Outline for February 22, 2006

Reading: text, §9.2

1. Greetings and felicitations!
   1. Puzzle of the day
2. Classical Cryptography
   1. Polyalphabetic: Vigenère, f_i(a) = a + k_i mod n
   2. Cryptanalysis: first do index of coincidence to see if it’s monoalphabetic or polyalphabetic, then Kasiski method.
   3. Problem: eliminate periodicity of key
3. Long key generation
   1. Running-key cipher: M = THETREASUREISBURIED; K = THESECONDCIPHERISAN; C = MOILVGOFXTMXZFLZAEQ; wedge is that (plaintext, key) letter pairs are not random (T/T, H/H, E/E, T/S, R/E, A/O, S/N, etc.)
   2. Perfect secrecy: when the probability of computing the plaintext message is the same whether or not you have the ciphertext
   3. Only cipher with perfect secrecy: one-time pads; C = AZPR; is that DOIT or DONT?
4. DES