SYSTEMS AND METHODS FOR CLIENT AUTHENTICATION

A server computer queries a client device using a test fingerprint that does not correspond to any document stored in the client device, and restricts access by the client device to the server computer if the query result is incorrect. In some implementations, the server selects a set of first fingerprints associated with documents stored in the client device, generates a set of test fingerprints, none of which are associated with any document stored in the client device, and queries the client device using fingerprints from the set of first fingerprints and the set of test fingerprints to produce query results. The server computer restricts access by the client device to the server computer if at least one of the query results is incorrect, and enables the client device to access the server computer if at least a predefined number of the query results are correct and none are incorrect.