doc/idea.doc

The IDEA library.
IDEA is a block cipher that operates on 64bit (8 byte) quantities.  It
uses a 128bit (16 byte) key.  It can be used in all the modes that DES can
be used.  This library implements the ecb, cbc, cfb64 and ofb64 modes.

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

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

All of the encryption functions take what is called an IDEA_KEY_SCHEDULE as an 
argument.  An IDEA_KEY_SCHEDULE is an expanded form of the idea key.
For all modes of the IDEA algorithm, the IDEA_KEY_SCHEDULE used for
decryption is different to the one used for encryption.

The define IDEA_ENCRYPT is passed to specify encryption for the functions
that require an encryption/decryption flag. IDEA_DECRYPT is passed to
specify decryption.  For some mode there is no encryption/decryption
flag since this is determined by the IDEA_KEY_SCHEDULE.

So to encrypt you would do the following
idea_set_encrypt_key(key,encrypt_ks);
idea_ecb_encrypt(...,encrypt_ks);
idea_cbc_encrypt(....,encrypt_ks,...,IDEA_ENCRYPT);

To Decrypt
idea_set_encrypt_key(key,encrypt_ks);
idea_set_decrypt_key(encrypt_ks,decrypt_ks);
idea_ecb_encrypt(...,decrypt_ks);
idea_cbc_encrypt(....,decrypt_ks,...,IDEA_DECRYPT);

Please note that any of the encryption modes specified in my DES library
could be used with IDEA.  I have only implemented ecb, cbc, cfb64 and
ofb64 for the following reasons.
- ecb is the basic IDEA 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 IDEA, 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
  idea_encrypt; an exercise left for the paranoid reader :-).

The functions are as follows:

void idea_set_encrypt_key(
unsigned char *key;
IDEA_KEY_SCHEDULE *ks);
	idea_set_encrypt_key converts a 16 byte IDEA key into an
	IDEA_KEY_SCHEDULE.  The IDEA_KEY_SCHEDULE is an expanded form of
	the key which can be used to perform IDEA encryption.
	An IDEA_KEY_SCHEDULE is an expanded form of the key which is used to
	perform actual encryption.  It can be regenerated from the IDEA key
	so it only needs to be kept when encryption is about
	to occur.  Don't save or pass around IDEA_KEY_SCHEDULE's since they
	are CPU architecture dependent, IDEA keys are not.
	
void idea_set_decrypt_key(
IDEA_KEY_SCHEDULE *encrypt_ks,
IDEA_KEY_SCHEDULE *decrypt_ks);
	This functions converts an encryption IDEA_KEY_SCHEDULE into a
	decryption IDEA_KEY_SCHEDULE.  For all decryption, this conversion
	of the key must be done.  In some modes of IDEA, an
	encryption/decryption flag is also required, this is because these
	functions involve block chaining and the way this is done changes
	depending on which of encryption of decryption is being done.
	Please note that there is no quick way to generate the decryption
	key schedule other than generating the encryption key schedule and
	then converting it.

void idea_encrypt(
unsigned long *data,
IDEA_KEY_SCHEDULE *ks);
	This is the IDEA encryption function that gets called by just about
	every other IDEA routine in the library.  You should not use this
	function except to implement 'modes' of IDEA.  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 ks is the
	IDEA_KEY_SCHEDULE to use.  Encryption or decryption depends on the
	IDEA_KEY_SCHEDULE.

void idea_ecb_encrypt(
unsigned char *input,
unsigned char *output,
IDEA_KEY_SCHEDULE *ks);
	This is the basic Electronic Code Book form of IDEA (in DES this
	mode is called Electronic Code Book so I'm going to use the term
	for idea as well :-).
	Input is encrypted into output using the key represented by
	ks.  Depending on the IDEA_KEY_SCHEDULE, encryption or
	decryption occurs.  Input is 8 bytes long and output is 8 bytes.
	
void idea_cbc_encrypt(
unsigned char *input,
unsigned char *output,
long length,
IDEA_KEY_SCHEDULE *ks,
unsigned char *ivec,
int enc);
	This routine implements IDEA 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 idea_cbc_encrypt().
	
void idea_cfb64_encrypt(
unsigned char *in,
unsigned char *out,
long length,
des_key_schedule ks,
des_cblock *ivec,
int *num,
int enc);
	This is one of the more useful functions in this IDEA library, it
	implements CFB mode of IDEA 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.
	Enc is used to indicate encryption or decryption.
	One very important thing to remember is that when decrypting, use
	the encryption form of the key.
	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.  What this means is that to encrypt
	idea_set_encrypt_key(key,ks);
	idea_cfb64_encrypt(...,ks,..,IDEA_ENCRYPT)
	do decrypt
	idea_set_encrypt_key(key,ks)
	idea_cfb64_encrypt(...,ks,...,IDEA_DECRYPT)
	Note: The same IDEA_KEY_SCHEDULE but different encryption flags.
	For idea_cbc or idea_ecb, idea_set_decrypt_key() would need to be
	used to generate the IDEA_KEY_SCHEDULE for decryption.
	The reason I'm stressing this point is that I just wasted 3 hours
	today trying to decrypt using this mode and the decryption form of
	the key :-(.
	
void idea_ofb64_encrypt(
unsigned char *in,
unsigned char *out,
long length,
des_key_schedule ks,
des_cblock *ivec,
int *num);
	This functions implements OFB mode of IDEA 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.  The same key and iv should be used to
	encrypt and decrypt.
	
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
idea_set_encrypt_key().

=====
For more information about the specific IDEA 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 IDEA.