Codebook

Tags

cryptography

A book of codes, I.E. a list of strings/keywords which can be substituted for other words to encrypt a message.

Essentially a mapping from plain-text blocks to cipher-text blocks.

Note: Codebooks are symmetric, you can swap the inputs and outputs (encryption and decryption) but if you apply both in any order in the same input you get the initial input back out \(\text{ENC}(\text{DEC}(X)) = \text{DEC}(\text{ENC}(X)) = X\).

Vulnerabilities

• We can use code-books to forge a message pretending to be a legitimate communicant.
• Code books also allow easy replacement of parts of a message, that you may not have planned to encrypt.