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ctf-crypto skill

by ljagiello·ljagiello/ctf-skills·3.4k stars·MIT

Provides cryptography attack techniques for CTF challenges. Use when attacking encryption, hashing, signatures, ZKP, PRNG, or mathematical crypto problems involving RSA, AES, ECC, lattices, LWE, CVP, number theory, Coppersmith, Pollard, Wiener, padding oracle, GCM, key derivation, or stream/block cipher weaknesses.

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Install the ctf-crypto skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description. 

git clone --depth 1 https://github.com/ljagiello/ctf-skills.git /tmp/ctf-skills
mkdir -p ~/.claude/skills
cp -r /tmp/ctf-skills/ctf-crypto ~/.claude/skills/ctf-crypto
available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

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):

pip install pycryptodome z3-solver sympy gmpy2 hashpumpy fpylll py_ecc
# Coppersmith (optional): pip install coppersmith
# Alternative Coppersmith lib: git clone https://github.com/jvdsn/crypto-attacks ~/.ctf-tools/crypto-attacks && pip install -r ~/.ctf-tools/crypto-attacks/requirements.txt

Linux (apt):

apt install hashcat

macOS (Homebrew):

brew install hashcat

Manual install:

  • SageMath (optional — only for legacy Sage fallback snippets (collapsed sections)) — Linux: apt install sagemath, macOS: brew install --cask sage
  • RsaCtfTool — git clone https://github.com/RsaCtfTool/RsaCtfTool (automated RSA attacks)
  • crypto-attacks (Coppersmith) — git clone https://github.com/jvdsn/crypto-attacks ~/.ctf-tools/crypto-attacks + pip install -r ~/.ctf-tools/crypto-attacks/requirements.txt (alternative to pip install coppersmith)

Note: gmpy2 requires libgmp — Linux: apt install libgmp-dev, macOS: brew install gmp.

Additional Resources

  • 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 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 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 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
  • modern-ciphers-4.md - Modern cipher attacks (Part 4): ChaCha20-Poly1305 nonce reuse forbidden attack over $2^{130}-5$ (RFC 8439, CTR $C1\oplus C2=P1\oplus P2$, Poly1305 $\sum c_i r^{n-i}$ via galois/sympy, 2-msg $ad=""$ vectors), partitioning-oracle / key-committing AEAD splitting lattice, sponge generality (SHA-3/Keccak $0x06$ vs $0x01$, Ascon/Gimli/Sparkle rate/capacity/rounds/pad table + endianness workflow), eSTREAM Trivium 1152-round warmup & cube attack outline (Grain)
  • 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: 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 (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: 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, X25519 low-order points + all-zero check
  • dh-attacks.md - Classic finite-field DH: trivial g (0/1/p-1), Pohlig-Hellman when p-1 smooth, small-subgroup confinement / Lim-Lee static key recovery via CRT, static vs ephemeral + Logjam downgrade triage
  • 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 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

When to Pivot

  • If the real blocker is understanding a binary, obfuscated client, or weird VM, switch to /ctf-reverse.
  • If the challenge is mostly packet carving, disk recovery, or stego extraction before any decryption starts, switch to /ctf-forensics.
  • If the task is just implementing an exploit against a vulnerable network service after the crypto part is solved, switch to /ctf-pwn or /ctf-web.
  • If the crypto challenge involves adversarial ML, model extraction, or neural-network-based ciphers, switch to /ctf-ai-ml.
  • If the challenge is really an encoding puzzle, esoteric cipher, or polyglot trick rather than true cryptanalysis, switch to /ctf-misc.

Quick Start Commands

# Identify cipher type
python3 -c "from Crypto.Util.number import *; n=<N>; print(f'bits={n.bit_length()}')"

# RSA quick check
python3 -c "from sympy import factorint; print(factorint(<n>))"  # Small factors?
openssl rsa -pubin -in key.pub -text -noout  # Extract n, e from PEM

# Quick factorization tools
python3 RsaCtfTool.py -n <n> -e <e> --uncipher <c>

# XOR analysis
python3 -c "from pwn import xor; print(xor(bytes.fromhex('<hex>'), b'flag{'))"

# Hash identification
hashid '<hash>'
hashcat --identify '<hash>'

# Quick factorization (sympy, primary)
python3 -c "from sympy import factorint; print(factorint(<n>))"
# Sage alternative: sage -c "print(factor(<n>))"

Classic Ciphers

  • Caesar: Frequency analysis or brute force 26 keys
  • Vigenere: Known plaintext attack with flag format prefix; derive key from (ct - pt) mod 26. Kasiski examination for unknown key length (GCD of repeated sequence distances)
  • Atbash: A<->Z substitution; look for "Abashed" hints in challenge name
  • Substitution wheel: Brute force all rotations of inner/outer alphabet mapping
  • Multi-byte XOR: Split ciphertext by key position, frequency-analyze each column independently; score by English letter frequency (space = 0x20)
  • Cascade XOR: Brute force first byte (256 attempts), rest follows deterministically
  • XOR rotation (power-of-2): Even/odd bits never mix; only 4 candidate states
  • Weak XOR verification: Single-byte XOR check has 1/256 pass rate; brute force with enough budget
  • Deterministic OTP: Known-plaintext XOR to recover keystream; match load-balanced backends
  • OTP key reuse (many-time pad): C1 XOR C2 XOR knownP = unknownP; crib dragging when no plaintext known
  • Homophonic (variable-length): Multi-character ciphertext groups map to single plaintext chars. Find n-grams with identical sub-n-gram frequencies, replace with symbols, solve as monoalphabetic. See classic-ciphers.md.
  • Grid permutation cipher: 5x5 grid with independent row/column permutations collapses keyspace to 5! x 5! = 14,400; brute-force in milliseconds. See classic-ciphers.md.

See classic-ciphers.md for full code examples.

Modern Cipher Attacks

  • AES-ECB: Block shuffling, byte-at-a-time chosen-plaintext suffix recovery (256 queries per byte, tool: FeatherDuster ecbcpadecrypt); image ECB preserves visual patterns. ECB cut-and-paste: splice ciphertext blocks to forge JSON fields (e.g., is_admin: true). See modern-ciphers-2.md.
  • AES-CBC: Bit flipping to change plaintext; padding oracle for decryption without key. IV bit-flip: flip specific bits in the IV to change first plaintext block (requires no MAC). See modern-ciphers-2.md.
  • CBC IV forgery + block truncation: XOR IV bytes to change decrypted block 0; strip trailing ciphertext blocks (no length integrity in CBC). Forges authenticated tokens when MAC is embedded in the ciphertext. See modern-ciphers-3.md.
  • Padding oracle to CBC bitflip RCE: Chain padding oracle (recover plaintext) with CBC bitflipping (inject shell metacharacters) for command injection via encrypted parameters. See modern-ciphers-3.md.
  • AES-CFB-8: Static IV with 8-bit feedback allows state reconstruction after 16 known bytes
  • CBC-MAC/OFB-MAC: XOR keystream for signature forgery: newsig = oldsig XOR block_diff
  • S-box collisions: Non-permutation S-box (len(set(sbox)) < 256) enables 4,097-query key recovery
  • GF(2) elimination: Linear hash functions (XOR + rotations) solved via Gaussian elimination over GF(2)
  • Padding oracle: Byte-by-byte decryption by modifying previous block and testing padding validity
  • LFSR stream ciphers: Berlekamp-Massey recovers feedback polynomial from 2L keystream bits; correlation attack breaks combined generators with biased combining functions
  • Galois LFSR tap recovery: XOR known file header (PNG/PDF/ZIP) with ciphertext to get keystream; split into N-bit windows, compute (state >> 1) XOR next_state for LSB=1 transitions to directly recover tap mask. Autocorrelation sliding finds correct length. See stream-ciphers.md.
  • OFB with invertible RNG: Known plaintext in any block leaks RNG state; if state transition is bijective, run RNG backwards to decrypt all blocks. See modern-ciphers-2.md.

More skills from ljagiello/ctf-skills

  • Fctf-ai-mlProvides 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.
  • Cctf-forensicsProvides 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.
  • Actf-malwareProvides 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.
  • Fctf-miscProvides 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.
  • Actf-osintProvides 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.
  • Fctf-pwnProvides 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.
  • Actf-reverseProvides 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.
  • Fctf-webProvides 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.
  • Actf-writeupGenerates a single standardized submission-style CTF writeup for competition handoff and organizer review. Use after solving a CTF challenge to document the solution steps, tools used, and lessons learned in a structured format.
  • Asolve-challengeSolves CTF challenges by performing first-pass triage, identifying the dominant category, and routing execution to the right specialized ctf-* skill. Use when the user gives you a challenge bundle, a remote service, a suspicious file, or only a vague challenge description and you must determine where to start. Do not use it when the category is already clear and a specialized skill can be invoked directly; this is the dispatcher and recon entrypoint, not the deepest reference for category-specific techniques.

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