Cryptography (Domain 3)

Vigenere system – polyalphabetic substitution cipher, key is a word

Vernan cipher – one-time pad, key is the length of the message

  • randomly generated
  • physically protected against disclosure
  • used only once
  • key at least as long as the message

Running key cipher (book cipher) – use a book text as the running key

Confusion – complicated relationship between plain and cipher text

Diffusion – one change in plain text = several changes in cipher text

Keys required: n = number of participants

  • Symmetric = (n * (n-1)) / 2
  • Asymmetric = 2 * n

Data Encryption Standard (DES) – 56-bit key, 16 rounds of XOR

  • Electronic codebook (ECB) – encrypts each block with the same key
  • Cipher Block Chaining (CBC) – each new block is XORed with previous block of cipher text before being encrypted (con: errors propagate)
  • Cipher Feedback (CFB) – streaming version of CBC – data collects in a buffer until full, then encrypts – uses Initialization Vector (IV)
  • Output Feedback (OFB) – similar to CFB but XORs with a seed value instead of previous cipher text – prevents error propagaion
  • Counter (CTR) – stream cipher, XOR with simple counter – no propagation errors – allows encryption to be broken into parts, so good for parallel computing