Short description:
The Playfair cipher is a digraph substitution cipher that encrypts pairs of letters using a 5x5 key table (usually combining I/J). It was historically used for manual encryption and provides better security than simple monoalphabetic substitution.
Implementation:
- Implementations and tests may be present in C++ and Rust subdirectories.
Algorithm (brief):
- Build a 5x5 key table from a keyword, filling remaining letters.
- Split the plaintext into digraphs (pairs), inserting filler characters if needed.
- For each digraph, apply Playfair rules (same row, same column, rectangle swap) to produce the ciphertext digraph.
Example:
Input: "HELLO" Output: ciphertext produced by the digraph substitutions using the chosen key (implementation-dependent).