code: function generatePskIdentityField(preAmble, beacon) {
    var hash = crypto.createHash('sha256');
    hash.update(Buffer.concat([preAmble, beacon]));
    return urlSafeBase64.encode(hash.digest());
  }
docs: jscs:disable jsDoc 
  Creates the PSK_Identity_Field to identify a TLS client establishing a PSK
  connection using beacon data (see
  http://thaliproject.org/PresenceProtocolBindings/).
  @param {Buffer} preAmble
  @param {Buffer} beacon
  @returns {string}
   jscs:enable jsDoc