The Forged Signet

CACTF2026

Task: HTTP-fronted quantum oracle implementing a 2-to-1 function with a hidden 64-bit XOR period s (f(x)=f(x XOR s)); you choose the middle single-qubit layer. Solution: choose L=H to complete Simon's algorithm, collect samples y satisfying y·s=0 over GF(2), Gaussian-eliminate to a rank-63 system, and extract the unique nonzero null-space vector s.