SimpleWebAuthn "old format" public key upgrader

updateOldPubKey.js

import base64url from 'base64url';
import cbor from 'cbor';

/**
 * @simplewebauthn/server
 * September 2020
 *
 * Add additional data to a barebones "old format" public key. Additional information includes:
 *
 * - "ECC" key type { kty: 2 }
 * - "P-256" curve  { alg: -7, crv: 1 }
 *
 * "Old format" is the following bas64url-encoded byte array:
 *
 * [0x04, x: 32 bytes, y: 32 bytes]
 */
const oldPubKey = 'BGmaxR4mBbukc2QhtW2ldhAAd555r-ljlGQN8MbcTnPP9CyUlE-0AB2fbzZbNgBvJuRa7r6o2jPphOmtyNPR_kY';
let oldBuffer = base64url.toBuffer(oldPubKey);

// Skip the "uncompressed" byte
oldBuffer = oldBuffer.slice(1);

const x = oldBuffer.slice(0, 32);
oldBuffer = oldBuffer.slice(32);

const y = oldBuffer.slice(0, 32);
oldBuffer = oldBuffer.slice(32);

const newMap = new cbor.Map([
  [1, 2],   // kty
  [3, -7],  // alg
  [-1, 1],  // crv
  [-2, x],  // x
  [-3, y],  // y
]);

const newBuffer = Buffer.from(cbor.encode(newMap));
const newPubKey = base64url.encode(newBuffer);

// This string is the "new format" value for the old "authenticator.publicKey" value
console.log(newPubKey);