code: function(callback) {
          if (!res.parsedHeader.signature) {
            callback('unsigned');
          } else {
            var signVerifier = new Crypter({
              cert: res.session.cert
            });
            if (signVerifier.verifySignature(JSON.stringify(res.parsedHeader.content), res.parsedHeader.signature)) {
              callback(null);
            } else {
              callback('forgery');
            }
          }
        }
docs: verify signature using sender certificate