/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
define(['sjcl', './hkdf'], function (sjcl, hkdf) {
  'use strict';
 
  var PREFIX_NAME = 'identity.mozilla.com/picl/v1/';
  var bitSlice = sjcl.bitArray.bitSlice;
  var salt = sjcl.codec.hex.toBits('');
 
  /**
   * @class hawkCredentials
   * @method deriveHawkCredentials
   * @param {String} tokenHex
   * @param {String} context
   * @param {int} size
   * @returns {Promise}
   */
  function deriveHawkCredentials(tokenHex, context, size) {
    var token = sjcl.codec.hex.toBits(tokenHex);
    var info = sjcl.codec.utf8String.toBits(PREFIX_NAME + context);
 
    return hkdf(token, info, salt, size || 3 * 32)
      .then(function(out) {
        var authKey = bitSlice(out, 8 * 32, 8 * 64);
        var bundleKey = bitSlice(out, 8 * 64);
 
        return {
          algorithm: 'sha256',
          id: sjcl.codec.hex.fromBits(bitSlice(out, 0, 8 * 32)),
          key: authKey,
          bundleKey: bundleKey
        };
      });
  }
 
  return deriveHawkCredentials;
});