Now that I think about it, one wouldn't have to sacrifice any bandwidth whatsoever! As, the stego program could be made to do wrap-around encoding.

Indeed, that's what I implemented. The current man page is appended. --- Jef pnmstego(1) USER COMMANDS pnmstego(1) NAME pnmstego - insert a stegonographic message into a portable anymap SYNOPSIS pnmstego [-offset n] [-components rgb] [-bitsched 10010...] file [pnmfile] DESCRIPTION Reads a portable anymap as input. Hides another file within the anymap by changing the low-order bits of each pixel, and writes the modified anymap to standard output. Stegonography means "hidden writing" - it's for storing a message in such a way that an opponent won't even be sure that the message is there, let alone be able to extract, decrypt, and read it. Pnmstego lets you store a file in the low-order bits of a PPM or PGM image. For a typical maxval of 256, this means the pixel values will be changed by only 1/256, which should not be noticable to someone viewing the image. Only PGM and PPM images may be used - trying to hide a file in a PBM image would be easily visible, so it's not allowed. OPTIONS The program has a bunch of flags that let you specify varia- tions on exactly how the bits are hidden in the image. If you don't specify any flags, the default options give you a reasonable storage method, to wit: -offset 0 -components b -bitsched 1 Letting the options default like this has the advantage that you can send a stegoed file to someone without having to specify what flags you used. You can also put the file on a public FTP server for people to retrieve, without needing any other communication. However, there is a disadvantage: people who the file is not intended for will be able to extract it too. If the resulting file is recognizable in some way, for instance if it's encrypted by PGP, then some of the purpose of the stegonographic storage will have been compromised. (A PGP file has a standard header and stores, in cleartext, the keyid needed to decrypt a file.) So, if you want the file to be absolutely positively hidden, you can use non-default settings for the options. Just don't forget what settings you used, and remember that if you want to send the file to someone else you will have to communicate the necessary settings somehow. Now, as for what the options do: -offset Specifies how far into the image to start. If the file runs off the end of the image, it wraps around back to the beginning, so don't worry about specifying too large an offset. In fact, you can even specify an offset larger than the image size - the initial value gets wrapped around too (modulus). Basically, any 32-bit number can be used. -components This lets you specify which of the red, green, or blue color components is used for storing bits. You give it a string made up from one or more of the letters r, g, and b. The default is b, use only the blue component. See the section below on quantization for why the default isn't rgb. If the input image is a PGM file, instead of PPM, this option is silently ignored. -bitsched This lets you skip using some possible bit posi- tions in the image. If the file you're hiding is not so big that you need the whole image to store it, you can use this option to hide it better. The argument is just an arbitrary- length string of 1's and 0's - the 1's say which bit positions should be used, and the 0's which should be skipped. The default is to use all bit positions. QUANTIZATION Normally, to store an image in a colormapped file format such as GIF, you first run it through ppmquant. However, if you try to quantize, then stego, and then convert to GIF, you will find that the ppmquant step will fail with an error about too many colors. This is because the stego step increased the number of colors. You might think, ok, stego first, then quantize; nope, the quantization would destroy the stegonographic message. Here's what you do instead: quantize down to half the number of colors you want in the final file, and then use only one of the three color com- ponents to store the hidden file. Using only one bit per pixel, the number of colors can at most double. This is why the default for the -components flag is to use only blue. So, the pipeline to use is: ppmquant 128 image.ppm | pnmstego message.txt | ppmtogif > hidden.ppm SEE ALSO pnmdestego(1), pnm(5) AUTHOR Copyright (C) 1994 by Jef Poskanzer.