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Solitaire (also called Pontifex) is a stream cipher designed by cryptographer Bruce Schneier to be operated by hand using an ordinary deck of playing cards plus 2 jokers — it appeared in Neal Stephenson's novel Cryptonomicon. This tool faithfully simulates the documented card-based keystream algorithm digitally.
How it works
A simulated 54-card deck (optionally shuffled by a keyword) generates a keystream number for each letter of your message via 5 documented steps: move joker A down 1, move joker B down 2, a triple cut around both jokers, a count cut based on the bottom card's value, and reading a keystream number from the top card's position. The keystream is added to (encrypt) or subtracted from (decrypt) each letter, wrapping modulo 26.
- Enter mode.
- Enter text (letters only).
- Enter keyword (optional — letters only).
- Click Calculate to see your results.
Examples
Encrypting with a keyword
Encrypting "ATTACK AT DAWN" with the keyword "CRYPTO" produces ciphertext that only decrypts correctly back to the original message using that same keyword.
Who should use it
- Learning how card-based stream ciphers work.
- Solving or creating a Cryptonomicon-style puzzle.
Industry applications
- Cryptography education
- Puzzle/ARG design
Advantages
- Faithfully simulates a real, documented hand-cipher algorithm.
- Optional keyword support.
Limitations
- Not a modern, vetted cryptographic standard — for real security use AES or similar.
Common mistakes to avoid
- Using a different keyword for decryption than was used for encryption — Solitaire requires the exact same keyword (or none) on both ends.
Best practices
- Use this for learning about card-based stream ciphers or a Cryptonomicon-inspired puzzle, not for real-world message security.
Tips
- For real-world message security, use the AES Encryption Tool instead.