john@anasazi.UUCP (John Moore) (03/27/86)
In article <12192278397.8.MRC@PANDA> MRC%PANDA@SUMEX-AIM.ARPA (Mark Crispin) writes: [original article posted only to net.video - discussion of MACOM VIDEOCYPHER II] [Since the VC-II is a new crypto system which will no doubt be subject to a lot of attacks in the real, non-classified world, it seems that it might be a suitable topic for discussion in net.crypt also.] > So then, what stops a clever group of conspirators from buying every >possible service for one box, then modifying lots of other boxes so they >have the same serial number? Unlike cable boxes, you'll own the satellite >box so you'll never have to worry about giving it back. I have seen lots of similar such speculations. My question: does anyone know how key management is done in the VC-II? I would propose the following scheme, which if it was in fact implemented, should be secure. The question is: is this scheme what was used? Is it really reasonably secure? Scheme: Each box has a unique DES master key which is known on the database of the service provider. When a service is enabled (or re-enabled monthly?), a working key is sent to the box (via Vertical Blanking Interval) and is encrypted using the master key. The working key is used to decrypt the digitized autio. It seems to me that this scheme is commercially secure assuming the following additional measures: (1) The method of selection of a master key is randomized so that knowing the master key of one device does not aid in guessing the master key of another one. (2) [not very critical] The working keys are changed regularly, and are different for each service. This is likely to be the case since desubscribing should cancel one's service. (3) [not very critical] The decrypting device is physically encapsulated so that it is difficult or impossible to simply xerox the master key from one device and proliferate it to many others. An alternative scheme which may have been used (and would be pretty stupid) is that all services use the same, permanent key, and the over-the-air transmissions simply enable/disable reception. I would appreciate comments or speculation on all this. Finally this seems a case for more than the usual USENET disclaimers, so: Please don't respond with anything that is a trade secret or would otherwise compromise the net integrity. Thanks. -- John Moore (NJ7E/XE1HDO) {decvax|ihnp4|hao}!noao!terak!anasazi!john {hao!noao|decvax|ihnp4|seismo}!terak!anasazi!john terak!anasazi!john@SEISMO.CSS.GOV (602) 861-7607 (day or evening) 7525 Clearwater Pkwy, Paradise Valley, AZ, 85253 (Home Address) The opinions expressed here are obviously not mine, so they must be someone else's.