Tuesday January 17, 2006 by Ed Felten

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

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

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.

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