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The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:16:51 ADT
Dave Del Torto (ddt@LSD.com)
Sun, 5 Apr 1998 13:53:46 -0700
At 11:49 pm -0700 4/2/98, Marty Levy wrote:
> [snip] My main concern is that I don't
>want the files to be distributed beyond the recipient (such as
>posted on the internet), and I want to be able to trace them
>back to the source if they are.
No offense, but talk about hopeless causes... modest (and easily defeated,
ala Anderson) traceability is the only possibility ...unless you purport to
have any mailhost add a timestamp to the image as it passes (cf. Schneier's
hash-lattices), but imagine how many artistes will thrill to that idea.
>Q. Is Acrobat's instantiation of 40b RC4 reasonably difficult to break?
Any algo at 40bits is reasonably easy to break, including RC4. I predict
that there will soon be tools widely available for even
moderately-spohisticated attackers (eg. copyright infringers with
sufficient funds in foreign ports) to crack targets like CD-ROM photo
collections.
Conversion to text-format files (PS, PDF, etc) adds the possibility of
twiddling commands in various identifiable ways, but that can only last so
long unless you constantly change patterns.
One scheme I discussed recently elsewhere involves accepting a public key
from a customer (on a per-transaction basis) and marking files using that,
so that non-repudiation is made more possible. You still have
marking-defeat attacks to contend with, but it's something to consider.
>Q. If not, is there any watermarking method likely to allow me to
>keep traceability?
IMHO, likely not, beyond something involving the participation of many
different interested parties. Any way you cut this, until there are a few
major breakthroughs in internetworking, it's more of a social engineering
problem than a cryptographic or software engineering one, given the
copy-proliferation that's inherently possible with digital media.
Right now, you might be as well off just telling naive users there's a
copyright protection scheme in place as actually having one.
dave
_____________________________________________________________________
"Oh, indeed, I've no doubt that you've made wonderful improvements to
the soundtrack ...though one would have to be a dog to hear them."
The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:16:51 ADT