Use the Mandelbrot function. It's a very simple function that can be implemented on the clientside, is essentially (though not exactly) one-way, but is easy reversible so that you can have a list of mappings for encryption. It maps (X,Y) coordinates into values in the range 0-N (where N is normally 255).

This is, essentially, a variation on Limbic~Region's shared-book approach. You can then vary a number of things:

• Order of coordinates sent. You could send messages in a NxM character grid and send them in col-row order (vs. the normal row-col) order.
• You can send every Nth coordinate as (Y,X) vs. (X,Y).
• You can vary the translation of the result of the Mandelbrot from an entry into the ASCII table to an entry into your own table.
• You can vary the Mandelbrot from a quadratic to a cubic (or quartic) equation. I did this in a course on fractals and instead of the normal Mandelbrot picture, I ended up with a cool picture of two wizards fighting each other. :-)