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diff --git a/doc/blowfish.doc b/doc/blowfish.doc new file mode 100644 index 0000000000..3a7291f371 --- /dev/null +++ b/doc/blowfish.doc @@ -0,0 +1,149 @@ +The Blowfish library. + +Blowfish 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. + +Blowfish is quite a bit faster that DES, and much faster than IDEA or +RC2. It is one of the faster block ciphers. + +For all calls that have an 'input' and 'output' variables, they can be the +same. + +This library requires the inclusion of 'blowfish.h'. + +All of the encryption functions take what is called an BF_KEY as an +argument. An BF_KEY is an expanded form of the Blowfish key. +For all modes of the Blowfish algorithm, the BF_KEY used for +decryption is the same one that was used for encryption. + +The define BF_ENCRYPT is passed to specify encryption for the functions +that require an encryption/decryption flag. BF_DECRYPT is passed to +specify decryption. + +Please note that any of the encryption modes specified in my DES library +could be used with Blowfish. I have only implemented ecb, cbc, cfb64 and +ofb64 for the following reasons. +- ecb is the basic Blowfish 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 Blowfish, 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 + BF_encrypt; an exercise left for the paranoid reader :-). + +The functions are as follows: + +void BF_set_key( +BF_KEY *ks; +int len; +unsigned char *key; + BF_set_key converts an 'len' byte key into a BF_KEY. + A 'ks' is an expanded form of the 'key' which is used to + perform actual encryption. It can be regenerated from the Blowfish key + so it only needs to be kept when encryption or decryption is about + to occur. Don't save or pass around BF_KEY's since they + are CPU architecture dependent, 'key's are not. Blowfish 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 by me, but blowfish can use upto + 72 bytes. As a warning, blowfish has a very very slow set_key + function, it actually runs BF_encrypt 521 times. + +void BF_encrypt( +unsigned long *data, +BF_KEY *key, +int encrypt); + This is the Blowfish encryption function that gets called by just about + every other Blowfish routine in the library. You should not use this + function except to implement 'modes' of Blowfish. + 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 + BF_KEY to use. Encryption or decryption is indicated by 'encrypt'. + which can have the values BF_ENCRYPT or BF_DECRYPT. + +void BF_ecb_encrypt( +unsigned char *in, +unsigned char *out, +BF_KEY *key, +int encrypt); + This is the basic Electronic Code Book form of Blowfish (in DES this + mode is called Electronic Code Book so I'm going to use the term + for blowfish 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 BF_cbc_encrypt( +unsigned char *in, +unsigned char *out, +long length, +BF_KEY *ks, +unsigned char *ivec, +int encrypt); + This routine implements Blowfish 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 BF_cbc_encrypt(). + +void BF_cfb64_encrypt( +unsigned char *in, +unsigned char *out, +long length, +BF_KEY *schedule, +unsigned char *ivec, +int *num, +int encrypt); + This is one of the more useful functions in this Blowfish library, it + implements CFB mode of Blowfish 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 BF_ofb64_encrypt( +unsigned char *in, +unsigned char *out, +long length, +BF_KEY *schedule, +unsigned char *ivec, +int *num); + This functions implements OFB mode of Blowfish 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 +BF_set_key(). + +===== +For more information about the specific Blowfish 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 Blowfish. + |