Outline for October 7, 2024

Reading: text, § 10.2
Due: Homework 1, due October 9; Project selection, due October 9

1. Transposition, substitution ciphers
   1. Product cipher

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 M = DOIT or M = DONT?

3. Product ciphers
   1. DES
      1. DES modes
   2. AES

The Vigenère Tableau

The Vigenère tableau, below, is used for a variety of ciphers.

The Vigenère Cipher

The leftmost column has the keys; the top alphabet has the plaintext. So:

The Beaufort Cipher

Find the plaintext in the left alphabet, and trace in until you find the key letter. Then turn a right angle and go up. The letter at the top alphabet is the ciphertext. So:

The Variant Beaufort Cipher

Find the key in the left alphabet, and trace in until you find the plaintext letter. Then turn a right angle and go up. The letter at the top alphabet is the ciphertext. So:

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ECS 235A, Computer and Information Security Version of October 4, 2024 at 9:26PM

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