If a radio station was taken over by an adversary, say as a result of some sort of censorship action, how could a listener even know unless drastic changes were made to the programming? Digital signatures are a solved problem for emails, but could it be made to work on a broadcast medium such as radio in a way that is minimally audible to listeners and revokable in some way? Callsign is an attempt to answer that question.

Todo List:

• Watermark creation method
• Watermark detection method
• Simulating the effects of radio transmission
• Testing suite (e.g., perceptibility, fault tolerance)
• Make watermarking fault-tolerant
• Encrypt/Decrypt Watermark
• SDR pipline (likely HackRF)
• Make watermark some sort of TOTP (possibly backed by a Yubikey)