
 [No title] (Site not responding. Last check: 20071106) 
  Formally, it is a 5tuple (E, D, M, K, C), where M is the set of plaintext messages, C is the set of ciphertext messages, K is a set of keys, and E: MxK > C (enciphering functions) and D: CxK > M (deciphering functions) Example: Caesar cipher. 
  Substitution cipher: A map from characters in the plaintext to the ciphertext. 
  Ciphertext = plaintext XOR key Plaintext = key XOR ciphertext It turns out that as long as key is truly random, ciphertext is also random  no frequency analysis possible. 
 www.eecs.umich.edu /~aprakash/eecs498/handouts/02cryptobackground.txt (668 words) 
