US Patent 7848746 Authentication methods and apparatus utilizing hash chains

A first processing device, which may be, for example, a wireless authentication token or an RFID tag, comprises a memory, a processor coupled to the memory, and interface circuitry coupled to the processor. The processor is operative to control the output of authentication information via the interface circuitry, where the authentication information comprises a sequence of values corresponding to images of a hash chain. A given one of the values of the sequence is utilized by a second processing device to generate a modified value suitable for providing joint authentication of the first and second processing devices.