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Shannon Cipher

links: AC1 TOC - Security & Cryptography - Index

  • Key space
  • Message space
  • Cipher space
  • Encryption function
  • Decryption function
  • Uniform
  • Probability

Shanon Cipher: \(ε = {(K, M, C)(E: K × M → C, D: K × C → M)}\)

Encrypt & Decrypt in detail:

Shannon_cipher.png

If the Shanon Cipher is defined like in the image above it provides perfect secrecy. The Shanon Cipher does not imply that directly, it depends on how \(E\) and \(D\) are defined.

Decorrelation

  • Ciphertext is drawn with a uniform probability over the ciphertext space for every key / message input.
  • The probability that two different messages using two different keys result in the same ciphertext is equally likely to any other outcome.

Confusion

  • Intended to make the relationship between the key and ciphertext as complex as possible.
  • The same message with a different key should result in a strictly different ciphertext.
  • (Confusion can be achieved by applying substitutions)

Diffusion

  • Refers to rearranging or spreading out the bits in the message so that any redundancy in the message is spread out over the ciphertext.
  • A different message with the same key should result in a strictly different ciphertext.
  • (Diffusion can be achieved by applying permutations)

Recorrelation

  • Refers to the decryption of the message.

links: AC1 TOC - Security & Cryptography - Index