code: function getHmacKey(key, blockIndex) {
      var shaSrc = new Uint8Array(8 + key.byteLength);
      shaSrc.set(new Uint8Array(key), 8);
      var view = new DataView(shaSrc.buffer);
      view.setUint32(0, blockIndex.lo, true);
      view.setUint32(4, blockIndex.hi, true);
      return CryptoEngine.sha512(ByteUtils.arrayToBuffer(shaSrc)).then(function(sha) {
          ByteUtils.zeroBuffer(shaSrc);
          return sha;
      });
  }
docs: Computes HMAC-SHA key
  @param {ArrayBuffer} key
  @param {Int64} blockIndex
  @returns {Promise.<ArrayBuffer>}