Block and Stream Ciphers1 Reference –Matt Bishop, Computer Security, Addison Wesley, 2003.

Block and Stream Ciphers2 Block Ciphers Block ciphers break messages into fixed length blocks, and encrypt each block using the same key. The Data Encryption Standard (DES) is an example of a block cipher, where blocks of 64 bits are encrypted using a 56-bit key.

Block and Stream Ciphers3 Formal Definition of a Block Cipher Let E be an encipherment algorithm, and let E k (b) be the encipherment of the message b with key k. Let a message m=b 1 b 2...where each b i is of a fixed length. A block cipher is a cipher for which E k (m) = E k (b 1 )E k (b 2 )...

Block and Stream Ciphers4 Stream Ciphers Stream ciphers, like block ciphers, break message into fixed length blocks, but use a sequence of keys to encrypt the blocks. The Vigenère cipher is an example of a stream cipher.

Block and Stream Ciphers5 Formal Definition of a Stream Cipher Let E be an encipherment algorithm, and let E k (b) be the encipherment of the message b with key k. Let a message m=b 1 b 2...where each b i is of a fixed length, and let k = k 1 k A stream cipher is a cipher for which E k (m) = E k 1 (b 1 )E k 2 (b 2 )...

Block and Stream Ciphers6 Synchronous Stream Ciphers The one-time pad is also a stream cipher, and can be implemented by exclusive-oring each bit of the key with one bit of the message. A synchronous stream cipher generates the key string from a source other than the message itself. A source for the key could come from an randomly generated binary sequence.

Block and Stream Ciphers7 Self-Synchronous Stream Ciphers Self-synchronous stream ciphers obtain a key from the message itself. An autokey cipher is a self-synchronous cipher that uses the message itself as a key.