paillier-bigint

An implementation of the Paillier cryptosystem relying on the native JS implementation of BigInt.

It can be used by any Web Browser or webview supporting BigInt and with Node.js (>=10.4.0). In the latter case, for multi-threaded primality tests, you should use Node.js v11 or newer or enable at runtime with node --experimental-worker with Node.js version >= 10.5.0 and < 11.

The operations supported on BigInts are not constant time. BigInt can be therefore unsuitable for use in cryptography. Many platforms provide native support for cryptography, such as Web Cryptography API or Node.js Crypto.

The Paillier cryptosystem, named after and invented by Pascal Paillier in 1999, is a probabilistic asymmetric algorithm for public key cryptography. A notable feature of the Paillier cryptosystem is its homomorphic properties.

Homomorphic properties

Homomorphic addition of plaintexts

The product of two ciphertexts will decrypt to the sum of their corresponding plaintexts,

D( E(m₁) · E(m₂) ) mod n² = m₁ + m₂ mod n

The product of a ciphertext with a plaintext raising g will decrypt to the sum of the corresponding plaintexts,

D( E(m₁) · g^m₂ ) mod n² = m₁ + m₂ mod n

(pseudo-)homomorphic multiplication of plaintexts

An encrypted plaintext raised to the power of another plaintext will decrypt to the product of the two plaintexts,

D( E(m₁)^m₂ mod n² ) = m₁ · m₂ mod n,

D( E(m₂)^m₁ mod n² ) = m₁ · m₂ mod n.

More generally, an encrypted plaintext raised to a constant k will decrypt to the product of the plaintext and the constant,

D( E(m₁)^k mod n² ) = k · m₁ mod n.

However, given the Paillier encryptions of two messages there is no known way to compute an encryption of the product of these messages without knowing the private key.

Key generation

Define the bit length of the modulus n, or keyLength in bits.
Choose two large prime numbers p and q randomly and independently of each other such that gcd( p·q, (p-1)(q-1) )=1 and n=p·q has a key length of keyLength. For instance:
1. Generate a random prime p with a bit length of keyLength/2 + 1.
2. Generate a random prime q with a bit length of keyLength/2.
3. Repeat until the bitlength of n=p·q is keyLength.
Compute parameters λ, g and μ. Among other ways, it can be done as follows:
1. Standard approach:
  1. Compute λ = lcm(p-1, q-1) with lcm(a, b) = a·b / gcd(a, b).
  2. Generate randoms α and β in Z* of n, and select generator g in Z* of n**2 as g = ( α·n + 1 ) β**n mod n**2.
  3. Compute μ = ( L( g^λ mod n**2 ) )**(-1) mod n where L(x)=(x-1)/n.
2. If using p,q of equivalent length, a simpler variant would be:
  1. λ = (p-1, q-1)
  2. g = n+1
  3. μ = λ**(-1) mod n

The public (encryption) key is (n, g).

The private (decryption) key is (λ, μ).

Encryption

Let m in [0, n) be the clear-text message,

Select random integer r in Z* of n.
Compute ciphertext as: c = g**m · r**n mod n**2

Decryption

Let c be the ciphertext to decrypt, where c in (0, n**2).

Compute the plaintext message as: m = L( c**λ mod n**2 ) · μ mod n

Usage

paillier-bigint can be imported to your project with npm:

npm install paillier-bigint

Then either require (Node.js CJS):

const paillierBigint = require('paillier-bigint')

or import (JavaScript ES module):

import * as paillierBigint from 'paillier-bigint'

The appropriate version for browser or node is automatically exported.

You can also download the IIFE bundle, the ESM bundle or the UMD bundle and manually add it to your project, or, if you have already imported paillier-bigint to your project, just get the bundles from node_modules/paillier-bigint/dist/bundles/.

An example of usage could be:

async function paillierTest () {
  // (asynchronous) creation of a random private, public key pair for the Paillier cryptosystem
  const { publicKey, privateKey } = await paillierBigint.generateRandomKeys(3072)

  // Optionally, you can create your public/private keys from known parameters
  // const publicKey = new paillierBigint.PublicKey(n, g)
  // const privateKey = new paillierBigint.PrivateKey(lambda, mu, publicKey)

  const m1 = 12345678901234567890n
  const m2 = 5n

  // encryption/decryption
  const c1 = publicKey.encrypt(m1)
  console.log(privateKey.decrypt(c1)) // 12345678901234567890n

  // homomorphic addition of two ciphertexts (encrypted numbers)
  const c2 = publicKey.encrypt(m2)
  const encryptedSum = publicKey.addition(c1, c2)
  console.log(privateKey.decrypt(encryptedSum)) // m1 + m2 = 12345678901234567895n

  // multiplication by k
  const k = 10n
  const encryptedMul = publicKey.multiply(c1, k)
  console.log(privateKey.decrypt(encryptedMul)) // k · m1 = 123456789012345678900n
}
paillierTest()

Consider using bigint-conversion if you need to convert from/to bigint to/from unicode text, hex, buffer.

API reference documentation

Check the API

juanelas/paillier-bigint