feat(dsa): implement DSA digital signature scheme with key generation…

[Gabry022003]

Add DSA Digital Signature Algorithm Implementation

Summary

Added complete DSA (Digital Signature Algorithm) implementation following FIPS 186-4 standard.

Changes

Core Implementation

• New module: src/dsa/ with algorithms and key structures
• New trait: traits::digital_signature::DigitalSignature for signature schemes
• Security levels: 80, 112, 128, 192, 256 bits (FIPS 186-4 compliant)
• Dependencies: Added rug v1.24 for arbitrary precision arithmetic

Testing & Examples

• Example: dsa_example.rs demonstrating complete usage workflow
• Tests: 11 comprehensive test suites with mathematical property validation
• Benchmarks: Performance testing for keygen/sign/verify operations
• Documentation: Updated README

Utilities

• Utils module: src/utils/rand.rs are adapted from the pke_rust

Key Features

• Cryptographically secure domain parameter generation
• Miller-Rabin primality testing with configurable rounds
• Secure random number generation via OS entropy
• Mathematical property validation (g^q ≡ 1 mod p, q | p-1)
• Attack detection (invalid signatures, tampered messages)

Integration

• Dual API design: Maintains existing BLS trait while adding DSA trait
• No breaking changes to existing BLS implementation
• Consistent error handling and testing patterns