js-libp2p-crypto/src/crypto/ecdh.js

'use strict'

const crypto = require('./webcrypto')()
const nodeify = require('nodeify')
const BN = require('asn1.js').bignum

const util = require('./util')
const toBase64 = util.toBase64
const toBn = util.toBn

exports.generateEphmeralKeyPair = function (curve, callback) {
  nodeify(crypto.subtle.generateKey(
    {
      name: 'ECDH',
      namedCurve: curve
    },
    true,
    ['deriveBits']
  ).then((pair) => {
    // forcePrivate is used for testing only
    const genSharedKey = (theirPub, forcePrivate, cb) => {
      if (typeof forcePrivate === 'function') {
        cb = forcePrivate
        forcePrivate = undefined
      }

      const privateKey = forcePrivate || pair.privateKey
      nodeify(crypto.subtle.importKey(
        'jwk',
        unmarshalPublicKey(curve, theirPub),
        {
          name: 'ECDH',
          namedCurve: curve
        },
        false,
        []
      ).then((publicKey) => {
        return crypto.subtle.deriveBits(
          {
            name: 'ECDH',
            namedCurve: curve,
            public: publicKey
          },
          privateKey,
          256
        )
      }).then((bits) => {
        // return p.derive(pub.getPublic()).toBuffer('be')
        return Buffer.from(bits)
      }), cb)
    }

    return crypto.subtle.exportKey(
      'jwk',
      pair.publicKey
    ).then((publicKey) => {
      return {
        key: marshalPublicKey(publicKey),
        genSharedKey
      }
    })
  }), callback)
}

const curveLengths = {
  'P-256': 32,
  'P-384': 48,
  'P-521': 66
}

// Marshal converts a jwk encodec ECDH public key into the
// form specified in section 4.3.6 of ANSI X9.62. (This is the format
// go-ipfs uses)
function marshalPublicKey (jwk) {
  const byteLen = curveLengths[jwk.crv]

  return Buffer.concat([
    Buffer([4]), // uncompressed point
    toBn(jwk.x).toBuffer('be', byteLen),
    toBn(jwk.y).toBuffer('be', byteLen)
  ], 1 + byteLen * 2)
}

// Unmarshal converts a point, serialized by Marshal, into an jwk encoded key
function unmarshalPublicKey (curve, key) {
  const byteLen = curveLengths[curve]

  if (!key.slice(0, 1).equals(Buffer([4]))) {
    throw new Error('Invalid key format')
  }
  const x = new BN(key.slice(1, byteLen + 1))
  const y = new BN(key.slice(1 + byteLen))

  return {
    kty: 'EC',
    crv: curve,
    x: toBase64(x),
    y: toBase64(y),
    ext: true
  }
}