Notes for October 29, 1999
1. Greetings and Felicitations!
2. Puzzle of the Day
3. Long key generation
a. Running-key cipher: M=THETREASUREISBURIED; K=THESECONDCIPHERISAN; C=MOIL-
VGOFXTMXZFLZAEQ; 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) com-
mand.
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?
4. DES
a. Go through the algorithm
5. Public-Key Cryptography
a. Basic idea: 2 keys, one private, one public
b. Cryptosystem must satisfy:
i. given public key, CI to get private key;
ii. cipher withstands chosen plaintext attack;
iii. encryption, decryption computationally feasible [note: commutativity not required]
c. Benefits: can give confidentiality or authentication or both