How to convert raw representations of ECDH key pair into a JSON Web Key?

user_7766787 0
Viewed 1571

Currently, I am trying to pass a public/private pair of keys generated via ECDH, represented as hex strings, into the importKey function of the Web Crypto API.

I am receiving these keys from an external source, but I have generated similar keys via node.js for testing. The curve is prime256v1. For reference, the public key I am using to test that I obtained is 04b71388fced2daee34793f74a7dfa982e37ce539a728233bcadaec298fc4ee422165b8db13e657f9c7b27b35364f523ad11fab29d717606140cc6312ec2c685cc, and the private key is 4bd22700ec3450b5f27e47ba70c233a680c981ab02c1432a859ae23111bef377.

const crypto = require('crypto');
const ecdh = crypto.createECDH('prime256v1');
ecdh.generateKeys();
console.log('ecdh p256 pubkey', ecdh.getPublicKey('hex'));
console.log('ecdh p256 prvkey', ecdh.getPrivateKey('hex'));

Importing the public key is successfully done via the raw option of importKey.

const hexToUintArray = hex => {
  const a = [];
  for (let i = 0, len = hex.length; i < len; i += 2) {
    a.push(parseInt(hex.substr(i, 2), 16));
  }
  return new Uint8Array(a);
}
const importedKey = await crypto.subtle.importKey(
  'raw',
 hexToUintArray('04b71388fced2daee34793f74a7dfa982e37ce539a728233bcadaec298fc4ee422165b8db13e657f9c7b27b35364f523ad11fab29d717606140cc6312ec2c685cc'),
  {
    name: 'ECDH',
    namedCurve: 'P-256'
  },
  true,
  []
);

However, the private key cannot be imported via the same method, as it fails with the error DataError: Data provided to an operation does not meet requirements, since the "raw" option only accepts EC public keys.

const importedPrvKey = await crypto.subtle.importKey(
  'raw',
  hexToUintArray('4bd22700ec3450b5f27e47ba70c233a680c981ab02c1432a859ae23111bef377'),
  {
    name: 'ECDH',
    namedCurve: 'P-256'
  },
  true,
  []
);

I am aware that I am able to import keys easily if it is in JSON Web Key format, but I am not aware of a method to convert it from the raw format to JWK format, or any other importable format that the Web Crypto API accepts.

jwk cryptography javascript
1 Answers

Managed to solve this myself, by looking at the source code to the the pem-to-jwk library source code. The library itself provides conversion from PEM to JWK.

The "d" parameter is the private key's ArrayBuffer, url-encoded in Base64. The "x" parameter is the first half of the uncompressed public key in an ArrayBuffer, url-encoded as a Base64 string. The "y" parameter is the second half of the uncompressed public key in an ArrayBuffer, url-encoded as a Base64 string.

const publicKeyHex = '04b71388fced2daee34793f74a7dfa982e37ce539a728233bcadaec298fc4ee422165b8db13e657f9c7b27b35364f523ad11fab29d717606140cc6312ec2c685cc';
const privateKeyHex = '4bd22700ec3450b5f27e47ba70c233a680c981ab02c1432a859ae23111bef377';

const hexToUintArray = hex => {
  const a = [];
  for (let i = 0, len = hex.length; i < len; i += 2) {
    a.push(parseInt(hex.substr(i, 2), 16));
  }
  return new Uint8Array(a);
}

const hexToArrayBuf = hex => {
  return hexToUintArray(hex).buffer;
}

const arrayBufToBase64UrlEncode = buf => {
  let binary = '';
  const bytes = new Uint8Array(buf);
  for (var i = 0; i < bytes.byteLength; i++) {
      binary += String.fromCharCode(bytes[i]);
  }
  return window.btoa(binary)
    .replace(/\//g, '_')
    .replace(/=/g, '')
    .replace(/\+/g, '-');
}

const jwkConv = (prvHex, pubHex) => ({
  kty: "EC",
  crv: "P-256",
  d: arrayBufToBase64UrlEncode(hexToArrayBuf(prvHex)),
  x: arrayBufToBase64UrlEncode(hexToArrayBuf(pubHex).slice(1, 33)),
  y: arrayBufToBase64UrlEncode(hexToArrayBuf(pubHex).slice(33, 66))
});

const importedPrivateKey = await crypto.subtle.importKey(
  'jwk',
  jwkConv(privateKeyHex, publicKeyHex),
  {
    name: 'ECDH',
    namedCurve: 'P-256'
  },
  true,
  []
);
user_77667870
Related