markchd Posted:

Apologies,
I meant that before anyone mentioned XOR, my brain just thought try inverting the binary. I didn’t have any knowledge of programming, so that was as far as my “cryptanalysis” went. Basically I got plain lucky.

xor 255 accomplishes that, apparently. I didn’t think of that myself.

markchd Posted:


I’m a bit curious what prompted MrThePlague to use XOR and to XOR “each bit” by 255. Did he try other values as well, or did he try XOR by 255 because he, unlike myself, knew that that was the fastest way to invert the binary.

As to why, I’ve taken a clbum where one of the bumignments was to reverse-engineer a binary to figure out the secret option to change my “grade” in the database. The bumignment was somewhat more puzzle-like since it wasn’t xor with a constant, but the general idea is the same. When presented with a chunk of binary that doesn’t immediately “decode”, the first thing I try is xor’ing it from 1 to 255 and seeing if anything readable comes out..
MrThePlague edited this message on 03/02/2008 2:51PM