A hand-held token can be operated to generate an acoustic signal representing the digital signature generated by a private key of a public key/private key pair. Verifiers that might be located at, e.g., buildings, in vehicles, at bank ATMs, etc. receive the signal and retrieve the corresponding public...http://www.google.com.au/patents/US7401224?utm_source=gb-gplus-sharePatent US7401224 - System and method for managing sonic token verifiers
