code: function Verifier(givenMasterKey) {
    var masterKey = givenMasterKey || MASTER_KEY;
    if (!masterKey) {
      throw new errors.ValidationFailed('invalid master key');
    }
    this.masterKey = base64url.toBuffer(masterKey);
  }
docs: Create a new verifier instance for the given master key
  
  @constructor
  @param {string} masterKey - Master key base64url string