feat: add Hill Cipher implementation

[AliAlimohammadi]

Description

This PR adds a Hill Cipher implementation to the ciphers module.

About Hill Cipher

The Hill Cipher is a polygraphic substitution cipher based on linear algebra. It encrypts blocks of letters using matrix multiplication modulo 36 (for alphanumeric characters A-Z, 0-9).

Implementation Details

• Pure Rust implementation with zero external dependencies
• All matrix operations (determinant, inverse, cofactor) implemented manually
• Supports any $N \times N$ key matrix
• Validates that key matrix determinant is coprime with 36
• Comprehensive test coverage

Testing

All tests pass:

cargo test hill_cipher

Example Usage

use ciphers::hill_cipher::HillCipher;

let key = vec![vec![2, 5], vec![1, 6]];
let cipher = HillCipher::new(key).unwrap();

let encrypted = cipher.encrypt("HELLO");
// Returns: "85FF00"

let decrypted = cipher.decrypt(&encrypted);
// Returns: "HELLOO" (with padding)

References

• Hill Cipher Tutorial
• Video Explanation

[AliAlimohammadi]

@siriak, this is ready to be merged.

[AliAlimohammadi]

The issue is not related to my changes.