Techniques are disclosed for authenticating users to a computing application. A relying application transmits a login page to a user requesting access to the application. The login page may include a QR code (or other barcode) displayed to the user. The QR code may encode a nonce along with a URL address indicating where a response to the login challenge should be sent. In response, the user scans the barcode with an app on a mobile device (e.g., using a camera on a smart phone) to recover both the nonce and the URL address. The mobile device may also include a certificate store containing a private key named in a PKI certificate. The app signs the nonce using the private key and sends the signed nonce in to the URL in a response message.