A system for detecting whether a device seeking communication with a server is a returning device that previously communicated with the server includes a database that stores groups of device attributes based on observable device characteristics and unique identifiers. The database is generally not accessible to the devices. Each attribute group and the associated device identifier (DID) can uniquely identify a particular device, and the associated DID is generally not derivable from the attributes. The database may satisfy a uniqueness property so that each attribute value in the database may also uniquely identify a device.