Cryptography

BroncoCTF 2026 (bronco flag prefix, hosted by Cal Poly Pomona’s Cyber Security Club) shipped a web track built entirely around one lesson repeated four different ways: client-side checks are not access control. Forbidden Archives blows past a naive keyword-stripping WAF by using the app’s own is_secret column against it. Lovely Login puts the entire authentication story in robots.txt (a disallowed /security page + a base64 user list in a comment). Super Secure Server exposes plaintext credentials at /api/config for a client-side compare, and separately its /login endpoint blindly trusts a client-supplied {"authenticated": true} JSON body. Unblur Me implements a “solve 500 derivatives to unlock the image” gate as pure CSS filter: blur(20px) on an <img> that already has the sharp bytes. The crypto track pairs a repeat-encrypt XOR OTP whose 64-character key alphabet leaks structure into every sample (Probably Unbreakable) with a command dispatcher whose MD5-hash-vs-string mismatch inside a program recursion turns a classic 128-byte collision pair into an unlock primitive (Blorg Multiplier). ...

Junior.Crypt 2026 (grodno flag prefix) shipped a web track with two well-designed misuse challenges and a crypto track built almost entirely around a single Portal / Aperture Science theme where every flag payload spells out its own vulnerability class in leetspeak: “still alive but GLaDOS keeps rewriting messages” is Franklin-Reiter over e = 3; “this was a triumph but the seed was too small” is a 20-bit truncated LCG; “companion cube this is strictly a many-time pad” is a template-driven multiset-match recovery; “neurotoxin diagnostics leak through timing alone” is an offline Vaudenay CBC padding-oracle attack replayed from a recorded 98,304-record timing trace. The web track’s Pulse challenge lets you inject a shell command through a validator that only checks the first line of a batch input, while Wrodle ships an HS256 game-state JWT whose secret is butterfly from any modern wordlist. ...

TraceBash CTF 2026 is a Jeopardy-style CTF with a clean, well-curated challenge set. The crypto track has four challenges, three at 100 points and one at 440 points. This writeup covers all four step-by-step. Each challenge is a different shape of cryptographic mistake. state-desync hides a 16-bit-seed stream cipher behind a noisy update function. broken-trust-protocol is a textbook Diffie-Hellman implementation that forgets to validate the peer public value. harmonic-cipher hides an 8-byte XOR key inside an audio file. quantum-echo ships two RSA-1024 public keys that share a 512-bit prime, where a single gcd factors both moduli in milliseconds. None of them requires sage, lattice work, or anything more exotic than a careful read and a small Python brute. All four exist in production code somewhere in the wild. ...

Fourth post in the Anti-Slop CTF 2026 series. The web writeup covered HTTP parsers. The reverse writeup covered a quadratic ECDSA nonce and a SHA-256 length extension. The pwn writeup covered a Bellcore CRT fault, a leak-then-overwrite, and a five-stage GCM forge chain. This one walks the two crypto challenges in the same step-by-step format. ...

Platform GPN CTF 2026 (kitctf) Difficulty Mixed (Easy → Hard) OS Jeopardy — Reverse, Crypto, Web, Pwn, Misc Tags AVX2 lane-swap miscompilation discovery + Kannan-embedding SIS lattice attack, NTRU mod-q reduction bug (c mod p == m), ECDSA nonce reuse from MD5(uuid3) collisions via fastcoll, eBPF signed-comparison verifier bypass with patched bzImage, JVM AOT cache override of bytecode, PHP 7.4 PHAR deserialization across two TCP races, Pydantic ForwardRef eval in create_model, CSS attribute-selector cookie exfiltration through Link: rel=stylesheet, holpy proof-checker thm re-axiomatization, knitout front/back-bed bitmap, ternary amplitude-modulated UART, Hamiltonian path on 250-node FSM extracted from jump tables, RFC 5424 syslog stream demux, Rust setuid TOCTOU symlink swap GPN CTF 2026 is the Gulaschprogrammiernacht CTF hosted annually by KITCTF at the GPN hacker camp in Karlsruhe, Germany. The 2026 edition runs a Jeopardy board across reverse engineering, crypto, web, pwn, and misc, with a sharp lean toward low-level systems bugs — a missing mod q in an NTRU implementation, a 4-way AVX2 lane-swap in a gcc -O3 -mavx2 build, a deleted BPF_ADJ_END_FROM_* check in a custom kernel, a JVM AOT cache that silently overrides a JAR method. The flavour throughout is kitchen — recipes, ovens, pots — and the flags universally read like Bavarian beer-tent slogans. ...

Platform BhAcKAri CTF 2026 Difficulty Mixed (Easy → Hard) OS Jeopardy — Web, Misc, Crypto, Reverse (Italian event) Tags JavaScript deobfuscation + AES-256-CBC cookie C2 + sed shell-glob bypass, lighttpd HTTP CONNECT tunneling past url.access-deny, patched d8 V8 sandbox eval escape via shop-trusted credits, seed-keyed LSB steganography with shuffle order, Minecraft 1.21.10 .mcfunction Vigenère with floor-mod, Coppersmith partial-prime small-roots via Howgrave-Graham lattice, deterministic Python C-extension stage chain with SHA-256/CRC32 key derivation, manual Windows PE loader with 4-byte patches into 7-Zip's GetHandlerProperty2 BhAcKAri CTF 2026 is an Italian-themed jeopardy event whose infrastructure lives on the .it TLD (challs.ctf.bhackari.it) and whose challenges drip with Venetian flavour — the name itself is a play on bacari, the small wine-and-cicchetti taverns of Venice. The 2026 edition runs eight challenges across four categories (Web, Misc, Crypto, Reverse) and rewards careful reading of source code, binary disassembly, and protocol logs in roughly equal measure. ...

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. ...

Platform BYUCTF 2026 Difficulty Mixed (Easy → Hard) OS Jeopardy (Crypto, Forensics, Reverse, Web) Tags Coppersmith small-roots, multi-prime RSA, Euler totient tower reduction, YARA constraint solving, NTP/ICMP/HTTP pcap forensics, Go reverse engineering, Python prototype pollution, CSP bypass BYUCTF 2026 is Brigham Young University’s annual capture-the-flag, hosted on the Cyber Jousting infrastructure (chals.cyberjousting.com). The 2026 edition runs fifteen challenges across four classic jeopardy categories — Crypto, Forensics, Reverse, and Web — with a heavy Portal-flavoured forensics arc starring GLaDOS, Wheatley, and the cake-is-a-lie ICMP/NTP/HTTP capture. ...

Platform Hackastra CTF 2026 Difficulty Mixed (Easy → Hard) OS Jeopardy (Web, Crypto, Reverse, Misc, Forensics) Tags JWT, RS256/HS256 confusion, DSA known-nonce, Coppersmith, Feistel inversion, ARM64/x86_64 RE, WASM RE, AWS Cognito, blind SQLi, XSS, LSB stego Hackastra CTF 2026 ran as a jeopardy-style competition on CTFtime (event #3270) with fifteen challenges spanning reverse engineering, web exploitation, cryptography, forensics, and miscellaneous infrastructure bugs. The event’s name plays on the Sanskrit word अस्त्र (astra, meaning “weapon” or “missile”), and the challenges live up to it — every flag in this set rewards a specific, named technique rather than rote tooling. ...