[extropy-chat] Re: codes in scam letters

Emlyn emlynoregan at gmail.com
Wed Sep 28 02:42:23 UTC 2005


> My method does not necessarily detect changed headers or corrupted
> images.  I am talking about any added bits in the picture that would
> normally get smoothed out by the compression method but didn't.  I.E.,
> they add the message after the image is compressed.  Adding it before
> wouldn't work because the message might get optimized out.  This is the
> key to my method.  I can detect optimization changes between what was
> posted and what the original graphics program and compression should
> have produced.  Bits that should have been optimized out but weren't
> obviously were added in after the optimization.  This is what I am
> detecting with this method.
>
> --
> Harvey Newstrom <HarveyNewstrom.com>
> CISSP CISA CISM CIFI NSA-IAM GSEC ISSAP ISSMP ISSPCS IBMCP

This assumes that a compressed image I produced by program A will be
bitwise identical to Compress(Uncompress(I)) using the same program.
It isn't clear to me that with lossy compression, you can safely say I
== Compress(Uncompress(I)). DTP professionals will agree I think.

I just tried this with a commercial program. I opened a bitmap image
and saved it as a jpeg. I closed the program, then reopened the
program. I opened the jpeg, did nothing to it, saved it as a new file,
with exactly the same compression settings (which had stayed as
defaults, so in effect I touched nothing).

Then I compared the two images. The windows program "comp" told me to
go away because they were different sizes. It turns out that they were
24.8KB and 24.7KB respectively (I didn't look any closer than the file
properties dialog).

So in this case, I != Compress(Uncompress(I)).

--
Emlyn

http://emlynoregan.com   * blogs * music * software *



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