# Using ROT13 to circumvent Gmail’s .exe filter

Wiebe reminded me by mail to send him a Windows executable. I use Gmail. Gmail doesn’t allow me to send archives containing .exe files. Something to do with viruses on Windows. Wiebe doesn’t use Windows. He uses Wine. I just want to send the damn executable.

Last time I wanted to do this, I emerged net-mail/email. But that just wasn’t very cool. A while ago someone also suggested using a password-protected zip file, but I find that even less cool because I hate zip files. (I usually prefer bzipped tarballs.)

ROT13 is a variation of ROT3, the Ceasar Cipher, which is as old as it is insecure. For computer use ROT13 is even cooler, because you can decrypt just as you encrypt. Decrypt and encrypt are the same. Let’s grab the simplestshortest C implementation we can find, rot13-shortest.c:

main(a){while(a=~getchar())putchar(~a-1/(~(a|32)/13*2-11)*13);}

Compile it and use it as a filter to encode a file:

rot13-shortest < suspicious.tar.bz2 > suspicious.tar.bz2.r13

To decode (to “decrypt” is too grand a verb), just swap the input and output files:

rot13-shortest < suspicious.tar.bz2.r13 > suspicious.tar.bz2

Now, it’s time for me to hand in my Geek Card. Firstly, I confused ROT13 with ROT3 (even spending some time trying to find a ROT3 implementation). Secondly, I had never used ROT13 before, not even to by-pass a forum filter or to make a joke which is funnier if you wear a pen-protector. EBG13 wbxrf whfg nera’g shaal.