If you look at the data sheets for satellite MPEG decoder chips, you will see that those for use in Videoguard receivers include some extra circuitry over and above the standard DVB CA (conditional access) hardware. This extra hardware is variously described as an NDS verifier or NDS ICAM (integrated conditional access module). What it is doing is anybody's guess. My guess is that it is processing ECM's, maybe storing extra temporary keys and keeping a beady eye on the smartcard. Presumably it also improves the security of the encryption system by preventing any interception of the data flowing between the decoder chip and an external CA system.
So anyone who wants to build a Videoguard plug in CAM is going to have to reproduce the NDS verifier in some additional secure hardware on the CAM. Not only that, but the data flowing out of the decoder via the CA interface and back again to the decoder becomes much more vunerable to monitoring, etc. This is maybe why NDS are so against a Videoguard CAM.
I thought I read somewhere that SCM had designed a Videoguard CAM, so it is technically feasible. However, I don't know if there are any serious plans to market it. Building the Videoguard CAM function into the MPEG decoder chip costs peanuts, but when you put it on an external CAM it is going to cost you a lot of money.
Personally, I think the way to go is to ensure that the Videoguard system is integrated into other receivers and not just dedicated BSkyB Digiboxes rather than necessarily demanding a Videoguard CAM. If you could buy, say, a Humax 5400 with integrated Videoguard with slots available for other CAMs, people would probably be happy to pay for it.
