doc/rc2.doc

The RC2 library.

RC2 is a block cipher that operates on 64bit (8 byte) quantities.  It
uses variable size key, but 128bit (16 byte) key would normally be considered
good.  It can be used in all the modes that DES can be used.  This
library implements the ecb, cbc, cfb64, ofb64 modes.

I have implemented this library from an article posted to sci.crypt on
11-Feb-1996.  I personally don't know how far to trust the RC2 cipher.
While it is capable of having a key of any size, not much reseach has
publically been done on it at this point in time (Apr-1996)
since the cipher has only been public for a few months :-)
It is of a similar speed to DES and IDEA, so unless it is required for
meeting some standard (SSLv2, perhaps S/MIME), it would probably be advisable
to stick to IDEA, or for the paranoid, Tripple DES.

Mind you, having said all that, I should mention that I just read alot and
implement ciphers, I'm a 'babe in the woods' when it comes to evaluating
ciphers :-).

For all calls that have an 'input' and 'output' variables, they can be the
same.

This library requires the inclusion of 'rc2.h'.

All of the encryption functions take what is called an RC2_KEY as an 
argument.  An RC2_KEY is an expanded form of the RC2 key.
For all modes of the RC2 algorithm, the RC2_KEY used for
decryption is the same one that was used for encryption.

The define RC2_ENCRYPT is passed to specify encryption for the functions
that require an encryption/decryption flag. RC2_DECRYPT is passed to
specify decryption.

Please note that any of the encryption modes specified in my DES library
could be used with RC2.  I have only implemented ecb, cbc, cfb64 and
ofb64 for the following reasons.
- ecb is the basic RC2 encryption.
- cbc is the normal 'chaining' form for block ciphers.
- cfb64 can be used to encrypt single characters, therefore input and output
  do not need to be a multiple of 8.
- ofb64 is similar to cfb64 but is more like a stream cipher, not as
  secure (not cipher feedback) but it does not have an encrypt/decrypt mode.
- If you want triple RC2, thats 384 bits of key and you must be totally
  obsessed with security.  Still, if you want it, it is simple enough to
  copy the function from the DES library and change the des_encrypt to
  RC2_encrypt; an exercise left for the paranoid reader :-).

The functions are as follows:

void RC2_set_key(
RC2_KEY *ks;
int len;
unsigned char *key;
int bits;
        RC2_set_key converts an 'len' byte key into a RC2_KEY.
        A 'ks' is an expanded form of the 'key' which is used to
        perform actual encryption.  It can be regenerated from the RC2 key
        so it only needs to be kept when encryption or decryption is about
        to occur.  Don't save or pass around RC2_KEY's since they
        are CPU architecture dependent, 'key's are not.  RC2 is an
	interesting cipher in that it can be used with a variable length
	key.  'len' is the length of 'key' to be used as the key.
	A 'len' of 16 is recomended.  The 'bits' argument is an
	interesting addition which I only found out about in Aug 96.
	BSAFE uses this parameter to 'limit' the number of bits used
	for the key.  To use the 'key' unmodified, set bits to 1024.
	This is what old versions of my RC2 library did (SSLeay 0.6.3).
	RSAs BSAFE library sets this parameter to be 128 if 128 bit
	keys are being used.  So to be compatable with BSAFE, set it
	to 128, if you don't want to reduce RC2's key length, leave it
	at 1024.
	
void RC2_encrypt(
unsigned long *data,
RC2_KEY *key,
int encrypt);
	This is the RC2 encryption function that gets called by just about
	every other RC2 routine in the library.  You should not use this
	function except to implement 'modes' of RC2.  I say this because the
	functions that call this routine do the conversion from 'char *' to
	long, and this needs to be done to make sure 'non-aligned' memory
	access do not occur.
	Data is a pointer to 2 unsigned long's and key is the
	RC2_KEY to use.  Encryption or decryption is indicated by 'encrypt'.
	which can have the values RC2_ENCRYPT or RC2_DECRYPT.

void RC2_ecb_encrypt(
unsigned char *in,
unsigned char *out,
RC2_KEY *key,
int encrypt);
	This is the basic Electronic Code Book form of RC2 (in DES this
	mode is called Electronic Code Book so I'm going to use the term
	for rc2 as well.
	Input is encrypted into output using the key represented by
	key.  Depending on the encrypt, encryption or
	decryption occurs.  Input is 8 bytes long and output is 8 bytes.
	
void RC2_cbc_encrypt(
unsigned char *in,
unsigned char *out,
long length,
RC2_KEY *ks,
unsigned char *ivec,
int encrypt);
	This routine implements RC2 in Cipher Block Chaining mode.
	Input, which should be a multiple of 8 bytes is encrypted
	(or decrypted) to output which will also be a multiple of 8 bytes.
	The number of bytes is in length (and from what I've said above,
	should be a multiple of 8).  If length is not a multiple of 8, bad 
	things will probably happen.  ivec is the initialisation vector.
	This function updates iv after each call so that it can be passed to
	the next call to RC2_cbc_encrypt().
	
void RC2_cfb64_encrypt(
unsigned char *in,
unsigned char *out,
long length,
RC2_KEY *schedule,
unsigned char *ivec,
int *num,
int encrypt);
	This is one of the more useful functions in this RC2 library, it
	implements CFB mode of RC2 with 64bit feedback.
	This allows you to encrypt an arbitrary number of bytes,
	you do not require 8 byte padding.  Each call to this
	routine will encrypt the input bytes to output and then update ivec
	and num.  Num contains 'how far' we are though ivec.
	'Encrypt' is used to indicate encryption or decryption.
	CFB64 mode operates by using the cipher to generate a stream
	of bytes which is used to encrypt the plain text.
	The cipher text is then encrypted to generate the next 64 bits to
	be xored (incrementally) with the next 64 bits of plain
	text.  As can be seen from this, to encrypt or decrypt,
	the same 'cipher stream' needs to be generated but the way the next
	block of data is gathered for encryption is different for
	encryption and decryption.
	
void RC2_ofb64_encrypt(
unsigned char *in,
unsigned char *out,
long length,
RC2_KEY *schedule,
unsigned char *ivec,
int *num);
	This functions implements OFB mode of RC2 with 64bit feedback.
	This allows you to encrypt an arbitrary number of bytes,
	you do not require 8 byte padding.  Each call to this
	routine will encrypt the input bytes to output and then update ivec
	and num.  Num contains 'how far' we are though ivec.
	This is in effect a stream cipher, there is no encryption or
	decryption mode.
	
For reading passwords, I suggest using des_read_pw_string() from my DES library.
To generate a password from a text string, I suggest using MD5 (or MD2) to
produce a 16 byte message digest that can then be passed directly to
RC2_set_key().

=====
For more information about the specific RC2 modes in this library
(ecb, cbc, cfb and ofb), read the section entitled 'Modes of DES' from the
documentation on my DES library.  What is said about DES is directly
applicable for RC2.