From: Smigrodzki, Rafal (SmigrodzkiR@MSX.UPMC.EDU)
Date: Thu Sep 20 2001 - 12:15:58 MDT

Louis Newstrom wrote:

Another picture technique, is to use a known picture, and modify the colors
slightly. The difference between the base picture and the one sent would
contain the message. This works well for one-time getting around security.
Sending the same picture more than once is a clear flag that something funny
is going on, and comparing two such pictures would instantly show which bits
are changing and which are static.

> If there are only 100 bytes encoded in a 2 MB photo, I would imagine it
> might be actually impossible to detect the message, without having the
> telling you at which particular bits to look.
While this is true, it has nothing to do with the picture technique. It's
just a variation of the old rule that the chance of cracking a code is
proportional to the number of examples. (A one symbol message is always
uncrackable. An infinitely long message is always crackable.)

### Actually, if you have means of exchanging large amounts of data by
secure means intermittently (as in having personal meetings once every ten
years or so, and exchanging CD-ROMs full of pictures), you could afford to
use a different code for each message, and to maintain an encrypted and
undetectable communication channel for a long time.

This again underscores the futility of any attempts to ferret out smart
terrorists by banning private encryption.

Needless to say, dumb terrorists do not warrant such a ban either.

Rafal Smigrodzki

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