Ctf crypto

CTF Cryptography

Quick reference for crypto CTF challenges. Each technique has a one-liner here; see supporting files for full details with code.

Additional Resources

• classic-ciphers.md — Vigenere/Kasiski, XOR variants, OTP reuse, homophonic
• modern-ciphers.md — AES/CBC/GCM, padding oracle, Bleichenbacher, LFSR, length extension
• modern-ciphers-2.md — 2024-26: S-box collision, AES-GCM derived, Ascon diff, linear MAC, FFT Shamir
• rsa-attacks.md — small e/d, common mod, Wiener, Hastad, Fermat, Coppersmith, polynomial-prime
• rsa-attacks-2.md — 2025-26: TLS blinder-squaring + Coppersmith partial-d
• rsa-attacks-oracle.md — Manger's OAEP, padding/timing/blinding oracles
• ecc-attacks.md — invalid curve, Smart's, Pohlig-Hellman, ECDSA nonce, genus-1 variety
• zkp-and-advanced.md — Groth16/PLONK/halo2/Noir ZK, Z3, SSS, LogUp/ProtoStar
• prng.md — MT19937, LCG, V8 XorShift128+, time-seed, ChaCha20 key recovery
• prng-2.md — 2024-26: GF(2) matrix, middle-square, logistic, Legendre bit oracle
• historical.md — Lorenz SZ40, book cipher
• advanced-math.md — CVP/Babai, LLL, Coppersmith, HNP, Pohlig-Hellman, Quaternion RSA
• advanced-math-2.md — 2024-26: LWE, clock group, CSIDH trace, Joux, GEA LFSR, Hill off-by-one
• exotic-crypto.md — braid-group DH, tropical, ePrint scheme killers, CSIDH sign-leak, PQ (Kyber/UOV)

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Pattern Recognition Index

Map observable signals (not challenge names) to the right technique. Scan this first.

Signal in challenge	Technique → file
pow(m, e, n) == m for some m ∉ {0,1,n-1}; timing outlier on one input	RSA fixed-point factoring → rsa-attacks.md
Close primes (`	p - q
Small d (Wiener bound), or small e + small m	Wiener / cube-root → rsa-attacks.md
Same m under same e to several n_i	Hastad broadcast / CRT → rsa-attacks.md
PKCS#1 v1.5 error side-channel on an RSA decrypt endpoint	Bleichenbacher / Manger → rsa-attacks-oracle.md, modern-ciphers.md
Two faulted RSA signatures, TLS renegotiation between them	Blinder A → A² bit recovery → rsa-attacks-oracle.md
Many linear eqs mod q with bounded error (ECDSA partial nonce, truncated LCG, HNP)	CVP/Babai (rkm0959 template) → advanced-math.md
Power/EM traces with uniform length but shape clustering	Waveform-morphology analysis (sliding min/max/mean) → advanced-math-2.md
CSIDH / isogeny + small secret vector + AES oracle	Brute force 419-element shared-secret space → advanced-math-2.md
Padding-oracle endpoint on CBC	Byte-by-byte CBC padding oracle → modern-ciphers.md
ECDSA with partial-nonce leak / same nonce reused	Pohlig-Hellman / nonce lattice → ecc-attacks.md, advanced-math.md
MT19937 outputs visible (624 words)	State recovery via Python randcrack or GF(2) matrix → prng.md
Boolean predicate + "find x such that f(x)=1" + N small	Qiskit Grover k = π/4√(N/M) → ctf-misc/ai-ml.md
Model weights file + accuracy-gate grader	Federated label-flip poisoning → ctf-misc/ai-ml.md
Two parsers for same URL/path/HTML (different libs in deps)	Parser differential → ctf-web/auth-and-access.md
halo2 circuit: advice_values[…] fill without RNG, ≥ N proofs same secret	Blinding-omission Lagrange recovery → zkp-and-advanced.md
LogUp/ProtoStar-style lookup over F_p with p ≤ 2^32	Characteristic-repetition bypass → zkp-and-advanced.md
Noir/Circom sha256_var(buf, len) with trailing buf unconstrained	Trailing-byte under-constraint → zkp-and-advanced.md
Obfuscated projective embedding, degree-2 coord relations, group order = small·large	Genus-1 variety → Weierstrass + BSGS/MOV/NFS → ecc-attacks.md
Jacobian Point class without is_on_curve check	Invalid-curve small-order pts → ecc-attacks.md
Scheme quoting ePrint, "homomorphism learning" / "entropic operator"	Linear-algebra scheme-killer → exotic-crypto.md
CSIDH/group-action KEM exposing group_action(e, ±1)	Sign-leak oracle → exotic-crypto.md
Distinguishable "invalid padding" vs "invalid message" on OAEP endpoint	Manger's attack → rsa-attacks-oracle.md
Modulus hex shows repeated-block structure (u·2^k + u·v + w)	Polynomial factorisation of n → rsa-attacks.md
Shamir t-of-n with x_i^t = 1 (roots of unity)	FFT collapse recovery → modern-ciphers-2.md
AES with Nr=1 literal or one-round helper	Linear inversion from one PT/CT pair → modern-ciphers-2.md
Hill/classical cipher mod = printable-ASCII range (94)	Try N-1 and N-2 → advanced-math-2.md
Dual hash suffix constraint MD5 + SHA1 (3-byte)	Joux multicollision cascade → advanced-math-2.md
GEA-1/GEA-2 LFSR with known keystream prefix	Rank-deficient key MITM → advanced-math-2.md
Bit oracle on (s_i / p) Legendre symbol	Z3/lattice over GF(p) state → prng-2.md
Falcon-512/1024 ref-impl signing with double-based FPU math + many signature samples	FP subnormal / rounding leakage → exotic-crypto.md#falcon
ML-DSA / Dilithium signatures with hints h (ω-bounded) or filtered z	Hint-leak lattice primal attack → exotic-crypto.md#ml-dsa
SPHINCS+ / SLH-DSA signing where FORS idx can repeat (non-atomic counter)	Tree-reuse forgery → exotic-crypto.md#slh-dsa
CSIDH / CTIDH KEM accepts attacker-supplied Montgomery A with no twist check	Invalid-curve Pohlig-Hellman → exotic-crypto.md#invalid-curve-pq
Custom KDF iterated N times before encrypt, entropy-shrinking op inside; PCAP has hundreds of packets	Null-key fixed-point: try key=0 first → modern-ciphers-2.md
AES ref-impl in Python with m[-8]/m[-4] negative indices + accepts len > 16	Extended-block linear byte relation → modern-ciphers-2.md
Merkle tree leaves = bcrypt(fixed_salt, user_payload) with variable-length payload	72-byte truncation collision → modern-ciphers-2.md
TLS server with RSA signing + OOB-byte primitive corrupting d; two renegotiations per session	Blinder squaring + Coppersmith partial-d → rsa-attacks-2.md

For each row the point is: if you see the signal, go to the file — you never need to know the challenge's name.

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For inline code/cheatsheet quick references (grep patterns, one-liners, common payloads), see quickref.md. The Pattern Recognition Index above is the dispatch table — always consult it first; load quickref.md only if you need a concrete snippet after dispatch.