Outline for October 3, 2022

Reading: text, §10.1–10.2
Due: Homework 1, due October 5; Project selection, due Oct 7

1. Cryptography
   1. Codes vs. ciphers
   2. Attacks: ciphertext only, known plaintext, chosen plaintext
   3. Types: substitution, transposition

2. 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 DOIT or 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 4, 2022 at 4:19PM

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