- Greetings and Felicitations!
- Midterm moved to Friday, November 5, 1999
- Example program put out in ~cs153/bin; it's dec-where, hp-where, pc-where, sgi-where (one per type of system)

- Puzzle of the Day
- Classical
- monoalphabetic (simple substitution):
*f*(*a*) = (*a*+*k*)**mod***n* - example: Cæsar with k = 3, RENAISSANCE -> UHQDLVVDQFH
- polyalphabetic: Vigenère,
*f*(_{i}*a*) = (*a*+*k*)_{i}**mod***n* - cryptanalysis: first do index of coincidence to see if it's monoalphabetic or polyalphabetic, then Kasiski method.
- problem: eliminate periodicity of key

- monoalphabetic (simple substitution):
- 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.)
- 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?

- DES
- Go through the algorithm

- Public-Key Cryptography
- Basic idea: 2 keys, one private, one public
- Cryptosystem must satisfy:
- given public key, CI to get private key;
- cipher withstands chosen plaintext attack;
- encryption, decryption computationally feasible [note: commutativity not required]

- Benefits: can give confidentiality or authentiction or both

Send email to cs153@csif.cs.ucdavis.edu.

Department of Computer Science

University of California at Davis

Davis, CA 95616-8562