ff3 - Format Preserving Encryption in Node.js

An implementation of the NIST approved FF3 and FF3-1 Format Preserving Encryption (FPE) algorithm in Node.js.

This package implements the FF3 algorithm for Format Preserving Encryption as described in the March 2016 NIST publication 800-38G Methods for Format-Preserving Encryption, and revised on February 28th, 2019 with a draft update for FF3-1.

• NIST Recommendation SP 800-38G (FF3)
• NIST Recommendation SP 800-38G Revision 1 (FF3-1)

Changes to minimum domain size and revised tweak length have been implemented in this package with suport for both 64-bit and 56-bit tweaks. NIST has only published official test vectors for 64-bit tweaks, but draft ACVP test vectors have been used for testing FF3-1. It is expected the final NIST standard will provide updated test vectors with 56-bit tweak lengths.

Installation

npm install ff3

Usage

FF3 is a Feistel cipher, and Feistel ciphers are initialized with a radix representing an alphabet. The number of characters in an alphabet is called the radix. Practical radix limits of 36 in Node.js means the following radix values are typical:

• radix 10: digits 0..9
• radix 36: alphanumeric 0..9, a-z

Special characters and international character sets, such as those found in UTF-8, would require a larger radix, and are not supported. Also, all elements in a plaintext string share the same radix. Thus, an identification number that consists of a letter followed by 6 digits (e.g. A123456) cannot be correctly encrypted by FPE while preserving this convention.

Input plaintext has maximum length restrictions based upon the chosen radix (2 * floor(96/log2(radix))):

• radix 10: 56
• radix 36: 36

To work around string length, its possible to encode longer text in chunks.

As with any cryptographic package, managing and protecting the key(s) is crucial. The tweak is generally not kept secret. This package does not protect the key in memory.

Code Example

The example code below can help you get started.

Using default domain [0-9]

const FF3Cipher = require('ff3/lib/FF3Cipher');

const key = "EF4359D8D580AA4F7F036D6F04FC6A94"
const tweak = "D8E7920AFA330A73"
const c = new FF3Cipher(key, tweak)

const plaintext = "4000001234567899"
let ciphertext = c.encrypt(plaintext)
let decrypted = c.decrypt(ciphertext)

console.log("%s -> %s -> %s", plaintext, ciphertext, decrypted)

Requires

This project was built and tested with Node.js 12 and later versions. It requires the 'crypto' library.

Testing

There are official test vectors for FF3 provided by NIST, which are used for testing in this package.

To run unit tests on this implementation, including all test vectors from the NIST specification, run the command:

1. node test/FF3CipherTest.js

The FF3 Algorithm

The FF3 algorithm is a tweakable block cipher based on an eight round Feistel cipher. A block cipher operates on fixed-length groups of bits, called blocks. A Feistel Cipher is not a specific cipher, but a design model. The encryption process consisting of eight rounds of processing of the plaintext, each round applies an internal round function followed by transformation steps.

The FF3 round function uses AES encryption in ECB mode, which is performed each iteration on alternating halves of the text being encrypted. The key value in FF3 is used only to initialize the AES cipher. Thereafter the tweak is used together with the intermediate encrypted text as input to the round function.

Other FPE Algorithms

Only FF1 and FF3 have been approved by NIST for format preserving encryption. There are patent claims on FF1 which allegedly include open source implementations. Given the issues raised in "The Curse of Small Domains: New Attacks on Format-Preserving Encryption" by Hoang, Tessaro and Trieu in 2018, it is prudent to be very cautious about using any FPE that isn't a standard and hasn't stood up to public scrutiny.

Implementation Notes

This implementation follows the algorithm as outlined in the NIST specification as closely as possible, including naming.

FPE can be used for sensitive data tokenization, especially with PCI and cryptographically reversible tokens. This implementation does not provide any guarantees regarding PCI DSS or other validation.

While all test vectors pass, this package has not otherwise been extensively tested.

The standard built-in BigInt package supports radices/bases up to 36. Therefore, this release supports a max base of 36, which can contain numeric digits 0-9 and lowercase alphabetic characters a-z.

The cryptographic library used is crypto for AES encryption. FF3 uses a single-block with an IV of 0, which is effectively ECB mode. AES ECB is the only block cipher function which matches the requirement of the FF3 spec.

The domain size was revised in FF3-1 to radixminLen >= 1,000,000 and is represented by the constant DOMAIN_MIN in ff3.py. FF3-1 is in draft status and updated 56-bit test vectors are not yet available.

The tweak is required in the initial FF3Cipher constructor, but can optionally be overridden in each encrypt and decrypt call. This is similar to passing an IV or nonce when creating an encrypter object.

Author

Brad Schoening

License

This project is licensed under the terms of the Apache 2.0 license.