[londons_explore]

This may be the way SirusXM works, but it is far from optimal. The best scheme is to give the radios simple sequential (or near sequential) id's, and have the station transmit a bitmap of which id's are enabled/disabled. Then it's one bit per subscriber, and can be transmitted in a few seconds. That bitmap is authenticated by a public key stored in the radio. You might want to run length encode the bitmap if you assume that in 30 years there might still be a handful of ancient radios still alive you wish to service.

Satellite bandwidth is expensive... Activation time is annoying for customers... The above scheme saves either (or both).

[tshaddox]

Just a random thought regarding activation: they could also have a channel broadcasting all new activations, so that you could put your radio into an activation mode while you're finalizing your subscription. Or they could give you an activation code that you could enter manually somehow (it could just be a simple message signed with Sirius' private key).

[InitialLastName]

Yeah, TBH I just assumed it was a unique ID baked into hardware because sequential IDs are a pain to manage, but your idea would be more efficient in the base case. As far as I can tell they also transmit things like channel-specific activations, but the sequential concept still works if that is a bitmap.