Notes for January 13, 1997

1. Hello
   1. Office hours on MW moved to 10:00-11:00AM; office hours for Wednesday are cancelled
2. Puzzle of the day
   1. Thought some ruminations on what is possible would be appropriate in light of Friday's lecture
3. Classical
   1. monoalphabetic (simple substitution): f(a) = a + k mod n
   2. example: Cæsar with k = 3, RENAISSANCE -> UHQDLVVDQFH
   3. polyalphabetic: Vigenère, f_i(a) = a + k_i mod n
   4. cryptanalysis: first do index of coincidence to see if it's monoalphabetic or polyalphabetic, then Kasiski method.
   5. problem: eliminate periodicity of key
4. 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. Enigma/rotor systems; wheels, 3 rotors and a reflecting one. Go through it; UNIX uses this for crypt(1) command.
   3. Perfect secrecy: when the probability of computing the plaintext message is the same whether or not you have the ciphertext
   4. Only cipher with perfect secrecy: one-time pads; C=AZPR; is that DOIT or DONT?
5. DES
   1. Go through the algorithm