cheat

spbctf

Task: crack a license/activation check in a not-stripped x86-64 ELF; key form xxxx-xxxx-xxxx-xxxx, 4 checks each XOR a scaled sum against a constant. Solution: static-analyze the four check functions, invert the additive/multiplicative arithmetic in Python (modular inverse, plus a 2-adic congruence for the even multiplier), and account for main negating three of the four arguments.