Notes for January 14, 1998
1. Greetings and felicitations!
a. Reading: Pfleeger, pp. 2146; Garfinkel & Spafford, pp. 139159, 175179
2. Puzzle of the day
a. Just to get you thinking; I¼ll ask this one again later on to see how your thinking has
changed.
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 it¼s monoalphabetic or polyalphabetic,
then Kasiski method.
e. problem: eliminate periodicity of key
[ended here ]
4. Long key generation
a. Running-key cipher: M=THETREASUREISBURIED; K=THESECONDCIPHERISAN;
C=MOILVGOFXTMXZFLZAEQ; wedge is that (plaintext,key) letter pairs are not ran-
dom (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) command.
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. Breaking UNIX crypt(1)
a. Purely statistical attack is possible (me) but it takes gobs of ciphertext
b. Known plaintext attack: that¼s Reeds and Weinberger¼s attack, with a nice suggestion by
Bob Morris