Notes for January 13, 1997
Office hours on MW moved to 10:00-11:00AM;
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
monoalphabetic (simple substitution):
f(a) = a + k mod n
example: Cæsar with k = 3, RENAISSANCE -> UHQDLVVDQFH
fi(a) = a + ki 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
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.)
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: one-time pads; C=AZPR; is that DOIT or DONT?
Go through the algorithm
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Page last modified on 1/23/97