Bellcore Fault Attack

Third post in the Anti-Slop CTF 2026 series. The web writeup covered HTTP parsers. The reverse writeup covered an ECDSA nonce attack and a SHA-256 length extension. This one walks the three pwn challenges in the same step-by-step format. The order below is roughly easiest to hardest. Paper Lantern is a clean single-chain CRT-fault attack against an RSA-FDH signer. Graceful Exit composes a negative-offset leak with a heap-object overwrite to convert an address disclosure into a controlled read through the legitimate output path. Anchorpoint is the marathon: a tiny stack-VM overflow unlocks ECDSA nonce recovery, a BIP340-style shadow proof, and an AES-GCM nonce-reuse GHASH forge, all chained into one connection. All three rewarded reading the binary and modelling the state machine before writing any exploit code. ...

Platform DalCTF 2026 (dalctf2026.com) Difficulty Mixed (Easy → Medium) OS Jeopardy — Crypto, Reverse Engineering, Web, Android Tags RSA modulus with small prime factor recovered by trial division, Bellcore CRT fault attack as the verification path, Playfair decryption against an un-keyed alphabet square, Huffman tree decode with inverted tiebreaker convention, LCG state recovery from one known plaintext byte, IEEE-754 bit-pattern reinterpretation via Quake-style float pointer cast, UPX-packed ELF unpacked into 44 per-byte check functions, Android APK static-string mining across MainActivity + strings.xml + ColorKt, HTML hidden attribute as a flag-hiding sink DalCTF 2026 is the DalCTF Jeopardy event with challenges spread across Crypto, Reverse Engineering, Web, and Android. The 2026 edition leans heavily into classical cryptography mistakes wrapped in misdirection — six of the nine challenges are crypto, and almost every one of them tries to push you toward a harder attack than the one that actually works. The flag format is dalctf{...} (occasionally DalCTF{...}), and the challenge names are explicit hints once you’ve solved them. ...