Whisper is an attendance-managing app which exchanges secure tokens via sound.
A server is planted on a table which listens for Whispers from devices of attendees, employees and volunteers. Whenever a device throws in a Whisper; the data (token) is decoded into a byte array and the hash of the token is generated and sent back to the user device, again, through a Whisper. This hash is compared with a local hash generated by the device. If the two hashes equate, the device is marked present.