Outline for October 8, 2025

Reading: text, §10.1–10.3
Assignments: Homework 1, due October 8; Project selection, due Oct 10

1. Class overview

2. Cryptography

3. Transposition, substitution ciphers
   1. Product cipher

4. Symmetric Cryptography
   1. Monoalphabetic (simple substitution): f(a) = a + k mod n
   2. Example: Caesar (shift) cipher with k = 3, RENAISSANCE → UHQDLVVDQFH
   3. Polyalphabetic: Vigenère, f_i(a) = a + k_i mod n
   4. Cryptanalysis: use index of coincidence to see if it is monoalphabetic or polyalphabetic; Kasiski method.
   5. Problem: eliminate periodicity of key
   6. 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 M = DOIT or M = DONT?

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

ECS 235A, Computer and Information Security Version of October 8, 2025 at 4:09PM

You can also obtain a PDF version of this.