Notes for October 27, 1999

1.	Greetings and Felicitations!

a.	Midterm moved to Friday, November 5, 1999

b.	Example program put out in ~cs153/bin; it's dec-where, hp-where, pc-where, sgi-where (one per type of sys-
tem)

2.	Puzzle of the Day

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 it's monoalphabetic or polyalphabetic, then Kasiski 
method.

e.	problem: eliminate periodicity of key

4.	Long key generation

a.	Running-key cipher: M=THETREASUREISBURIED; K=THESECONDCIPHERISAN; C=MOIL-
VGOFXTMXZFLZAEQ; 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.)

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

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.	Public-Key Cryptography

a.	Basic idea: 2 keys, one private, one public

b.	Cryptosystem must satisfy:

i.	given public key, CI to get private key;

ii.	cipher withstands chosen plaintext attack;

iii.	encryption, decryption computationally feasible [note: commutativity not required]

c.	Benefits: can give confidentiality or authentiction or both