Description

This PR adds an implementation of the Trifid cipher to the ciphers module.

The Trifid cipher is a classical cipher that uses a table to fractionate each plaintext letter into a trigram, mixes the constituents of the trigrams, and then applies the table in reverse to turn these mixed trigrams into ciphertext letters.

Changes

• Added src/ciphers/trifid.rs with encryption and decryption functions
• Updated src/ciphers/mod.rs to include the new module and exports
• Includes comprehensive tests with 13 test cases covering:
  • Basic encryption and decryption
  • Custom alphabets
  • Round-trip encryption/decryption
  • Error handling for invalid inputs (invalid alphabet length, invalid characters)
  • Edge cases (empty messages, single characters, unmapped characters, empty input)
  • Internal helper functions with full code coverage

Features

• trifid_encrypt(message, alphabet, period) - Encrypts a message using the Trifid cipher
• trifid_decrypt(message, alphabet, period) - Decrypts a Trifid cipher encrypted message
• Proper error handling with Result<String, String>
• Configurable alphabet (must be 27 characters)
• Configurable period for grouping characters
• Type aliases for improved code clarity and reduced complexity

Testing

All tests pass with 100% code coverage:

cargo test trifid

References

• Wikipedia: Trifid Cipher

Checklist

• Code follows the repository's style guidelines
• All tests pass
• Added comprehensive test coverage (13 tests, 100% coverage)
• Updated mod.rs with new module
• Added documentation comments
• No clippy warnings
• Code formatted with cargo fmt