Notes for October 27, 1999
1. Greetings and Felicitations!
a. Midterm moved to Friday, November 5, 1999
b. Example program put out in ~cs153/bin; it's dec-where, hp-where, pc-where, sgi-where (one per type of sys-
tem)
2. Puzzle of the Day
3. Classical
a. monoalphabetic (simple substitution): f(a) = a + k mod n
b. example: Caesar with k = 3, RENAISSANCE UHQDLVVDQFH
c. polyalphabetic: VigenĖre, fi(a) = (a + ki) mod n
d. cryptanalysis: first do index of coincidence to see if it's monoalphabetic or polyalphabetic, then Kasiski
method.
e. problem: eliminate periodicity of key
4. 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?
5. DES
a. Go through the algorithm
6. 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 authentiction or both