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.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.71795% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.11%. Comparing base (a6f9ffd) to head (666c99b).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ciphers/hill_cipher.rs	98.71%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1006      +/-   ##
==========================================
+ Coverage   96.09%   96.11%   +0.01%     
==========================================
  Files         375      376       +1     
  Lines       26674    26908     +234     
==========================================
+ Hits        25632    25862     +230     
- Misses       1042     1046       +4     

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