Andrew J Bromage wrote:
A cipher is an algorithm for turning an n-bit block into another n-bit (and back again) block using an m-bit key. It does not cover encoding messages longer or shorter than an n-bit block. At best it's a "codec" for a certain fixed-size binary object where the size depends, in general, on the algorithm.
This table may help clarify the thinking:
| Algorithm | Codec | Format ----------------+---------------+---------------+---------------- Cipher | DES | CBC-DES | SSL MAC | SHA-1 | HMAC-SHA-96 | SNMPv3 Text compress | LZ77 | Deflate | GZip Image compress | DCT | JPEG | JFIF
This is great. I would just change one thing: in my opinion, SSL and SNMPv3 are not formats, but protocols. HTML is a format, HTTP is a protocol. The distinction should be that a format is a way of structuring data that is designed for storage, whereas a protocol is used to transfer data where there is a sender and a reciever, or a peer relationship in which multiple entities interact. Protocols are interactive, where formats are passive. An example of a crypto format might be the OpenPGP file format. An example of a MAC format (or at least, one involving a secure hash) might be the unix /etc/shadow file. Thanks, Matt Harden