Techniques are disclosed for authenticating users accessing computing applications, e.g., applications hosted in a cloud environment accessed using a variety of computing systems. As disclosed, an authentication process is performed using a certificate and private key installed on a mobile device and a nonce generated on the server. To authenticate a user, a server generates a nonce, encrypts the nonce with a public key associated with the user, and encodes the encrypted nonce in a barcode graphic (e.g., a QR code). The resulting barcode graphic is displayed to the user, and a mobile device scans the barcode graphic to recover the encrypted nonce. The encrypted nonce is decrypted using a private key stored on the mobile device. The clear text nonce is then displayed on the screen of the mobile device and used as a one-time password (OTP) for authentication.