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.