System and method for computer authentication with user modification of an image using a shared secret

Computers can be authenticated using a shared secret. During an authentication process, a server transmits an image to a client. A mobile device captures and analyzes the image. If the image contains the shared secret known only to the authentic server and the authentic mobile communication device, the mobile device can authenticate the server. The secret in the image can be readily analyzed. A single image may contain multiple shared secrets. Once the server has been authenticated, the user must modify the image in accordance with a shared modification secret to thereby authentic the user. The modified image is transmitted back to the authenticated server. If the image was properly modified, the user is authenticated.