C-FAST

A C implementation of the FAST (Format-preserving, Additive, Symmetric Translation) cipher.

Overview

FAST is a format-preserving encryption scheme designed for arbitrary radix values and word lengths. This implementation provides a complete cryptographic library for performing format-preserving encryption while maintaining the structure and format of input data.

This implementation is fully compatible with the FAST implementation in Zig.

Features

• Format-preserving encryption for arbitrary radix (4-256)
• Configurable word lengths and security parameters
• S-box pool generation using AES-based PRNG
• Multi-layer SPN (Substitution-Permutation Network) structure
• Comprehensive test suite with edge case handling
• Tweak support for domain separation
• AES-CMAC based PRF for key derivation

Requirements

• GCC or compatible C compiler
• OpenSSL 3.x
• Make build system

On macOS with Homebrew:

brew install openssl@3

Building

Build the library and test suite:

make

Run tests:

make test             # Run basic test suite
make test_edge_cases  # Build edge case tests (run with ./test_edge_cases)
make diffusion        # Run diffusion tests
make benchmark        # Run performance benchmarks

Clean build artifacts:

make clean

Usage

Basic Example

#include "fast.h"

// Initialize parameters
fast_params_t params;
calculate_recommended_params(&params, 10, 16);  // radix=10, word_length=16

// Create context with key
uint8_t key[16] = { /* 128-bit key */ };
fast_context_t *ctx;
fast_init(&ctx, &params, key);

// Encrypt data
uint8_t plaintext[16] = { /* input data */ };
uint8_t ciphertext[16];
uint8_t tweak[8] = { 0, 1, 2, 3, 4, 5, 6, 7 };
fast_encrypt(ctx, tweak, sizeof(tweak), plaintext, ciphertext, 16);

// Decrypt data
uint8_t decrypted[16];
fast_decrypt(ctx, tweak, sizeof(tweak), ciphertext, decrypted, 16);

// Cleanup
fast_cleanup(ctx);

Configuration Parameters

• radix: Base of the numeral system (4-256)
• word_length: Length of plaintext/ciphertext words
• sbox_count: Number of S-boxes in the pool (default: 256)
• num_layers: Number of SPN layers for security
• branch_dist1, branch_dist2: Branch distances for diffusion
• security_level: Target classical security in bits (default 128)

References

• FAST Paper
• The Next Generation of Performant Data Protection: a New FPE Algorithm
• Format-Preserving Encryption