CGMS-A + VEIL = SDMI ? Tuesday January 17, 2006 by Ed Felten http://www.freedom-to-tinker.com/?p=955
I wrote last week about the Analog Hole Bill, which would require almost all devices that handle analog video signals to implement a particular anti-copying scheme called CGMS-A + VEIL. Today I want to talk about how that scheme works, and what we can learn from its design. CGMS-A + VEIL is, not surprisingly, a combination of two discrete signaling technologies called CGMS-A and VEIL. Both allow information to be encoded in an analog video signal, but they work in different ways. CGMS-A stores a few bits of information in a part of the analog video signal called the vertical blanking interval (VBI). Video is transmitted as a series of discrete frames that are displayed one by one. In analog video signals, there is an empty space between the frames. This is the VBI. Storing information there has the advantage that it doesn¹t interfere with any of the frames of the video, but the disadvantage that the information, being stored in part of the signal that nobody much cares about, is easily lost. (Nowadays, closed captioning information is stored in the VBI; but still, VBI contents are easily lost.) For example, digital video doesn¹t have a VBI, so straight analog-to-digital translation will lose anything stored in the VBI. The problem with CGMS-A, then, is that it is too fragile and will often be lost as the signal is stored, processed, and translated. There¹s one other odd thing about CGMS-A, at least as it is used in the Analog Hole Bill. It¹s remarkably inefficient in storing information. The version of CGMS-A used there (with the so-called RCI bit) stores three bits of information (if it is present), so it can encode eight distinct states. But only four distinct states are used in the bill¹s design. This means that it¹s possible, without adding any bits to the encoding, to express four more states that convey different information about the copyright owner¹s desires. For example, there could be a way for the copyright owner to signal that the customer was free to copy the video for personal use, or even that the customer was free to retransmit the video without alteration. But our representatives didn¹t see fit to support those options, even though there are unused states in their design. The second technology, VEIL, is a watermark that is inserted into the video itself. VEIL was originally developed as a way for TV shows to send signals to toys. If you pointed the toy at the TV screen, it would detect any VEIL information encoded into the TV program, and react accordingly. Then somebody got the idea of using VEIL as a ³rights signaling² technology. The idea is that whenever CGMS-A is signaling restrictions on copying, a VEIL watermark is put into the video. Then if a signal is found to have a VEIL watermark, but no CGMS-A information, this is taken as evidence that CGMS-A information must have been lost from that signal at some point. When this happens, the bill requires that the most restrictive DRM rules be applied, allowing viewing of the video and nothing else. Tellingly, advocates of this scheme do their best to avoid calling VEIL a ³watermark², even though that¹s exactly what it is. A watermark is an imperceptible (or barely perceptible) component, added to audio or video signal to convey information. That¹s a perfect description of VEIL. Why don¹t they call it a watermark? Probably because watermarks have a bad reputation as DRM technologies, after the Secure Digital Music Initiative (SDMI). SDMI used two signals, one of which was a ³robust² watermark, to encode copy control information in content. If the robust watermark was present but the other signal was absent, this was taken as evidence that something was wrong, and strict restrictions were to be enforced. Sound familiar? SDMI melted down after its watermark candidates all four of them were shown to be removable by an adversary of modest skill. And an adversary who could remove the watermark could then create unprotected copies of the content. Is the VEIL watermark any stronger than the SDMI watermarks? I would expect it to be weaker, since the VEIL technology was originally designed for an application where accidental loss of the watermark was a problem, but deliberate removal by an adversary was not an issue. So how does VEIL work? I¹ll write about that soon. You are a subscribed member of the infowarrior list. Visit www.infowarrior.org for list information or to unsubscribe. This message may be redistributed freely in its entirety. Any and all copyrights appearing in list messages are maintained by their respective owners.
