constructor(opts) {

    this.opts = opts || {};

    if (!this.opts.invalidSessionSecretMessage)
      this.opts.invalidSessionSecretMessage = 'Invalid session secret';

    if (!this.opts.invalidSessionSecretStatusCode)
      this.opts.invalidSessionSecretStatusCode = 403;

    if (!this.opts.invalidTokenMessage)
      this.opts.invalidTokenMessage = 'Invalid CSRF token';

    if (!this.opts.invalidTokenStatusCode)
      this.opts.invalidTokenStatusCode = 403;

    if (!this.opts.excludedMethods)
      this.opts.excludedMethods = [ 'GET', 'HEAD', 'OPTIONS' ];

    if (typeof this.opts.disableQuery !== 'boolean')
      this.opts.disableQuery = false;

    this.tokens = csrf(opts);

    return this.middleware;

}