Add RSA Cipher

[AliAlimohammadi]

Description

Adds the RSA (Rivest-Shamir-Adleman) cipher algorithm to the ciphers module.

This algorithm implements asymmetric encryption using public-key cryptography, where encryption and decryption use different keys. The implementation demonstrates core RSA concepts including key generation, modular exponentiation, and the Extended Euclidean Algorithm.

Implementation Details

• Key generation from two prime numbers (p, q)
• Modular exponentiation using square-and-multiply algorithm for efficiency
• Extended Euclidean Algorithm for computing modular inverse
• Public key encryption: C = M^e mod n
• Private key decryption: M = C^d mod n
• Text encryption/decryption by converting characters to numeric values
• Strong type safety with PublicKey and PrivateKey structs
• Time Complexity: O(log n) for encryption/decryption (modular exponentiation)
• Space Complexity: O(1)

Examples

// Generate keypair from two primes
let (public, private) = generate_keypair(61, 53);

// Encrypt and decrypt a number
let message = 65;
let encrypted = encrypt(message, &public);
let decrypted = decrypt(encrypted, &private);
assert_eq!(message, decrypted);

// Encrypt and decrypt text
let text = "HELLO";
let encrypted = encrypt_text(text, &public);
let decrypted = decrypt_text(&encrypted, &private);
assert_eq!(text, decrypted);

Security Warning

⚠️ This is an educational implementation demonstrating RSA concepts. DO NOT use for production cryptography! Use established libraries like ring, rust-crypto, or openssl for real-world applications.

Limitations:

• Small key sizes (educational purposes)
• No padding schemes (OAEP, PKCS#1)
• Vulnerable to timing attacks
• Not constant-time operations

Testing

• Added comprehensive unit tests covering:
  • GCD computation (Euclid's algorithm)
  • Modular inverse calculation (Extended Euclidean)
  • Modular exponentiation (square-and-multiply)
  • Key generation with various prime pairs
  • Number encryption/decryption
  • Text encryption/decryption
  • Key property verification
  • Edge cases and various input sizes
• All 12 tests pass with cargo test
• Code formatted with cargo fmt
• No clippy warnings

Checklist

• I have tested the code
• I have run cargo fmt
• I have run cargo clippy
• I have added documentation
• I have added tests
• I have included security warnings

[AliAlimohammadi]

@siriak, this is ready to be merged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.63014% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.64%. Comparing base (e877687) to head (ce661e5).

Files with missing lines	Patch %	Lines
src/ciphers/rsa_cipher.rs	98.63%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #967      +/-   ##
==========================================
+ Coverage   95.62%   95.64%   +0.01%     
==========================================
  Files         339      340       +1     
  Lines       22078    22224     +146     
==========================================
+ Hits        21113    21257     +144     
- Misses        965      967       +2     

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