Notes for January 16, 1998

1.	Greetings and felicitations!

a.	Reading: Pfleeger, pp. 21Ç46; Garfinkel & Spafford, pp. 139Ç159, 175Ç179

2.	Puzzle of the day

a.	Look for things the size of the shell ”

3.	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?

4.	Breaking UNIX crypt(1)

a.	Purely statistical attack is possible (me) but it takes gobs of ciphertext

b.	Known plaintext attack: thatºs Reeds and Weinbergerºs attack, with a nice suggestion by 
Bob Morris

[ ended here ]

5.	DES

a.	Go through the algorithm