A hand-held sonic token can be used as a pseudorandom oracle for a requesting application, which can generate a challenge that is sent to the token. The user of the token decides whether to allow the token to function as an oracle, and if so, the user causes the token to digitally sign the challenge and send it back to the requesting application, for use of the digitally signed challenge as an encryption key. After encryption the requesting application deletes the signed challenge, with subsequent decryption essentially following the encryption process.