Been tried before?

[Andrew Wiggin]

I had an idea for a key system using graphics files. Has that been tried 
before? The basic idea was to use a graphics file with a color depth of 
24-bit or higher. Then, using the rgb values of, say, every prime pixel 
(i.e. the first pixel, then the second....then the seventh, then the 
eleventh) or something like that to create a stream of numbers the feed 
into the cipher which would be highly erratic. A similar looking picture 
would not work, as the rgb values would be (I assume) hard to guess, and 
even if you came close, you would have to get the right order. If you 
used a graphics file format similar to PNG, for instance, you could even 
include alpha channels.
    Am I just shooting my mouth off, or does this sound like something 
that would work? I'm wondering if this has been tried before and if it 
would be worth programming out.

--Drew

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