Notes for January 14, 1998
[ ended here ]
- Greetings and felicitations!
- Reading: Pfleeger, pp. 21-46;
Garfinkel & Spafford, pp. 139-159, 175-179
- Puzzle of the day
- Just to get you thinking; I'll ask this one again later
on to see how your thinking has changed.
- monoalphabetic (simple substitution):
f(a) = a + k mod n
- example: Caesar with k = 3,
RENAISSANCE -> UHQDLVVDQFH
- polyalphabetic: Vigenère,
(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;
wedge is that (plaintext,key) letter pairs are not
random (T/T, H/H,
- 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: one-time pads;
C=AZPR; is that DOIT or DONT?
- Go through the algorithm
- Breaking UNIX crypt(1)
- Purely statistical attack is possible (me) but it takes gobs
- Known plaintext attack: that's Reeds and Weinberger's attack,
with a nice suggestion by Bob Morris
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