A digital data sampler operating in a computer processor selects and stores digital data samples from a data stream used for generating audio-visual output during a session with a client operated by a user. The session generates the data stream independently of the data sampler. The data sampler may collect parameter data correlated to a probability will be remembered by the user at some future time, for each sample. The data sampler may store the data samples and parameter data as shared secret data for use in a future authentication session. During a future authentication session, an authentication device selects test data from the shared secret data to generate sensible output in an authentication process. The authentication process grants access to a controlled resource in response to user input indicating specific knowledge of the shared secret data selected from a presentation of similar sensible outputs.