CTF Cryptography Quick Reference

Problem

CTF crypto challenges often block on recognizing the attack surface: an AES-looking service may expose nonce reuse, RSA may have small exponents, nearby primes, or partial key leakage, an unknown XOR blob may be recoverable from file magic headers, stream-cipher LFSR tasks may require Berlekamp-Massey, and lattice tasks need LLL/BKZ triage before deeper analysis. This skill organizes cryptography challenges by algorithm family as a quick reference for choosing the right attack path first.

How It Works

• Classic ciphers: start with Caesar, Vigenère, XOR, OTP key reuse, and file-header magic bytes to decide whether brute force, frequency analysis, or crib dragging applies.
• Modern ciphers: covers AES-ECB, AES-CBC, CFB, padding oracle, nonce reuse, CBC bit flipping, HMAC/MAC forgery, and SRP bypass, with focus on oracles, block boundaries, weak key derivation, and authentication structure flaws.
• RSA/ECC/lattices: maps common routes such as Wiener, Coppersmith, Hastad, partial-key recovery, fault attacks, invalid curves, ECDSA nonce reuse, and LLL/BKZ/LWE workflows.
• Supporting resources: the main text gives one-line conclusions, while companion files add code and details, such as rsa-attacks.md, modern-ciphers-2.md, and lattice-and-lwe.md.

Boundaries

It fits best when the cryptography part is already isolated, with ciphertext, protocol parameters, or implementation clues available. If the blocker is reversing, disk forensics, network exploit development, model extraction, or misc encoding puzzles, switch to the corresponding direction instead of treating the task as pure cryptanalysis.