ctf-crypto
ctf-crypto is a cryptography reference skill for capture-the-flag competitions covering attack techniques against symmetric ciphers (AES, DES, stream ciphers), asymmetric systems (RSA, ECC), hashing (MD5, SHA), authentication (HMAC, padding oracles), and mathematical weaknesses (Coppersmith, Wiener, lattice attacks). Use it when solving CTF challenges involving encryption, digital signatures, key derivation, or exploiting cipher implementation flaws.
git clone --depth 1 https://github.com/ljagiello/ctf-skills /tmp/ctf-crypto && cp -r /tmp/ctf-crypto/ctf-crypto ~/.claude/skills/ctf-cryptoSKILL.md
# CTF Cryptography Quick reference for crypto CTF challenges. Each technique has a one-liner here; see supporting files for full details with code. ## Prerequisites **Python packages (all platforms):** ```bash pip install pycryptodome z3-solver sympy gmpy2 hashpumpy fpylll py_ecc ``` **Linux (apt):** ```bash apt install hashcat sagemath ``` **macOS (Homebrew):** ```bash brew install hashcat ``` **Manual install:** - SageMath — Linux: `apt install sagemath`, macOS: `brew install --cask sage` - RsaCtfTool — `git clone https://github.com/RsaCtfTool/RsaCtfTool` (automated RSA attacks) > **Note:** `gmpy2` requires libgmp — Linux: `apt install libgmp-dev`, macOS: `brew install gmp`. ## Additional Resources - [classic-ciphers.md](classic-ciphers.md) - Classic ciphers: Vigenere (+ Kasiski examination), Atbash, substitution wheels, XOR variants (+ multi-byte frequency analysis), deterministic OTP, cascade XOR, book cipher, OTP key reuse / many-time pad, variable-length homophonic substitution, grid permutation cipher keyspace reduction, image-based Caesar shift ciphers, XOR key recovery via file format headers - [modern-ciphers.md](modern-ciphers.md) - Modern cipher attacks: AES (CFB-8, ECB leakage), CBC-MAC/OFB-MAC, padding oracle, S-box collisions, GF(2) elimination, LCG partial output recovery, affine cipher over composite modulus, AES-GCM with derived keys, AES-GCM nonce reuse (forbidden attack), Ascon-like reduced-round differential cryptanalysis, custom linear MAC forgery, CBC padding oracle (full block decryption), Bleichenbacher RSA PKCS#1 v1.5 padding oracle (ROBOT), birthday attack / meet-in-the-middle, CRC32 collision signature forgery, AES key recovery via byte-by-byte zeroing oracle, AES-CBC ciphertext forging via error-message decryption oracle - [modern-ciphers-2.md](modern-ciphers-2.md) - Modern cipher attacks (Part 2): Blum-Goldwasser bit-extension oracle, hash length extension, compression oracle (CRIME-style), hash function time reversal via cycle detection, OFB mode invertible RNG backward decryption, weak key derivation via public key hash XOR, HMAC-CRC linearity attack, DES weak keys in OFB mode, SRP protocol bypass, modified AES S-Box brute-force, square attack on reduced-round AES, AES-ECB byte-at-a-time chosen plaintext, AES-ECB cut-and-paste block manipulation, AES-CBC IV bit-flip auth bypass, Rabin LSB parity oracle, PBKDF2 pre-hash bypass, MD5 multi-collision via fastcol - [modern-ciphers-3.md](modern-ciphers-3.md) - Modern cipher attacks (Part 3): custom hash state reversal, CRC32 brute-force for small payloads, noisy RSA LSB oracle error correction, sponge hash MITM collision, CBC IV forgery + block truncation, padding oracle to CBC bitflip RCE, SPN S-box intersection attack, AES-CFB IV recovery from timestamp-seeded PRNG, three-round XOR protocol key cancellation, AES-CBC UnicodeDecodeError side-channel oracle, SHA-256 basis attack for XOR-aggregate hash bypass, custom MAC forgery via XOR block cancellation, HMAC key recovery via XOR+addition arithmetic - [stream-ciphers.md](stream-ciphers.md) - Stream cipher attacks: LFSR (Berlekamp-Massey, correlation attack, known-plaintext, Galois vs Fibonacci, Galois tap recovery via autocorrelation), RC4 second-byte bias, XOR consecutive byte correlation - [rsa-attacks.md](rsa-attacks.md) - RSA attacks: small e (cube root), common modulus, Wiener's, Pollard's p-1, Hastad's broadcast, Hastad with linear padding (Coppersmith), Franklin-Reiter related message (e=3), Coppersmith linearly-related primes, Fermat/consecutive primes, multi-prime, restricted-digit, Coppersmith structured primes, Manger oracle, polynomial hash - [rsa-attacks-2.md](rsa-attacks-2.md) - RSA attacks (specialized): RSA p=q validation bypass, cube root CRT gcd(e,phi)>1, factoring from phi(n) multiple, multiplicative homomorphism signature forgery, weak keygen via base representation, RSA with gcd(e,phi)>1 exponent reduction, batch GCD shared prime factoring, partial key recovery from dp/dq/qinv, RSA-CRT fault attack, homomorphic decryption oracle bypass, small prime CRT decomposition, Montgomery reduction timing attack, Bleichenbacher low-exponent signature forgery, RSA signature bypass with e=1 and crafted modulus - [ecc-attacks.md](ecc-attacks.md) - ECC attacks: small subgroup, invalid curve, Smart's attack (anomalous, with Sage code), fault injection, clock group DLP, Pohlig-Hellman, ECDSA nonce reuse, Ed25519 torsion side channel, DSA nonce reuse, DSA key recovery via MD5 collision on k-generation - [zkp-and-advanced.md](zkp-and-advanced.md) - ZKP/graph 3-coloring, Z3 solver guide, garbled circuits, Shamir SSS, bigram constraint solving, race conditions, Groth16 broken setup, DV-SNARG forgery, KZG pairing oracle for permutation recovery, Shamir SSS reused polynomial coefficients - [prng.md](prng.md) - PRNG attacks (foundational): MT19937, MT float recovery via GF(2) magic matrix for token prediction, LCG, GF(2) matrix PRNG, V8 XorShift128+ Math.random state recovery via Z3, middle-square, deterministic RNG hill climbing, random-mode oracle, time-based seeds, C srand/rand synchronization via ctypes, password cracking, logistic map chaotic PRNG - [prng-attacks.md](prng-attacks.md) - PRNG attacks (CTF-era, 2017+): MT subset-sum seed recovery, MT19937 constraint propagation, Rule 86 cellular automaton reversal via Z3, Java LCG meet-in-the-middle partial modulo, LCG backward stepping via modular inverse, LFSR bit-fold ASCII parity, Z3 solve-time timing oracle, randcrack DSA k prediction, format-string PRNG seed offset, NTP-poisoned PRNG UUID XOR - [historical.md](historical.md) - Historical ciphers (Lorenz SZ40/42, book cipher implementation) - [advanced-math.md](advanced-math.md) - Advanced mathematical attacks (isogenies, Pohlig-Hellman, baby-step giant-step (BSGS) for general DLP, LLL, Merkle-Hellman knapsack via LLL, Coppersmith, quaternion RSA, GF(2)[x] CRT, S-box collision code, LWE lattice CVP attack, affine cipher over non-prime modulus, intr
Provides AI and machine learning techniques for CTF challenges. Use when attacking ML models, crafting adversarial examples, performing model extraction, prompt injection, membership inference, training data poisoning, fine-tuning manipulation, neural network analysis, LoRA adapter exploitation, LLM jailbreaking, or solving AI-related puzzles.
Provides digital forensics and signal analysis techniques for CTF challenges. Use when analyzing disk images, memory dumps, event logs, network captures, cryptocurrency transactions, steganography, PDF analysis, Windows registry, Volatility, PCAP, Docker images, coredumps, side-channel power traces, DTMF audio spectrograms, packet timing analysis, CD audio disc images, or recovering deleted files and credentials.
Provides malware analysis and network traffic techniques for CTF challenges. Use when analyzing obfuscated scripts, malicious packages, custom crypto protocols, C2 traffic, PE/.NET binaries, RC4/AES encrypted communications, YARA rules, shellcode analysis, memory forensics for malware (Volatility malfind, process injection detection), anti-analysis techniques (VM/sandbox detection, timing evasion, API hashing, process injection, environment checks), or extracting malware configurations and indicators of compromise.
Provides miscellaneous CTF challenge techniques for problems that do not cleanly fit the main categories. Use for encoding puzzles, pyjails, bash jails, RF/SDR, DNS oddities, unicode tricks, esoteric languages, QR or audio puzzles, constraint solving, game theory, unusual sandbox escapes, and hybrid logic puzzles. Prefer a more specific skill first when the challenge is mainly web, pwn, reverse, forensics, malware, OSINT, or crypto. Treat this as the fallback skill for genuine cross-category or edge-case challenges, not the default starting point.
Provides open source intelligence techniques for CTF challenges. Use when gathering information from public sources, social media, geolocation, DNS records, username enumeration, reverse image search, Google dorking, Wayback Machine, Tor relays, FEC filings, or identifying unknown data like hashes and coordinates.
Provides binary exploitation techniques for CTF challenges. Use when you already have a vulnerable native target or service and need to turn memory corruption or low-level primitives into code execution or privilege escalation, such as buffer overflows, format strings, heap bugs, ROP, ret2libc, shellcode, kernel exploitation, seccomp bypass, sandbox escape, or Windows/Linux exploit chains. Do not use it when the main blocker is understanding what the binary does; use reverse engineering first. Do not use it for pure web bugs, disk or packet forensics, or standalone crypto/math challenges.
Provides reverse engineering techniques for CTF challenges. Use when the main job is to understand how a compiled, obfuscated, packed, or virtualized target works before exploiting or solving it, including binaries, APKs, WASM, firmware, custom VMs, bytecode, game clients, malware-like loaders, and anti-debug or anti-analysis logic. Do not use it when the vulnerability is already understood and the remaining task is exploitation; use pwn instead. Do not use it for pure web workflows, log or disk forensics, or standalone crypto problems unless reversing the implementation is the real blocker.
Provides web exploitation techniques for CTF challenges. Use when the target is primarily an HTTP application, API, browser client, template engine, identity flow, or smart-contract frontend/backend surface, including XSS, SQLi, SSTI, SSRF, XXE, JWT, auth bypass, file upload, request smuggling, OAuth/OIDC, SAML, prototype pollution, and similar web bugs. Do not use it for native binary memory corruption, reverse engineering of standalone executables, disk or memory forensics, or pure cryptanalysis unless the web flaw is still the main path to the flag.