[591] in Cypherpunks
Re: Steganography
daemon@ATHENA.MIT.EDU (wcs@anchor.ho.att.com)
Tue May 25 22:12:48 1993
From: wcs@anchor.ho.att.com
To: cypherpunks@toad.com
Date: Tue, 25 May 93 21:34:35 EDT
Original-From: anchor.ho.att.com!wcs (Bill_Stewart(HOY002)1305)
Original-To: toad.com!cypherpunks
> Here is a pitfall to be avoided in Steganography using
> low bits of AD converter output. Such converters may be biased
> in their low bit. [...] Falling too close to 50% would be a clue that the data
That's not too risky for graphics, since many scanned pictures aren't
based on raw A/D converter output; they've been processed and squashed down
to some smaller number of bits. A more serious concern is compressibility -
a real image file is probably more compressible than a file with the
low-order bit replaced by a crypto-bit, since the real data has moderate
correlation and the crypto-bits are random. I doubt the Feds will immediately
start looking to see if you're shipping GIF files that have significantly
worse compression than average, but they'd probably find something if they did it.
Bill Stewart
