Re: Steganography

[peter honeyman <honey@citi.umich.edu>]

>                                  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.

how many bits are we talking about here?  suppose it's two in sixteen.
7/8 of the compressible bits remain.  so if the normal compressibility
is 2:1, taking two out of sixteen bits would leave 1.75:1 compression.
is that a "notable" difference?

i haven't been paying close enough attention -- is two out of sixteen
a realistic amount?  it seems high to me.  if it's one out of sixteen,
the effect is only a 6.25% reduction in compression.  is that notable?

	peter