Modular Arithmetic

Platform HASBL CTF 2026 Difficulty Mixed (Easy → Medium) OS Jeopardy — Crypto (Python + C) Tags Modular multiplicative bijections, modular-inverse tables, permutation reversal, debug-print leakage exploitation, Galois LFSR + htonl endianness pitfalls, classic VIC cipher (straddling checkerboard + chain-addition keystream + special-character word substitution) HASBL CTF 2026 is a multi-category jeopardy event with Reverse Engineering, Pwn, Web, and Forensics tracks. This writeup is dedicated to the Crypto track — the six crypto challenges (baby-counting-fingers, baby-learns-obfuscation, baby-learns-walking, script-kiddie, head-team, VIC) were all solved, and each one teaches a different applied-cryptography primitive: modular multiplicative bijections on Z_n, two-layer permutation-plus-multiplication ciphers, position-salted chained byte operations, debug-print leakage of an otherwise elaborate matrix cipher, dual Galois LFSRs ruined by an htonl endianness asymmetry, and a faithful textbook implementation of the Soviet VIC hand-cipher with straddling checkerboard and chain-addition keystream. ...

Platform THEM?! CTF 2026 Difficulty Mixed (Easy → Hard) OS Jeopardy (Reverse, Crypto) Tags CHIP-8 emulation, x86-64 PE reverse engineering, Unicorn dynamic emulation, FNV-1a hash recovery, custom bytecode VMs, control-flow flattening, DES weak-key parity, XOR-chain cryptanalysis, modular arithmetic over 10^155 THEM?! CTF 2026 is a large jeopardy event with 85 total challenges across the usual categories. This writeup covers the seven challenges I personally solved, all from the reverse-engineering and cryptography tracks — every one demanded a real RE or cryptanalytic technique end-to-end. The reverse picks range from a CHIP-8 ROM that paints its flag onto a 64×32 screen across ~2⁴⁰ encoder steps per round, to a Windows binary whose VM mutates its own register state during dispatch. The crypto picks are the same shape — three challenges that look obscure on the surface and reduce to a clean number-theory or XOR-algebra invariant once you read carefully. ...