Fix security issues and improve input validation

[douglaz]

Summary

This PR addresses multiple security vulnerabilities and code quality issues identified during a comprehensive code review. The changes improve the robustness and usability of WSKDF while maintaining backward compatibility.

Security Fixes

1. Integer Overflow Protection

• Added checked_mul() for mem_limit_kbytes * 1024 calculation to prevent overflow
• Added checked_mul() for iterations * threads calculation in benchmark command
• These prevent potential panics or undefined behavior when using large values

2. Input Validation in Release Builds

• Changed debug_assert\! to assert\! for n_bits validation to ensure checks run in release builds
• Added explicit n_bits validation in the find-key command
• Prevents arithmetic underflow when n_bits = 0

3. Comprehensive Input Validation

• Added validation functions for common checks (threads, iterations, n_bits)
• Set reasonable limits: MAX_THREADS = 10,000, MAX_ITERATIONS = 1,000,000
• Added validation for empty/whitespace hex strings with clear error messages
• All hex parsing functions now trim input and check for empty strings

Code Quality Improvements

1. Reduced Code Duplication

• Refactored validation logic into reusable functions
• Centralized validation constants for easy maintenance
• Consistent error messages across all commands

2. Convention Compliance

• Fixed string interpolation to use named placeholders throughout
• Improved error messages with specific values included
• All code now follows project conventions for error handling and formatting

3. Enhanced CLI Documentation

• Added comprehensive help text for all commands (benchmark, generate-salt were missing)
• Documented expected formats (hex-encoded) and sizes (8-byte preimage, 16-byte salt, 32-byte key)
• Improved argument descriptions with constraints and usage instructions
• Added detailed program description explaining WSKDF's purpose

Changes Made

• wskdf-core/src/lib.rs: Fixed integer overflow and assertion issues
• wskdf-cli/src/main.rs: Added validation, improved documentation, fixed string interpolation
• CONVENTIONS.md: Added file documenting project coding standards
• flake.nix: Added gh to development environment

Testing

• All existing tests pass
• Code formatted with cargo fmt
• No warnings from cargo clippy
• Manually tested edge cases (n_bits=0, large thread counts, empty files)

Breaking Changes

None. All changes maintain backward compatibility.

Checklist

• Code follows project conventions
• All tests pass
• Documentation updated
• No new warnings from clippy
• Security vulnerabilities addressed