Patent attributes
A request is received from a user at a client to access a file of a set of files backed up to a backup server. Upon verifying a password provided by the user, the client is issued another request for authentication. A first data structure is received responsive to the request. The first data structure is generated using identifiers corresponding to a set of files at the client of which at least some presumably have been backed up to the server. A second data structure is generated. The second data structure is generated using identifiers corresponding to the set of files backed up to the server. The first and second data structures are compared to assess a degree of similarity between the files at the client and the files backed up to the backup server. The user is denied access when the degree of similarity is below a threshold.