Custos could encode the key in any number of ways, while keeping them secret. They could later (maybe some time into the future) announce the method, and catch

Custos could encode the key in any number of ways, while keeping them secret. They could later (maybe some time into the future) announce the method, and catch the source of the leak. As with any watermarking, the intention is not to make it physically impossible to make an unauthorised copy, but make it accountable. If the leaker can't be sure that they're aware of all the methods that could be used to catch them, they may decide against leaking their copy. One method may be to encode it cryptographically, in a way where it looks like noise until you have the key, similar to Code Division Multiple Access communication schemes.