diff options
Diffstat (limited to 'doc/idea.doc')
| -rw-r--r-- | doc/idea.doc | 176 |
1 files changed, 176 insertions, 0 deletions
diff --git a/doc/idea.doc b/doc/idea.doc new file mode 100644 index 0000000000..68a22a6cae --- /dev/null +++ b/doc/idea.doc @@ -0,0 +1,176 @@ +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. + |