GPN CTF 2026 — Organized: Ternary Amplitude-Modulated UART in Popcount Density
Platform GPN CTF 2026 (kitctf) Difficulty Medium OS Misc — signal recovery from high-entropy file, ternary amplitude modulation Tags rejecting the 'random noise' default hypothesis, computing per-window popcount means and run-lengths, plotting a 200-bin histogram to spot three peaks instead of two, recognising the structure as UART (start bit, 8 data bits LSB-first, stop bit) framed by a mid-amplitude idle marker, decoding 49 frames to ASCII Organized is the GPN CTF 2026 misc challenge whose entire trick is recognising the carrier’s organization. The handout is a 7,650,000-byte file that looks like noise — file(1) calls it data, every bit position is 1 with probability ≈ 0.287. The “organization” is hidden in the bit-density of windows, not in the bytes themselves: per-12,500-byte window popcount falls into one of three sharp levels, giving a 612-trit string. Past a 24-trit preamble, the rest is 49 UART-style frames of 12 trits each. Decode → ASCII → flag: ...