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 key to selectively grant access authorization to components served by the verifiers . methods and systems permit adding and removing a token from the access list of a verifier . other methods and systems enable the token to be used with several verifiers that are nearby each other , such as might be the case with multiple vehicles owned by the same user and parked nearby each other , without more than one verifier being operated to grant access .