Graphic encryption

[Bill Stewart]

At 08:17 AM 10/6/95 -0700, privsoft at ix.netcom.com (Steve ) wrote:
>Also if any of ya'll are famillar with graphic encryption, I am looking 
>for opinions as to its strengths / weaknesses.

While normally I'd agree with Lucky Green and say that a good encryption
algorithm's strength is independent of its input data, there are exceptions.

The description of Privasoft's method on their web page sounded like* it
broke up a fax image into little squares and shuffled them around based on
some proprietary (hence presumably weak) algorithm, somewhat like feeding
them to a shredder or puzzle-box, then put them in a file you can ship
with regular fax software or decrypt yourself.  If that's the case,
and if the squares are sufficiently big to have enough pixels for fax
software to compress decently, then it's weak no matter _how_ strong
the encryption algorithm that shuffles them is, because you can piece together
matching edges like a jigsaw puzzle, or like a bunch of Iranian students
with American Embassy shredded documents.  Using computers is a lot easier,
since you've got precise images of print - most edges will match pretty well
with the edges of the adjacent squares, and you can jigsaw the parts that
still need work if you get too many multiple matches.  

A much stronger way to encrypt faxes is to do the Group 3 (or G4) compression
first, then encrypt using a real algorithm, ship the (now-useless-for-fax)
encrypted bits, decrypt, and read with your favorite fax-image viewer.
Or, of course, take the raw file you would have done this with, which is much
smaller than the fax-compressed image, encrypt it, ship, decrypt, and view,
but that means the person on the far end needs the same word-processor.


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* The software was on the web page; if I'd had spare disk space that day and
some slack time I could have downloaded the demo version and tried it,
but this is what the description sounded like it meant.
#---
#                                       Thanks;  Bill
# Bill Stewart, Freelance Information Architect, stewarts at ix.netcom.com
# Phone +1-510-247-0664 Pager/Voicemail 1-408-787-1281
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