Outline for October 3, 2022
Reading: text, §10.1–10.2
Due: Homework 1, due October 5; Project selection, due Oct 7
- Codes vs. ciphers
- Attacks: ciphertext only, known plaintext, chosen plaintext
- Types: substitution, transposition
- Symmetric Cryptography
- Monoalphabetic (simple substitution): f(a) = a + k mod n
- Example: Caesar (shift) cipher with k = 3, RENAISSANCE → UHQDLVVDQFH
- Polyalphabetic: Vigenère, fi(a) = a + ki mod n
- Cryptanalysis: use index of coincidence to see if it is monoalphabetic or polyalphabetic; Kasiski method.
- Problem: eliminate periodicity of key
- 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?