Notes for January 13, 1997 1. Hello a. Office hours on MW moved to 10:00Ç11:00AM; office hours for Wednesday are cancelled 2. Puzzle of the day a. Thought some ruminations on what is possible would be appropriate in light of Fri- dayºs lecture ” 3. Classical a. monoalphabetic (simple substitution): f(a) = a + k mod n b. example: Caesar with k = 3, RENAISSANCE -> UHQDLVVDQFH c. polyalphabetic: VigenÀre, fi(a) = (a + ki) mod n\ d. cryptanalysis: first do index of coincidence to see if itºs monoalphabetic or polyal- phabetic, then Kasiski method. e. problem: eliminate periodicity of key 4. Long key generation a. Running-key cipher: M=THETREASUREISBURIED; K=THESECONDCIPHER- ISAN; 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.) b. Enigma/rotor systems; wheels, 3 rotors and a reflecting one. Go through it; UNIX uses this for crypt(1) command. c. Perfect secrecy: when the probability of computing the plaintext message is the same whether or not you have the ciphertext d. Only cipher with perfect secrecy: one-time pads; C=AZPR; is that DOIT or DONT? 5. DES a. Go through the algorithm