Insomni'hack GPGPU reversing

One of the reversing challenges was exotic: we were given a ciphertext (ohv'c~f3ehnw4byzzky), a GPGPU file (kernel.bin) of the encryption routine, and the Instruction Set Architecture (ISA) of this GPU.

Since I have never played with GPGPUs (code/assembly/whatever), it seemed hard at first glance. But actually not! Open kernel.bin and see that there's some kind of disassembly. Good! And pretty small, roughly 80 lines of code.

I will explain how I reversed it and reimplemented it in Python to have the same encryption routine, discovered that it was also the decryption routine, and just run it against the ciphertext to obtain the plaintext.