A mechanism is provided for unlocking a locked computing device based on a knowledge of a user. Responsive to detecting an intent to utilize the locked computing device, a subset of the images is randomly selecting in an identified tag category from a plurality of tagged images, the subset of images comprising a plurality of tags in the identified tag category. An order in which the user must gesture to the plurality of tags in the subset of images is randomly selected. The subset of images and the order in which the user must gesture to the plurality of tags in the subset of images to the user is presented to the user. Responsive to the user gesturing to a subset of tags in the subset of images in the selected order, the locked computing device is unlocked.