Notes for January 13, 1997

Hello

Office hours on MW moved to 10:0011:00AM;
office hours for Wednesday are cancelled

Puzzle of the day

Thought some ruminations on what is possible would be appropriate
in light of Friday's lecture

Classical

monoalphabetic (simple substitution):
f(a) = a + k mod n

example: Cæsar with k = 3, RENAISSANCE > UHQDLVVDQFH

polyalphabetic: Vigenère,
f_{i}(a) = a + k_{i} mod n

cryptanalysis: first do index of coincidence to see if it's
monoalphabetic or polyalphabetic, then Kasiski method.

problem: eliminate periodicity of key

Long key generation

Runningkey 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.)

Enigma/rotor systems;
wheels, 3 rotors and a reflecting one. Go through it; UNIX
uses this for crypt(1) command.

Perfect secrecy: when the probability of computing the plaintext message
is the same whether or not you have the ciphertext

Only cipher with perfect secrecy: onetime pads; C=AZPR; is that DOIT or DONT?

DES

Go through the algorithm
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Department of Computer Science
University of California at Davis
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Page last modified on 1/23/97