So, as promised, here's the story on how I cracked the trade code generator for Animal Crossing on the GameCube. Set the Wayback Machine to 2002/3 (I forget the exact year).
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This kicked off a 2-3 month period where I did little other than go to university, take my courses, get home, and work on painstakingly translating every function, instruction by instruction, into pseudo-C code.
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After having every function reversed, I converted it into proper C. It required a good bit of debugging - fortunately, I had Dolphin to check my results against on a per-function basis - but at the end of it all, I was able to generate arbitrary trade codes.
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It's now time for some info on the algorithm itself. It comprises many, many layers of obfuscation. A 21-byte buffer is assembled consisting of 8 bytes for the town, 8 bytes for the player, 2 item bytes, 2 info bytes, and a checksum byte.
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These 21 bytes are subjected to a transposition cipher, a substitution cipher, random bit shuffling, being shifted like a massive barrel shifter, a cut-down version of RSA encryption, and finally converted into 28 6-bit characters and printed on-screen.
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In summation, there was no way that this sort of algorithm could be brute-forced, simply due to the sheer number of transforms applied to the data. But it wasn't designed to withstand a university student with a surfeit of time and a working debugger. ~Fin
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