1. In cryptography, the word cipher means an encryption algorithm. A cipher transforms the original data/message into pseudo-random data/message of the same length. In order to decipher the message, a reverse transformation must be applied. Key point: A block cipher is one that encrypts a block of data at a time. For example, DES uses a block size of 64-bits. Each input block must correspond to exactly one output block (like a codebook). A block-cipher suffers from the fact the same data repeated in a message would be encoded in the same way. Consider a block size of 8-bit encrypting English text; you could therefore figure out all the letter 'e's in the cipher text because they are the most common letter used. Therefore, block-ciphers are often used in a chaining mode such that the same pattern will indeed be decrypted differently. Key point: A stream cipher is essentially a chained block cipher with a block size of 1 (either 1-bit or 1-byte). It generates a keystream against which it XORs the plaintext, operating much like a one-time pad, though less secure in theory but more secure in practice. Example: Some popular ciphers are: DES The original widely-used computer-based encryption cipher that spawned the industry, but easily crackable today. triple DES A more secure form of DES whereby data is simply encrypted three different times. RC4 One of the most widely used ciphers today because of its prevalent use within web browsers and SSL. RC2 A cipher similar to RC4. IDEA Gained popularity because it was used as the default cipher for PGP. Blowfish Popular cipher because of its open source and non-patented status. CAST-128 Alternate cipher in PGP. Skipjack Controversal cipher designed for the Clipper chip, a government program to encourage key recovery for law enforcement. GOST 28147 Russian standard with 256-bit key. AES The new American standard for replacing DES. From Hacking-Lexicon