Cipher

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Ciphers are defined over three spaces

  1. The set of all possible keys K
  2. The set of all possible messages M
  3. The set of all possible cipher texts C

A cipher consists of two algorithms

E: K x M → C

D: K x C → M

The algorithms must statisfy the Consistency Equation which is as follows

for all m in M and k in K : D(k, E(k,m)) = m

This means that all messages encrypted with a k must be able to be decrypted with the same key k and produce the same original message m .

Something interesting is that E might be a randomized algorithm which produces slightly different outputs for given inputs. However essentially, D must be deterministic!

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