=pod

=head1 NAME

DERlib - internal OpenSSL DER library

=head1 DESCRIPTION

OpenSSL contains an internal small DER reading and writing library,
as an alternative to the publicly known i2d and d2i functions.  It's
solely constituted of functions that work as building blocks to create
more similar functions to encode and decode larger structures.

All these functions have similar function signatures (C<something>
will vary depending on what the function will encode):

    int DER_w_something(WPACKET *pkt, int tag, ...);

=begin comment

When readers are added, add this:

    int DER_r_something(PACKET *pkt, int tag, ...);

=end comment

I<pkt> is the packet context used, and I<tag> should be the
context-specific tag value of the element being handled, or -1 if there
is no tag number for that element (you may use the convenience macro
B<DER_NO_CONTEXT> instead of -1).  Any argument following is the C
variable that's being encoded or decoded.

=head2 DER writers / encoders

DER writers are based in L<WPACKET(3)>, a generic packet writing
library, so before using any of them, I<pkt> must be initialized
using L<WPACKET_init_der(3)> or L<WPACKET_init_null_der(3)>

DER writers must be used in reverse order, except for the wrapping
functions that implement a constructed element.  The latter are easily
recognised by their function name including the words C<begin> and
C<end>.  As an example, we can look at the DSA signature structure,
which is defined like this in ASN.1 terms:

    -- Copied from RFC 3279, section 2.2.2
    Dss-Sig-Value  ::=  SEQUENCE  {
            r       INTEGER,
            s       INTEGER  }

With the DER library, this is the corresponding code, given two OpenSSL
B<BIGNUM>s I<r> and I<s>:

    int ok = DER_w_begin_sequence(pkt, -1)
          && DER_w_bn(pkg, -1, s)
          && DER_w_bn(pkg, -1, r)
          && DER_w_end_sequence(pkt, -1);

As an example of the use of I<tag>, an ASN.1 element like this:

    v [1] INTEGER OPTIONAL

Would be encoded like this:

    DER_w_bn(pkt, 1, v)

=begin comment

=head2 DER readers / decoders

TBA

=end comment

=head1 EXAMPLES

A more complex example, encoding the AlgorithmIdentifier with
RSASSA-PSS values.

As a reminder, the AlgorithmIdentifier is specified like this:

    -- From RFC 3280, section 4.1.1.2
    AlgorithmIdentifier  ::=  SEQUENCE  {
         algorithm               OBJECT IDENTIFIER,
         parameters              ANY DEFINED BY algorithm OPTIONAL  }    

And the RSASSA-PSS OID and parameters are specified like this:

    -- From RFC 3279, section 3.1
    id-RSASSA-PSS  OBJECT IDENTIFIER  ::=  { pkcs-1 10 }

    RSASSA-PSS-params  ::=  SEQUENCE  {
       hashAlgorithm      [0] HashAlgorithm DEFAULT
                                 sha1Identifier,
       maskGenAlgorithm   [1] MaskGenAlgorithm DEFAULT
                                 mgf1SHA1Identifier,
       saltLength         [2] INTEGER DEFAULT 20,
       trailerField       [3] INTEGER DEFAULT 1  }

The value we want to encode, written in ASN.1 syntax:

    {
        algorithm               id-RSASSA-PSS,
        parameters {
            hashAlgorithm       sha256Identifier,
            maskGenAlgorithm    mgf1SHA256Identifier,
            saltLength          20  -- unnecessarily explicit
        }
    }

Assuming that we have precompiled constants for C<id-RSASSA-PSS>,
C<sha256Identifier> and C<mgf1SHA256Identifier>, the DER writing code
looks as follows. This is a complete function to write that specific
value:

    int DER_w_AlgorithmIdentifier_RSASSA_PSS_special(WPACKET *pkt,
                                                     int tag,
                                                     RSA *rsa)
    {
        return DER_w_begin_sequence(pkt, tag)
            && (DER_w_begin_sequence(pkt, DER_NO_CONTEXT)
                && DER_w_ulong(pkt, 2, 20)
                && DER_w_precompiled(pkt, 1,
                                     der_mgf1SHA256Identifier,
                                     sizeof(der_mgf1SHA256Identifier))
                && DER_w_precompiled(pkt, 0,
                                     der_sha256Identifier,
                                     sizeof(der_sha256Identifier))
                && DER_w_end_sequence(pkt, DER_NO_CONTEXT))
            && DER_w_precompiled(pkt, DER_NO_CONTEXT,
                                 der_id_RSASSA_PSS,
                                 sizeof(der_id_RSASSA_PSS))
            && DER_w_end_sequence(pkt, tag);
    }

=head1 SEE ALSO

L<DER_w_bn(3)>, L<DER_w_begin_sequence(3)>, L<DER_w_precompiled(3)>

=head1 COPYRIGHT

Copyright 2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

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