Abstract: Methods and apparatus are provided for authenticating a user based on implicit user memory. Access to a protected resource is controlled by presenting a user with a plurality of stimuli during a priming phase; presenting the user with a plurality of degraded versions of the primed stimuli and with a plurality of degraded versions of non-primed stimuli during an authentication phase; receiving an identification from the user of the degraded versions of the primed stimuli and the degraded versions of the non-primed stimuli; and authenticating the user based upon a number of correct identifications of the degraded versions of the primed stimuli. The stimuli can comprise, for example, images and/or sounds. An authentication score can be based, for example, on a number of correctly labeled primed stimuli plus a number of incorrectly labeled non-primed stimuli.