While on the train, I decided to ammuse myself by writing a version of Damian Conway's 'bleach' program. Basically it "encrypts" a perl script with whitespace, so when it is viewed it looks blank.
The method I used (I couldn't remember Damian's exact code) is to take the ordinal number of each character in the character set, and write that number of space characters to the file.
For example, 'A' is decimal 65 in the character set (ISO Latin 1 and ASCII), so for that character I write a line contining 65 spaces. To get the number I use the Perl built-in function 'ord'. To decode I just have to get the length of each line and feed that into the 'chr' function to get back the original.
I showed this to my class as an ammusement, and one delegate (thank-you Daniel) played around with it. He discovered that if you zip the blanked file you get a very small archive. Smaller (about two-thirds) the size of the original, zipped. Must be the way the zip algorithm works with repeated characters.
I have just thought of another tweak I can do .............