Doing a pnmstego isn't quite the right approach, since it's a special-purpose tool that advertises its presence. A more appropriate general-use tool would be something like pnmbitplane, which would let you do something like extract a specific bitplane from a ppm or pgm as a pbm, and a reverse tool which would let you substitute or xor a pbm into a pgm or ppm, with appropriate options for the ppm versions that let you do one color, all three, etc., and for both ppm and pgm to let you pick which bitplane. You'd also have to add something appropriate for padding, like a random-fill vs. 0- or 1-fill option, or else provide a tool that makes a string of bytes into a bitmap shaped like a given pnm. Some advantages - it's more general, there are other uses for it (such as adding titles to ppm pictures), it's not much more work to write, and only the NSA agents reading cypherpunks will know that's why it's there :-). A disadvantage, of course, is that potential users may not guess, unless people tell them things like "Oh, yeah, you could use ppmbitplane to do that, of course". This also lets you reserve the name ppmstego for translating between ppm and Stego Mac format. Bill # Bill Stewart AT&T Global Information Solutions, aka NCR Corp # 6870 Koll Center Parkway, Pleasanton CA, 94566 Phone 1-510-484-6204 fax-6399 # email bill.stewart@pleasantonca.ncr.com billstewart@attmail.com # ViaCrypt PGP Key IDs 384/C2AFCD 1024/9D6465