noise-curve-secp/index.js

`secp256k1` elliptic curve operations for use with [`noise-handshake`](https://github.com/chm-diederichs/noise-handshake)

const secp = require('secp256k1-native')
const sodium = require('sodium-universal')
const assert = require('nanoassert')

const DHLEN = secp.secp256k1_SECKEYBYTES
const PKLEN = 33
const SKLEN = secp.secp256k1_SECKEYBYTES
const ALG = 'secp256k1'

module.exports = {
  DHLEN,
  PKLEN,
  SKLEN,
  ALG,
  name: ALG,
  generateKeyPair,
  dh
}

function generateKeyPair (privKey) {
  const ctx = secp.secp256k1_context_create(secp.secp256k1_context_SIGN)

  if (privKey) assert(secp.secp256k1_ec_seckey_verify(ctx, privKey))

  const keyPair = {}
  keyPair.secretKey = privKey || Buffer.alloc(SKLEN)
  keyPair.publicKey = Buffer.alloc(PKLEN)

  while (!secp.secp256k1_ec_seckey_verify(ctx, keyPair.secretKey)) {
    sodium.randombytes_buf(keyPair.secretKey)
  }

  const pk = Buffer.alloc(64)
  secp.secp256k1_ec_pubkey_create(ctx, pk, keyPair.secretKey)
  secp.secp256k1_ec_pubkey_serialize(ctx, keyPair.publicKey, pk, secp.secp256k1_ec_COMPRESSED)

  return keyPair
}

function dh (publicKey, { secretKey }) {
  assert(secretKey.byteLength === SKLEN)
  assert(publicKey.byteLength === PKLEN)

  const point = Buffer.alloc(secp.secp256k1_PUBKEYBYTES)
  const ctx = secp.secp256k1_context_create(secp.secp256k1_context_SIGN)
  secp.secp256k1_ec_pubkey_parse(ctx, point, publicKey)

  const output = Buffer.alloc(DHLEN)

  secp.secp256k1_ecdh(
    ctx,
    output,
    point,
    secretKey,
    Buffer.alloc(0)
  )

  return output
}