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xxxx is exactly right on this. Several years ago I posted to sci.crypt
my "novel" idea for packing bits into the essentially inaudible "least
significant bits" (LSBs) of digital recordings, such as DATs and CDs.
Ditto for the LSBs in an 8-bit image or 24-bit color image. I've since
seen this idea reinvented _several_ times on sci.crypt and
elsewhere...and I'm willing to bet I wasn't the first, either (so I
don't claim any credit).

A 2-hour DAT contains about 10 Gbits (2 hours x 3600 sec/hr x 2
channels x 16 bits/sample x 44K samples/sec), or about 1.2 Gbytes. A CD
contains about half this, i.e., about 700 Mbytes. The LSB of a DAT is
1/16th of the 1.2 Gbytes, or 80 Mbytes. This is a _lot_ of storage!

A home-recorded DAT--and I use a Sony D-3 DAT Walkman to make
tapes--has so much noise down at the LSB level--noise from the A/D and
D/A converters, noise from the microphones (if any), etc.--that the
bits are essentially random at this level. (This is a subtle, but
important, point: a factory recorded DAT or CD will have predetermined
bits at all levels, i.e., the authorities could in principle spot any
modifications. But home-recorded, or dubbed, DATs will of course not
be subject to this kind of analysis.) Some care might be taken to
ensure that the statistical properties of the signal bits resemble
what would be expected with "noise" bits, but this will be a minor

Adobe Photoshop can be used to easily place message bits in the
"noise" that dominates things down at the LSB level. The resulting GIF
can then be posted to UseNet or e-mailed. Ditto for sound samples,
using the ideas I just described (but typically requiring sound
sampling boards, etc.). I've done some experiments along these lines.

This doesn't mean our problems are solved, of course. Exchanging tapes
is cumbersome and vulnerable to stings. But it does help to point out
the utter futility of trying to stop the flow of bits.


Timothy C. May         | Crypto Anarchy: encryption, digital money,  
[email protected]       | anonymous networks, digital pseudonyms, zero
408-688-5409           | knowledge, reputations, information markets, 
W.A.S.T.E.: Aptos, CA  | black markets, collapse of governments.
Higher Power: 2^756839 | PGP 2.0 and MailSafe keys by arrangement.