       16: Copy the previous bit length 3 - 6 times.
           The next 2 bits indicate repeat length (0 = 3, ... ,3 = 6)
              Example:  Codes 8, 16 (+2 bits 11), 16 (+2 bits 10) will
                        expand to 12 bit lengths of 8 (1 + 6 + 5)
       17: Repeat a bit length of 0 for 3 - 10 times. (3 bits of length)
       18: Repeat a bit length of 0 for 11 - 138 times (7 bits of length)

The lengths of the bit length codes are sent packed 3 bits per value
(0 - 7) in the following order:

   16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15

The Huffman codes should be built as described in the Implode algorithm
except codes are assigned starting at the shortest bit length, i.e. the
shortest code should be all 0's rather than all 1's.  Also, codes with
a bit length of zero do not participate in the tree construction.  The
codes are then used to decode the bit lengths for the literal and
distance tables.

The bit lengths for the literal tables are sent first with the number
of entries sent described by the 5 bits sent earlier.  There are up
to 286 literal characters; the first 256 represent the respective 8
bit character, code 256 represents the End-Of-Block code, the remaining
29 codes represent copy lengths of 3 thru 258.  There are up to 30
distance codes representing distances from 1 thru 32k as described
below.

                             Length Codes
                             ------------
      Extra             Extra              Extra              Extra
 Code Bits Length  Code Bits Lengths  Code Bits Lengths  Code Bits Length(s)
 ---- ---- ------  ---- ---- -------  ---- ---- -------  ---- ---- ---------
  257   0     3     265   1   11,12    273   3   35-42    281   5  131-162
  258   0     4     266   1   13,14    274   3   43-50    282   5  163-194
  259   0     5     267   1   15,16    275   3   51-58    283   5  195-226
  260   0     6     268   1   17,18    276   3   59-66    284   5  227-258
  261   0     7     269   2   19-22    277   4   67-82    285   0    258
  262   0     8     270   2   23-26    278   4   83-98
  263   0     9     271   2   27-30    279   4   99-114
  264   0    10     272   2   31-34    280   4  115-130

                            Distance Codes
                            --------------
      Extra           Extra             Extra               Extra
 Code Bits Dist  Code Bits  Dist   Code Bits Distance  Code Bits Distance
 ---- ---- ----  ---- ---- ------  ---- ---- --------  ---- ---- --------
   0   0    1      8   3   17-24    16    7  257-384    24   11  4097-6144
   1   0    2      9   3   25-32    17    7  385-512    25   11  6145-8192
   2   0    3     10   4   33-48    18    8  513-768    26   12  8193-12288
   3   0    4     11   4   49-64    19    8  769-1024   27   12 12289-16384
   4   1   5,6    12   5   65-96    20    9 1025-1536   28   13 16385-24576
   5   1   7,8    13   5   97-128   21    9 1537-2048   29   13 24577-32768
   6   2   9-12   14   6  129-192   22   10 2049-3072
   7   2  13-16   15   6  193-256   23   10 3073-4096

The compressed data stream begins immediately after the
compressed header data.  The compressed data stream can be
interpreted as follows:

do
   read header from input stream.

   if stored block
      skip bits until byte aligned
      read count and 1's compliment of count
      copy count bytes data block
   otherwise
      loop until end of block code sent
         decode literal character from input stream
         if literal < 256
            copy character to the output stream
         otherwise
            if literal = end of block
               break from loop
            otherwise
               decode distance from input stream

               move backwards distance bytes in the output stream, and
               copy length characters from this position to the output
               stream.
      end loop
while not last block

if data descriptor exists
   skip bits until byte aligned
   check data descriptor signature
   read crc and sizes
endif

X. Enhanced Deflating - Method 9
--------------------------------

The Enhanced Deflating algorithm is similar to Deflate but
uses a sliding dictionary of up to 64K. Deflate64(tm) is supported
by the Deflate extractor.

[This description is inofficial.  It has been deduced by Info-ZIP from
close inspection of PKZIP 4.x Deflate64(tm) compressed output.]

The Deflate64 algorithm is almost identical to the normal Deflate algorithm.
Differences are:

- The sliding window size is 64k.

- The previously unused distance codes 30 and 31 are now used to describe
  match distances from 32k-48k and 48k-64k.
         Extra
    Code Bits Distance
    ---- ---- -----------
     ..   ..      ...
     29   13  24577-32768
     30   14  32769-49152
     31   14  49153-65536

- The semantics of the "maximum match length" code #258 has been changed to
  allow the specification of arbitrary large match lengths (up to 64k).
         Extra
    Code Bits Lengths
    ---- ---- ------
     ...  ..    ...
     284   5  227-258
     285  16  3-65538

Whereas the first two modifications fit into the framework of Deflate,
this last change breaks compatibility with Deflate method 8. Thus, a
Deflate64 decompressor cannot decode normal deflated data.

XI. BZIP2 - Method 12
---------------------

BZIP2 is an open-source data compression algorithm developed by
Julian Seward.  Information and source code for this algorithm
can be found on the internet.


XII. Traditional PKWARE Encryption
----------------------------------

The following information discusses the decryption steps
required to support traditional PKWARE encryption.  This
form of encryption is considered weak by today's standards
and its use is recommended only for situations with
low security needs or for compatibility with older .ZIP
applications.

XIII. Decryption
----------------

The encryption used in PKZIP was generously supplied by Roger
Schlafly.  PKWARE is grateful to Mr. Schlafly for his expert
help and advice in the field of data encryption.

PKZIP encrypts the compressed data stream.  Encrypted files must
be decrypted before they can be extracted.

Each encrypted file has an extra 12 bytes stored at the start of
the data area defining the encryption header for that file.  The
encryption header is originally set to random values, and then
itself encrypted, using three, 32-bit keys.  The key values are
initialized using the supplied encryption password.  After each byte
is encrypted, the keys are then updated using pseudo-random number
generation techniques in combination with the same CRC-32 algorithm
used in PKZIP and described elsewhere in this document.

The following is the basic steps required to decrypt a file:

1) Initialize the three 32-bit keys with the password.
2) Read and decrypt the 12-byte encryption header, further
   initializing the encryption keys.
3) Read and decrypt the compressed data stream using the
   encryption keys.


Step 1 - Initializing the encryption keys
-----------------------------------------

Key(0) <- 305419896
Key(1) <- 591751049
Key(2) <- 878082192

loop for i <- 0 to length(password)-1
    update_keys(password(i))
end loop


Where update_keys() is defined as:


update_keys(char):
  Key(0) <- crc32(key(0),char)
  Key(1) <- Key(1) + (Key(0) & 000000ffH)
  Key(1) <- Key(1) * 134775813 + 1
  Key(2) <- crc32(key(2),key(1) >> 24)
end update_keys


Where crc32(old_crc,char) is a routine that given a CRC value and a
character, returns an updated CRC value after applying the CRC-32
algorithm described elsewhere in this document.


Step 2 - Decrypting the encryption header
-----------------------------------------

The purpose of this step is to further initialize the encryption
keys, based on random data, to render a plaintext attack on the
data ineffective.


Read the 12-byte encryption header into Buffer, in locations
Buffer(0) thru Buffer(11).

loop for i <- 0 to 11
    C <- buffer(i) ^ decrypt_byte()
    update_keys(C)
    buffer(i) <- C
end loop


Where decrypt_byte() is defined as:


unsigned char decrypt_byte()
    local unsigned short temp
    temp <- Key(2) | 2
    decrypt_byte <- (temp * (temp ^ 1)) >> 8
end decrypt_byte


After the header is decrypted,  the last 1 or 2 bytes in Buffer
should be the high-order word/byte of the CRC for the file being
decrypted, stored in Intel low-byte/high-byte order, or the high-order
byte of the file time if bit 3 of the general purpose bit flag is set.
Versions of PKZIP prior to 2.0 used a 2 byte CRC check; a 1 byte CRC check is
used on versions after 2.0.  This can be used to test if the password
supplied is correct or not.


Step 3 - Decrypting the compressed data stream
----------------------------------------------

The compressed data stream can be decrypted as follows:


loop until done
    read a character into C
    Temp <- C ^ decrypt_byte()
    update_keys(temp)
    output Temp
end loop


XIV. Strong Encryption Specification (EFS)
------------------------------------------

Version 5.x of this specification introduced support for strong
encryption algorithms.  These algorithms can be used with either
a password or an X.509v3 digital certificate to encrypt each file.
This format specification supports either password or certificate
based encryption to meet the security needs of today, to enable
interoperability between users within both PKI and non-PKI
environments, and to ensure interoperability between different
computing platforms that are running a ZIP program.

Password based encryption is the most common form of encryption
people are familiar with.  However, inherent weaknesses with
passwords (e.g. susceptibility to dictionary/brute force attack)
as well as password management and support issues make certificate
based encryption a more secure and scalable option.  Industry
efforts and support are defining and moving towards more advanced
security solutions built around X.509v3 digital certificates and
Public Key Infrastructures(PKI) because of the greater scalability,
administrative options, and more robust security over traditional
password-based encryption.

Most standard encryption algorithms are supported with this
specification. Reference implementations for many of these
algorithms are available from either commercial or open source
distributors.  Readily available cryptographic toolkits make
implementation of the encryption features straight-forward.
This document is not intended to provide a treatise on data
encryption principles or theory.  Its purpose is to document the
data structures required for implementing interoperable data
encryption within the .ZIP format.  It is strongly recommended that
you have a good understanding of data encryption before reading
further.

The algorithms introduced in Version 5.0 of this specification
include:

    RC2 40 bit, 64 bit, and 128 bit
    RC4 40 bit, 64 bit, and 128 bit
    DES
    3DES 112 bit and 168 bit

Version 5.1 adds support for the following:

    AES 128 bit, 192 bit, and 256 bit

Version 6.1 introduces encryption data changes to support
interoperability with SmartCard and USB Token certificate storage
methods which do not support the OAEP strengthening standard.

Version 6.2 introduces support for encrypting metadata by compressing
and encrypting the central directory data structure to reduce information
leakage.   Information leakage can occur in legacy ZIP applications
through exposure of information about a file even though that file is
stored encrypted.  The information exposed consists of file
characteristics stored within the records and fields defined by this
specification.  This includes data such as a files name, its original
size, timestamp and CRC32 value.

Central Directory Encryption provides greater protection against
information leakage by encrypting the Central Directory structure and
by masking key values that are replicated in the unencrypted Local
Header.   ZIP compatible programs that cannot interpret an encrypted
Central Directory structure cannot rely on the data in the corresponding
Local Header for decompression information.

Extra Field records that may contain information about a file that should
not be exposed should not be stored in the Local Header and should only
be written to the Central Directory where they can be encrypted.  This
design currently does not support streaming.  Information in the End of
Central Directory record, the ZIP64 End of Central Directory Locator,
and the ZIP64 End of Central Directory record are not encrypted.  Access
to view data on files within a ZIP file with an encrypted Central Directory
requires the appropriate password or private key for decryption prior to
viewing any files, or any information about the files, in the archive.

Older ZIP compatible programs not familiar with the Central Directory
Encryption feature will no longer be able to recognize the Central
Directory and may assume the ZIP file is corrupt.  Programs that
attempt streaming access using Local Headers will see invalid
information for each file.  Central Directory Encryption need not be
used for every ZIP file.  Its use is recommended for greater security.
ZIP files not using Central Directory Encryption should operate as
in the past.

The details of the strong encryption specification for certificates
remain under development as design and testing issues are worked out
for the range of algorithms, encryption methods, certificate processing
and cross-platform support necessary to meet the advanced security needs
of .ZIP file users today and in the future.

This feature specification is intended to support basic encryption needs
of today, such as password support.  However this specification is also
designed to lay the foundation for future advanced security needs.

Encryption provides data confidentiality and privacy.  It is
recommended that you combine X.509 digital signing with encryption
to add authentication and non-repudiation.


Single Password Symmetric Encryption Method:
-------------------------------------------

The Single Password Symmetric Encryption Method using strong
encryption algorithms operates similarly to the traditional
PKWARE encryption defined in this format.  Additional data
structures are added to support the processing needs of the
strong algorithms.

The Strong Encryption data structures are:

1. General Purpose Bits - Bits 0 and 6 of the General Purpose bit
flag in both local and central header records.  Both bits set
indicates strong encryption.  Bit 13, when set indicates the Central
Directory is encrypted and that selected fields in the Local Header
are masked to hide their actual value.


2. Extra Field 0x0017 in central header only.

     Fields to consider in this record are:

     Format - the data format identifier for this record.  The only
     value allowed at this time is the integer value 2.

     AlgId - integer identifier of the encryption algorithm from the
     following range

         0x6601 - DES
         0x6602 - RC2 (version needed to extract < 5.2)
         0x6603 - 3DES 168
         0x6609 - 3DES 112
         0x660E - AES 128
         0x660F - AES 192
         0x6610 - AES 256
         0x6702 - RC2 (version needed to extract >= 5.2)
         0x6801 - RC4
         0xFFFF - Unknown algorithm

     Bitlen - Explicit bit length of key

          40
          56
          64
         112
         128
         168
         192
         256

     Flags - Processing flags needed for decryption

         0x0001 - Password is required to decrypt
         0x0002 - Certificates only
         0x0003 - Password or certificate required to decrypt

         Values > 0x0003 reserved for certificate processing


3. Decryption header record preceeding compressed file data.

         -Decryption Header:

          Value     Size     Description
          -----     ----     -----------
          IVSize    2 bytes  Size of initialization vector (IV)
          IVData    IVSize   Initialization vector for this file
          Size      4 bytes  Size of remaining decryption header data
          Format    2 bytes  Format definition for this record
          AlgID     2 bytes  Encryption algorithm identifier
          Bitlen    2 bytes  Bit length of encryption key
          Flags     2 bytes  Processing flags
          ErdSize   2 bytes  Size of Encrypted Random Data
          ErdData   ErdSize  Encrypted Random Data
          Reserved1 4 bytes  Reserved certificate processing data
          Reserved2 (var)    Reserved for certificate processing data
          VSize     2 bytes  Size of password validation data
          VData     VSize-4  Password validation data
          VCRC32    4 bytes  Standard ZIP CRC32 of password validation data

     IVData - The size of the IV should match the algorithm block size.
              The IVData can be completely random data.  If the size of
              the randomly generated data does not match the block size
              it should be complemented with zero's or truncated as
              necessary.  If IVSize is 0, then IV = CRC32 + Uncompressed
              File Size (as a 64 bit little-endian, unsigned integer value).

     Format - the data format identifier for this record.  The only
     value allowed at this time is the integer value 3.

     AlgId - integer identifier of the encryption algorithm from the
     following range

         0x6601 - DES
         0x6602 - RC2 (version needed to extract < 5.2)
         0x6603 - 3DES 168
         0x6609 - 3DES 112
         0x660E - AES 128
         0x660F - AES 192
         0x6610 - AES 256
         0x6702 - RC2 (version needed to extract >= 5.2)
         0x6801 - RC4
         0xFFFF - Unknown algorithm

     Bitlen - Explicit bit length of key

          40
          56
          64
         112
         128
         168
         192
         256

     Flags - Processing flags needed for decryption

         0x0001 - Password is required to decrypt
         0x0002 - Certificates only
         0x0003 - Password or certificate required to decrypt

         Values > 0x0003 reserved for certificate processing

     ErdData - Encrypted random data is used to generate a file
               session key for encrypting each file.  SHA1 is
               used to calculate hash data used to derive keys.
               File session keys are derived from a master session
               key generated from the user-supplied password.
               If the Flags field in the decryption header contains
               the value 0x4000, then the ErdData field must be
               decrypted using 3DES.

     Reserved1 - Reserved for certificate processing, if value is
               zero, then Reserved2 data is absent.  See the explanation
               under the Certificate Processing Method for details on
               this data structure.

     Reserved2 - If present, the size of the Reserved2 data structure
               is located by skipping the first 4 bytes of this field
               and using the next 2 bytes as the remaining size.  See
               the explanation under the Certificate Processing Method
               for details on this data structure.

     VSize - This size value will always include the 4 bytes of the
             VCRC32 data and will be greater than 4 bytes.

     VData - Random data for password validation.  This data is VSize
             in length and VSize must be a multiple of the encryption
             block size.  VCRC32 is a checksum value of VData.
             VData and VCRC32 are stored encrypted and start the
             stream of encrypted data for a file.

4. Single Password Central Directory Encryption

Central Directory Encryption is achieved within the .ZIP format by
encrypting the Central Directory structure.  This encapsulates the metadata
most often used for processing .ZIP files.  Additional metadata is stored for
redundancy in the Local Header for each file.  The process of concealing
metadata by encrypting the Central Directory does not protect the data within
the Local Header.  To avoid information leakage from the exposed metadata
in the Local Header, the fields containing information about a file are masked.

Local Header:

Masking replaces the true content of the fields for a file in the Local
Header with false information.  When masked, the Local Header is not
suitable for streaming access and the options for data recovery of damaged
archives is reduced.  Extra Data fields that may contain confidential
data should not be stored within the Local Header.  The value set into
the Version needed to extract field should be the correct value needed to
extract the file without regard to Central Directory Encryption. The fields
within the Local Header targeted for masking when the Central Directory is
encrypted are:

        Field Name                     Mask Value
        ------------------             ---------------------------
        compression method              0
        last mod file time              0
        last mod file date              0
        crc-32                          0
        compressed size                 0
        uncompressed size               0
        file name (variable size)       Base 16 value from the
                                        range 1 - FFFFFFFFFFFFFFFF
                                        represented as a string whose
                                        size will be set into the
                                        file name length field

The Base 16 value assigned as a masked file name is simply a sequentially
incremented value for each file starting with 1 for the first file.
Modifications to a ZIP file may cause different values to be stored for
each file.  For compatibility, the file name field in the Local Header
should never be left blank.  As of Version 6.2 of this specification,
the Compression Method and Compressed Size fields are not yet masked.

Encrypting the Central Directory:

Encryption of the Central Directory does not include encryption of the
Central Directory Signature data, the ZIP64 End of Central Directory
record, the ZIP64 End of Central Directory Locator, or the End
of Central Directory record.  The ZIP file comment data is never
encrypted.

Before encrypting the Central Directory, it may optionally be compressed.
Compression is not required, but for storage efficiency it is assumed
this structure will be compressed before encrypting.  Similarly, this
specification supports compressing the Central Directory without
requiring that it also be encrypted.  Early implementations of this
feature will assume the encryption method applied to files matches the
encryption applied to the Central Directory.

Encryption of the Central Directory is done in a manner similar to
that of file encryption.  The encrypted data is preceded by a
decryption header.  The decryption header is known as the Archive
Decryption Header.  The fields of this record are identical to
the decryption header preceding each encrypted file.  The location
of the Archive Decryption Header is determined by the value in the
Start of the Central Directory field in the ZIP64 End of Central
Directory record.  When the Central Directory is encrypted, the
ZIP64 End of Central Directory record will always be present.

The layout of the ZIP64 End of Central Directory record for all
versions starting with 6.2 of this specification will follow the
Version 2 format.  The Version 2 format is as follows:

The first 48 bytes will remain identical to that of Version 1.
The record signature for both Version 1 and Version 2 will be
0x06064b50.  Immediately following the 48th byte, which identifies
the end of the field known as the Offset of Start of Central
Directory With Respect to the Starting Disk Number will begin the
new fields defining Version 2 of this record.

New fields for Version 2:

Note: all fields stored in Intel low-byte/high-byte order.

          Value                 Size       Description
          -----                 ----       -----------
          Compression Method    2 bytes    Method used to compress the
                                           Central Directory
          Compressed Size       8 bytes    Size of the compressed data
          Original   Size       8 bytes    Original uncompressed size
          AlgId                 2 bytes    Encryption algorithm ID
          BitLen                2 bytes    Encryption key length
          Flags                 2 bytes    Encryption flags
          HashID                2 bytes    Hash algorithm identifier
          Hash Length           2 bytes    Length of hash data
          Hash Data             (variable) Hash data

The Compression Method accepts the same range of values as the
corresponding field in the Central Header.

The Compressed Size and Original Size values will not include the
data of the Central Directory Signature which is compressed or
encrypted.

The AlgId, BitLen, and Flags fields accept the same range of values
the corresponding fields within the 0x0017 record.

Hash ID identifies the algorithm used to hash the Central Directory
data.  This data does not have to be hashed, in which case the
values for both the HashID and Hash Length will be 0.  Possible
values for HashID are:

      Value         Algorithm
     ------         ---------
     0x0000          none
     0x0001          CRC32
     0x8003          MD5
     0x8004          SHA1

When the Central Directory data is signed, the same hash algorithm
used to hash the Central Directory for signing should be used.
This is recommended for processing efficiency, however, it is
permissible for any of the above algorithms to be used independent
of the signing process.

The Hash Data will contain the hash data for the Central Directory.
The length of this data will vary depending on the algorithm used.

The Version Needed to Extract should be set to 62.

The value for the Total Number of Entries on the Current Disk will
be 0.  These records will no longer support random access when
encrypting the Central Directory.

When the Central Directory is compressed and/or encrypted, the
End of Central Directory record will store the value 0xFFFFFFFF
as the value for the Total Number of Entries in the Central
Directory.  The value stored in the Total Number of Entries in
the Central Directory on this Disk field will be 0.  The actual
values will be stored in the equivalent fields of the ZIP64
End of Central Directory record.

Decrypting and decompressing the Central Directory is accomplished
in the same manner as decrypting and decompressing a file.


5. Useful Tips

Strong Encryption is always applied to a file after compression. The
block oriented algorithms all operate in Cypher Block Chaining (CBC)
mode.  The block size used for AES encryption is 16.  All other block
algorithms use a block size of 8.  Two ID's are defined for RC2 to
account for a discrepancy found in the implementation of the RC2
algorithm in the cryptographic library on Windows XP SP1 and all
earlier versions of Windows.

A pseudo-code representation of the encryption process is as follows:

Password = GetUserPassword()
RD  = Random()
ERD = Encrypt(RD,DeriveKey(SHA1(Password)))
For Each File
    IV = Random()
    VData = Random()
    FileSessionKey = DeriveKey(SHA1(IV + RD))
    Encrypt(VData + VCRC32 + FileData,FileSessionKey)
Done

The function names and parameter requirements will depend on
the choice of the cryptographic toolkit selected.  Almost any
toolkit supporting the reference implementations for each
algorithm can be used.  The RSA BSAFE(r), OpenSSL, and Microsoft
CryptoAPI libraries are all known to work well.


Certificate Processing Method:
-----------------------------

The Certificate Processing Method for ZIP file encryption remains
under development.  The information provided here serves as a guide
to those interested in certificate-based data decryption.  This
information may be subject to change in future versions of this
specification and is subject to change without notice.

OAEP Processing with Certificate-based Encryption:

Versions of PKZIP available during this development phase of the
certificate processing method may set a value of 61 into the
version needed to extract field for a file.  This indicates that
non-OAEP key wrapping is used.  This affects certificate encryption
only, and password encryption functions should not be affected by
this value.  This means values of 61 may be found on files encrypted
with certificates only, or on files encrypted with both password
encryption and certificate encryption.  Files encrypted with both
methods can safely be decrypted using the password methods documented.

OAEP stands for Optimal Asymmetric Encryption Padding.  It is a
strengthening technique used for small encoded items such as decryption
keys.  This is commonly applied in cryptographic key-wrapping techniques
and is supported by PKCS #1.  Versions 5.0 and 6.0 of this specification
were designed to support OAEP key-wrapping for certificate-based
decryption keys for additional security.

Support for private keys stored on Smart Cards or Tokens introduced
a conflict with this OAEP logic.  Most card and token products do
not support the additional strengthening applied to OAEP key-wrapped
data.  In order to resolve this conflict, versions 6.1 and above of this
specification will no longer support OAEP when encrypting using
digital certificates.

Certificate Processing Data Fields:

The Certificate Processing Method of this specification defines the
following additional data fields:


1. Certificate Flag Values

Additional processing flags that can be present in the Flags field of both
the 0x0017 field of the central directory Extra Field and the Decryption
header record preceding compressed file data are:

         0x0007 - reserved for future use
         0x000F - reserved for future use
         0x0100 - Indicates non-OAEP key wrapping was used.  If this
                  this field is set, the version needed to extract must
                  be at least 61.  This means OAEP key wrapping is not
                  used when generating a Master Session Key using
                  ErdData.
         0x4000 - ErdData must be decrypted using 3DES-168, otherwise use the
                  same algorithm used for encrypting the file contents.
         0x8000 - reserved for future use


2. CertData - Extra Field 0x0017 record certificate data structure

The data structure used to store certificate data within the section
of the Extra Field defined by the CertData field of the 0x0017
record are as shown:

          Value     Size     Description
          -----     ----     -----------
          RCount    4 bytes  Number of recipients.
          HashAlg   2 bytes  Hash algorithm identifier
          HSize     2 bytes  Hash size
          SRList    (var)    Simple list of recipients hashed public keys


     RCount    This defines the number intended recipients whose
               public keys were used for encryption.  This identifies
               the number of elements in the SRList.

     HashAlg   This defines the hash algorithm used to calculate
               the public key hash of each public key used
               for encryption. This field currently supports
               only the following value for SHA-1

               0x8004 - SHA1

     HSize     This defines the size of a hashed public key.

     SRList    This is a variable length list of the hashed
               public keys for each intended recipient.  Each
               element in this list is HSize.  The total size of
               SRList is determined using RCount * HSize.


3. Reserved1 - Certificate Decryption Header Reserved1 Data:

          Value     Size     Description
          -----     ----     -----------
          RCount    4 bytes  Number of recipients.

     RCount    This defines the number intended recipients whose
               public keys were used for encryption.  This defines
               the number of elements in the REList field defined below.


4. Reserved2 - Certificate Decryption Header Reserved2 Data Structures:


          Value     Size     Description
          -----     ----     -----------
          HashAlg   2 bytes  Hash algorithm identifier
          HSize     2 bytes  Hash size
          REList    (var)    List of recipient data elements


     HashAlg   This defines the hash algorithm used to calculate
               the public key hash of each public key used
               for encryption. This field currently supports
               only the following value for SHA-1

               0x8004 - SHA1

     HSize     This defines the size of a hashed public key
               defined in REHData.

     REList    This is a variable length of list of recipient data.
               Each element in this list consists of a Recipient
               Element data structure as follows:


    Recipient Element (REList) Data Structure:

          Value     Size     Description
          -----     ----     -----------
          RESize    2 bytes  Size of REHData + REKData
          REHData   HSize    Hash of recipients public key
          REKData   (var)    Simple key blob


     RESize    This defines the size of an individual REList
               element.  This value is the combined size of the
               REHData field + REKData field.  REHData is defined by
               HSize.  REKData is variable and can be calculated
               for each REList element using RESize and HSize.

     REHData   Hashed public key for this recipient.

     REKData   Simple Key Blob.  The format of this data structure
               is identical to that defined in the Microsoft
               CryptoAPI and generated using the CryptExportKey()
               function.  The version of the Simple Key Blob
               supported at this time is 0x02 as defined by
               Microsoft.

5. Certificate Processing - Central Directory Encryption:

Central Directory Encryption using Digital Certificates will
operate in a manner similar to that of Single Password Central
Directory Encryption.  This record will only be present when there
is data to place into it.  Currently, data is placed into this
record when digital certificates are used for either encrypting
or signing the files within a ZIP file.  When only password
encryption is used with no certificate encryption or digital
signing, this record is not currently needed. When present, this
record will appear before the start of the actual Central Directory
data structure and will be located immediately after the Archive
Decryption Header if the Central Directory is encrypted.

The Archive Extra Data record will be used to store the following
information.  Additional data may be added in future versions.

Extra Data Fields:

0x0014 - PKCS#7 Store for X.509 Certificates
0x0016 - X.509 Certificate ID and Signature for central directory
0x0019 - PKCS#7 Encryption Recipient Certificate List

The 0x0014 and 0x0016 Extra Data records that otherwise would be
located in the first record of the Central Directory for digital
certificate processing. When encrypting or compressing the Central
Directory, the 0x0014 and 0x0016 records must be located in the
Archive Extra Data record and they should not remain in the first
Central Directory record.  The Archive Extra Data record will also
be used to store the 0x0019 data.

When present, the size of the Archive Extra Data record will be
included in the size of the Central Directory.  The data of the
Archive Extra Data record will also be compressed and encrypted
along with the Central Directory data structure.

6. Certificate Processing Differences:

The Certificate Processing Method of encryption differs from the
Single Password Symmetric Encryption Method as follows.  Instead
of using a user-defined password to generate a master session key,
cryptographically random data is used.  The key material is then
wrapped using standard key-wrapping techniques.  This key material
is wrapped using the public key of each recipient that will need
to decrypt the file using their corresponding private key.

This specification currently assumes digital certificates will follow
the X.509 V3 format for 1024 bit and higher RSA format digital
certificates.  Implementation of this Certificate Processing Method
requires supporting logic for key access and management.  This logic
is outside the scope of this specification.


License Agreement:
-----------------

The features set forth in this Section XIV (the "Strong Encryption
Specification") are covered by a pending patent application. Portions of
this Strong Encryption technology are available for use at no charge
under the following terms and conditions.

1. License Grant.

   a. NOTICE TO USER. PLEASE READ THIS ENTIRE SECTION XIV OF THE
   APPNOTE (THE "AGREEMENT") CAREFULLY. BY USING ALL OR ANY PORTION OF THE
   LICENSED TECHNOLOGY, YOU ACCEPT ALL THE TERMS AND CONDITIONS OF THIS
   AGREEMENT AND YOU AGREE THAT THIS AGREEMENT IS ENFORCEABLE LIKE ANY
   WRITTEN NEGOTIATED AGREEMENT SIGNED BY YOU. IF YOU DO NOT AGREE, DO NOT
   USE THE LICENSED TECHNOLOGY.

   b. Definitions.

      i. "Licensed Technology" shall mean that proprietary technology now or
      hereafter owned or controlled by PKWare, Inc. ("PKWARE") or any
      subsidiary or affiliate  that covers or is necessary to be used to give
      software the ability to a) extract and decrypt data from zip files
      encrypted using any methods of data encryption and key processing which
      are published in this APPNOTE or any prior APPNOTE, as supplemented by
      any Additional Compatibility Information; and b) encrypt file contents
      as part of .ZIP file processing using only the Single Password Symmetric
      Encryption Method as published in this APPNOTE or any prior APPNOTE, as
      supplemented by any Additional Compatibility Information.  For purposes
      of this AGREEMENT, "Additional Compatibility Information" means, with
      regard to any method of data encryption and key processing published in
      this or any prior APPNOTE, any corrections, additions, or clarifications
      to the information in such APPNOTE that are required in order to give
      software the ability to successfully extract and decrypt zip files (or,
      but solely in the case of the Single Password Symmetric Encryption Method,
      to successfully encrypt zip files) in a manner interoperable with the
      actual implementation of such method in any PKWARE product that is
      documented or publicly described by PKWARE as being able to create, or
      to extract and decrypt, zip files using that method.

      ii. "Licensed Products" shall mean any products you produce that
      incorporate the Licensed Technology.

   c. License to Licensed Technology.

   PKWARE hereby grants to you a non-exclusive license to use the Licensed
   Technology for the purpose of manufacturing, offering, selling and using
   Licensed Products, which license shall extend to permit the practice of all
   claims in any patent or patent application (collectively, "Patents") now or
   hereafter owned or controlled by PKWARE in any jurisdiction in the world
   that are infringed by implementation of the Licensed Technology.  You have
   the right to sublicense rights you receive under the terms of this AGREEMENT
   for the purpose of allowing sublicensee to manufacture, offer, sell and use
   products that incorporate all or a portion of any of your Licensed Products,
   but if you do, you agree to i) impose the same restrictions on any such
   sublicensee as these terms impose on you and ii) notify the sublicensee,
   by means chosen by you in your unfettered discretion, including a notice on
   your web site, of the terms of this AGREEMENT and make available to each
   sublicensee the full text of this APPNOTE.   Further, PKWARE hereby grants to
   you a non-exclusive right to reproduce and distribute, in any form, copies of
   this APPNOTE, without modification.  Notwithstanding anything to the contrary
   in this AGREEMENT, you have the right to sublicense the rights, without any of
   the restrictions described above or elsewhere in this AGREEMENT, to use, offer
   to sell and sell Licensed Technology as incorporated in executable object code
   or byte code forms of your Licensed Products.  Any sublicense to use the
   Licensed Technology incorporated in a Licensed Product granted by you shall
   survive the termination of this AGREEMENT for any reason.  PKWARE warrants that
   this license shall continue to encumber the Licensed Technology regardless of
   changes in ownership of the Licensed Technology.

   d. Proprietary Notices.

      i. With respect to any Licensed Product that is distributed by you either
      in source code form or in the form of an object code library of externally
      callable functions that has been designed by you for incorporation into third
      party products, you agree to include, in the source code, or in the case of
      an object code library, in accompanying documentation, a notice using the
      words "patent pending" until a patent is issued to PKWARE covering any
      portion of the Licensed Technology or PKWARE provides notice, by means
      chosen by PKWARE in its unfettered discretion, that it no longer has any
      patent pending covering any portion of the Licensed Technology.  With respect
      to any Licensed Product, upon your becoming aware that at least one patent has
      been granted covering the Licensed Technology, you agree to include in any
      revisions made by you to the documentation (or any source code distributed
      by you) the words "Pat. No.", or "Patent Number" and the patent number or
      numbers of the applicable patent or patents.  PKWARE shall, from time to time,
      inform you of the patent number or numbers of the patents covering the
      Licensed Technology, by means chosen by PKWARE in its unfettered discretion,
      including a notice on its web site.  It shall be a violation of the terms of
      this AGREEMENT for you to sell Licensed Products without complying with the
      foregoing marking provisions.

      ii. You acknowledge that the terms of this AGREEMENT do not grant you any
      license or other right to use any PKWARE trademark in connection with the sale,
      offering for sale, distribution and delivery of the Licensed Products, or in
      connection with the advertising, promotion and offering of the Licensed Products.
      You acknowledge PKWARE's ownership of the PKZIP trademark and all other marks
      owned by PKWARE.

   e. Covenant of Compliance and Remedies.

   To the extent that you have elected to implement portions of the Licensed
   Technology, you agree to use reasonable diligence to comply with those portions
   of this Section XIV, as modified or supplemented by Additional Compatibility
   Information available to you, describing the portions of the Licensed Technology
   that you have elected to implement.  Upon reasonable request by PKWARE, you will
   provide written notice to PKWARE identifying which version of this APPNOTE you
   have relied upon for your implementation of any specified Licensed Product.

   If any substantial non-compliance with the terms of this AGREEMENT is determined
   to exist, you will make such changes as necessary to bring your Licensed Products
   into substantial compliance with the terms of this AGREEMENT.  If, within sixty
   days of receipt of notice that a Licensed Product fails to comply with the terms
   of this AGREEMENT, you fail to make such changes as necessary to bring your
   Licensed Products into compliance with the terms of this AGREEMENT, PKWARE may
   terminate your rights under this AGREEMENT.  PKWARE does not waive and expressly
   reserves the right to pursue any and all additional remedies that are or may
   become available to PKWARE.

   f. Warranty and Indemnification Regarding Exportation.

   You realize and acknowledge that, as between yourself and PKWARE, you are fully
   responsible for compliance with the import and export laws and regulations of
   any country in or to which you import or export any Licensed Products, and you
   agree to hold PKWARE harmless from any claim of violation of any such import
   or export laws.

   g. Patent Infringement.

   You agree that you will not bring or threaten to bring any action against PKWARE
   for infringement of the claims of any patent owned or controlled by you solely
   as a result of PKWARE's own implementation of the Licensed Technology.  As its
   exclusive remedy for your breach of the foregoing agreement, PKWARE reserves
   the right to suspend or terminate all rights granted under the terms of this
   AGREEMENT if you bring or threaten to bring any such action against PKWARE,
   effective immediately upon delivery of written notice of suspension or
   termination to you.

   h. Governing Law.

   The license granted in this AGREEMENT shall be governed by and construed under
   the laws of the State of Wisconsin and the United States.

   i. Revisions and Notice.

   The license granted in this APPNOTE is irrevocable, except as expressly set
   forth above.  You agree and understand that any changes which PKWARE determines
   to make to this APPNOTE shall be posted at the same location as the current
   APPNOTE or at a location which will be identified by means chosen by PKWARE,
   including a notice on its web site, and shall be available for adoption by you
   immediately upon such posting, or at such other time as PKWARE shall determine.
   Any changes to the terms of the license published in a subsequent version of
   this AGREEMENT shall be binding upon you only with respect to your products
   that (i) incorporate any Licensed Technology (as defined in the subsequent
   AGREEMENT) that is not otherwise included in the definition of Licensed
   Technology under this AGREEMENT, or (ii) that you expressly identify are to
   be licensed under the subsequent AGREEMENT, which identification shall be by
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   agrees to identify each change to this APPNOTE by using a unique version and
   release date identifier or other unique identifier.

   j. Warranty by PKWARE

   PKWare, Inc. warrants that it has the right to grant the license hereunder.

XV. Change Process
------------------

In order for the .ZIP file format to remain a viable definition, this
specification should be considered as open for periodic review and
revision.  Although this format was originally designed with a
certain level of extensibility, not all changes in technology
(present or future) were or will be necessarily considered in its
design.  If your application requires new definitions to the
extensible sections in this format, or if you would like to
submit new data structures, please forward your request to
zipformat@pkware.com.  All submissions will be reviewed by the
ZIP File Specification Committee for possible inclusion into
future versions of this specification.  Periodic revisions
to this specification will be published to ensure interoperability.
We encourage comments and feedback that may help improve clarity
or content.


XVI. Acknowledgements
---------------------

In addition to the above mentioned contributors to PKZIP and PKUNZIP,
I would like to extend special thanks to Robert Mahoney for suggesting
the extension .ZIP for this software.


XVII. References
----------------

    Fiala, Edward R., and Greene, Daniel H., "Data compression with
       finite windows",  Communications of the ACM, Volume 32, Number 4,
       April 1989, pages 490-505.

    Held, Gilbert, "Data Compression, Techniques and Applications,
       Hardware and Software Considerations", John Wiley & Sons, 1987.

    Huffman, D.A., "A method for the construction of minimum-redundancy
       codes", Proceedings of the IRE, Volume 40, Number 9, September 1952,
       pages 1098-1101.

    Nelson, Mark, "LZW Data Compression", Dr. Dobbs Journal, Volume 14,
       Number 10, October 1989, pages 29-37.

    Nelson, Mark, "The Data Compression Book",  M&T Books, 1991.

    Storer, James A., "Data Compression, Methods and Theory",
       Computer Science Press, 1988

    Welch, Terry, "A Technique for High-Performance Data Compression",
       IEEE Computer, Volume 17, Number 6, June 1984, pages 8-19.

    Ziv, J. and Lempel, A., "A universal algorithm for sequential data
       compression", Communications of the ACM, Volume 30, Number 6,
       June 1987, pages 520-540.

    Ziv, J. and Lempel, A., "Compression of individual sequences via
       variable-rate coding", IEEE Transactions on Information Theory,
       Volume 24, Number 5, September 1978, pages 530-536.