libffx - Format Preserving Encryption
A Python implementation of the FFX Mode of Operation for Format-Preserving Encryption (FPE).
Format-preserving encryption encrypts data while preserving its format. For example, a 16-digit credit card number encrypts to another 16-digit number, and a 9-digit SSN encrypts to another 9-digit number.
Quick Start
import ffx
# 128-bit key (as hex)
key = ffx.FFXInteger('2b7e151628aed2a6abf7158809cf4f3c', radix=16, blocksize=32)
# Create encrypter for decimal digits
ffx_obj = ffx.new(key.to_bytes(16), radix=10)
# Encrypt a credit card number
cc_number = ffx.FFXInteger('4111111111111111', radix=10, blocksize=16)
tweak = ffx.FFXInteger('0000000000', radix=10, blocksize=10)
encrypted = ffx_obj.encrypt(tweak, cc_number)
decrypted = ffx_obj.decrypt(tweak, encrypted)
print(f"Original: {cc_number}") # 4111111111111111
print(f"Encrypted: {encrypted}") # 3847592710482695
print(f"Decrypted: {decrypted}") # 4111111111111111
API Reference
ffx.new(key, radix)
Create a new FFX encrypter.
key: 16-byte AES-128 keyradix: Base for message alphabet (2-36)
FFXInteger(value, radix=2, blocksize=None)
Represent a value in a specific radix.
value: Integer, string representation, or another FFXIntegerradix: Base (2-36)blocksize: Minimum output length (zero-padded)
FFXEncrypter.encrypt(tweak, plaintext) / .decrypt(tweak, ciphertext)
Encrypt/decrypt with an optional tweak (public associated data).
Specification
This implementation follows the NIST FFX-A2 specification:
- Cipher: AES-128
- Mode: Maximally-balanced Feistel network
- Rounds: 10 (constant)
- Radix: 2–36 (binary through alphanumeric)
Security Considerations
- FFX is designed for format-preserving encryption of small domains
- Security depends on domain size; very small domains may be vulnerable to brute force
- Always use cryptographically random keys
- Tweaks should be unique per encryption when possible
Links
License
MIT License
1 maintainer
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