Solitaire Cipher

The Solitaire Cipher is a manual encryption technique developed by Bruce Schneier in 1999. It was designed as a way to allow cryptographic communication without the need for electronic devices, making it particularly useful for secure messaging in environments where electronic communications are restricted or monitored.

The cipher employs a deck of playing cards as the primary encryption tool, using a combination of card manipulation and a basic form of the Vigenère cipher. The method is notable for its clever use of a physical object to create a stream of keystream values, which are then used to encrypt a message. It combines elements of both classical ciphers and modern cryptographic principles, allowing users to generate a pseudo-random sequence of letters that correspond to plaintext letters.

To encrypt a message, the user must first prepare the deck of cards, which involves arranging the cards in a specific order. The keystream is generated by manipulating the deck according to a defined set of rules, and this keystream is then combined with the plaintext using a simple addition modulo 26 to produce the ciphertext.

Example:

Suppose the plaintext message is "HELLO". After generating a keystream from the shuffled deck, we might get the letters "D", "B", "E", "F", and "G" as our keystream values.

1. Encrypt each letter:
   • H (7) + D (3) = K (10)
   • E (4) + B (1) = F (5)
   • L (11) + E (4) = P (15)
   • L (11) + F (5) = Q (16)
   • O (14) + G (6) = U (20)
2. Resulting Ciphertext:
   • The plaintext "HELLO" can be encrypted to "KFPUQ".

Mapping Table:

Here’s how the Solitaire Cipher encryption works with a simplified keystream:

The Solitaire Cipher offers a unique blend of simplicity and security for manual encryption, leveraging the use of physical cards to generate a keystream in a way that is both practical and engaging. However, its reliance on manual processes can make it more susceptible to human error compared to electronic encryption methods.