shamir_share

A secure and efficient Rust library for Shamir's Secret Sharing, built with a security-first, constant-time design to prevent side-channel attacks. Split sensitive data into multiple shares where only a threshold number is needed for reconstruction.

Overview

shamir_share provides a robust implementation of Shamir's Secret Sharing scheme as a Rust library. It allows you to split sensitive data (such as database files) into multiple shares, where the original data can only be reconstructed when a predetermined number of shares are combined.

Key Features

• Flexible Share Generation:
  • Split data into customizable number of shares (n)
  • Define threshold (k) for minimum shares needed for reconstruction
  • Lazy iterator-based share generation with Dealer for memory efficiency
  • Support for up to 255 shares (GF(256) field limitation)
• Strong Security Features:
  • Secure random number generation using ChaCha20Rng
  • Constant-time cryptographic operations to prevent side-channel attacks
  • SHA-256 integrity verification of reconstructed secrets
  • Magic number and version checks for share file format
  • Comprehensive fuzzing for robustness testing
• Performance & Usability:
  • Zero-copy operations for efficient memory usage
  • Streaming support for large files
  • Builder pattern for ergonomic configuration
  • Cross-platform compatibility
  • Comprehensive error handling

Installation

Add this to your Cargo.toml:

[dependencies]
shamir_share = "0.1.0"

Quick Start

Basic Usage

use shamir_share::{ShamirShare, FileShareStore, ShareStore};

// Create a scheme with 5 shares and threshold 3
let mut scheme = ShamirShare::builder(5, 3).build().unwrap();

// Split a secret
let secret = b"my secret data";
let shares = scheme.split(secret).unwrap();

// Store shares
let temp_dir = tempfile::tempdir().unwrap();
let mut store = FileShareStore::new(temp_dir.path()).unwrap();
for share in &shares {
    store.store_share(share).unwrap();
}

// Reconstruct from 3 shares
let loaded_shares = vec![
    store.load_share(1).unwrap(),
    store.load_share(2).unwrap(),
    store.load_share(3).unwrap(),
];
let reconstructed = ShamirShare::reconstruct(&loaded_shares).unwrap();
assert_eq!(reconstructed, secret);

Lazy Share Generation with Dealer

use shamir_share::ShamirShare;

let mut scheme = ShamirShare::builder(10, 5).build().unwrap();
let secret = b"my secret data";

// Generate only the shares you need
let shares: Vec<_> = scheme.dealer(secret).take(5).collect();

// Or use iterator methods for advanced filtering
let even_shares: Vec<_> = scheme.dealer(secret)
    .filter(|share| share.index % 2 == 0)
    .take(5)
    .collect();

let reconstructed = ShamirShare::reconstruct(&shares).unwrap();
assert_eq!(reconstructed, secret);

Usage Examples

Basic Usage

use shamir_share::{ShamirShare, FileShareStore, ShareStore};
use std::path::Path;

// Create a new ShamirShare instance with 5 shares, threshold of 3
let mut shamir = ShamirShare::builder(5, 3).build()?;

// Split a secret
let secret = b"This is a secret message";
let shares = shamir.split(secret)?;

// Store shares in a directory
let store = FileShareStore::new(Path::new("/path/to/shares"))?;
for share in &shares {
    store.store_share(share)?;
}

// Later, reconstruct the secret using at least 3 shares
let mut reconstruction_shares = Vec::new();
for i in 1..=3 {
    reconstruction_shares.push(store.load_share(i)?);
}

let reconstructed = ShamirShare::reconstruct(&reconstruction_shares)?;
assert_eq!(&reconstructed, secret);

Security

This library is designed with security as the primary concern, implementing multiple layers of protection against various attack vectors:

Cryptographic Security Guarantees

• Constant-Time GF(2^8) Arithmetic: All finite field operations use constant-time algorithms with no lookup tables, preventing cache-timing side-channel attacks
• Resistant to Side-Channel Attacks: Russian Peasant Multiplication and Fermat's Little Theorem for inversion ensure consistent execution time regardless of input values
• Cryptographically Secure Random Number Generation: Uses ChaCha20Rng seeded from OsRng for polynomial coefficient generation
• Integrity Checking: SHA-256 hash verification with constant-time comparison prevents tampering detection
• No Information Leakage: Individual shares reveal no information about the secret without meeting the threshold

Implementation Security Features

• Memory Safety: Written in Rust with zero unsafe code blocks
• Constant-Time Hash Comparison: Prevents timing attacks during integrity verification
• Secure Share Format: Magic numbers and version checks prevent format confusion attacks
• Parallel Processing: Uses Rayon for safe parallel computation without compromising security

Security Considerations for Users

• Share Distribution: Distribute and store shares in different physical locations
• Access Control: Implement proper access controls for share storage locations
• Network Security: Use secure channels when transmitting shares over networks

Security-Performance Tradeoff

This library prioritizes security over raw performance. The constant-time implementations of cryptographic operations protect against side-channel attacks but are slower than lookup table-based approaches. For typical use cases (secrets under a few MB), the performance impact is minimal:

Operation	1KB Secret	10KB Secret	100KB Secret
Split	~160 us	~550 us	~3.4 ms
Reconstruct	~80 us	~170 us	~680 us
Total	~240 us	~720 us	~4.1 ms

API Documentation

For detailed API documentation, please visit docs.rs/shamir_share.

Core Types

• ShamirShare: Main implementation of Shamir's Secret Sharing scheme
• ShamirShareBuilder: Builder pattern for configuring ShamirShare instances
• Dealer: Lazy iterator for generating shares on-demand
• Share: Represents a single share of the secret
• FileShareStore: File-based storage for shares
• ShareStore: Trait for implementing custom storage backends
• Config: Configuration options for the sharing process
• SplitMode: Enum for specifying how data is split (Sequential or Parallel)

Development & Testing

Running Tests

# Run all tests
cargo test

# Run tests with coverage
cargo test --all-features

# Run benchmarks
cargo bench

Fuzzing for Security

This library includes comprehensive fuzzing targets to ensure robustness against malformed inputs:

# Install nightly Rust and cargo-fuzz
rustup install nightly
cargo +nightly install cargo-fuzz

# Run fuzzing targets
cd fuzz
cargo +nightly fuzz run fuzz_reconstruct    # Test reconstruction logic
cargo +nightly fuzz run fuzz_share_storage  # Test share parsing logic

# Or use the convenience script
./run_fuzz.sh all 300  # Run all targets for 5 minutes each

The fuzzing targets test:

• Reconstruction robustness: Invalid shares, corrupted data, edge cases
• Storage parsing: Malformed files, truncated data, invalid headers
• Memory safety: Buffer overruns, integer overflows, panic conditions

Performance Testing

# Run streaming benchmarks
./run_streaming_benchmarks.sh

# Compare with baseline
cargo run --release --bin performance_compare

Contributing

Contributions are welcome! Please see CONTRIBUTING.md for guidelines on how to contribute to this project.

Security Considerations for Contributors

• All cryptographic operations must be constant-time
• New features should include comprehensive fuzzing targets
• Performance changes should be benchmarked
• Security-sensitive code requires additional review

License

This project is licensed under the MIT License - see the LICENSE file for details.