Individual skill

ctf-crypto

CTF crypto challenge playbook — read the provided encryption script like a spec, attack RSA parameter weaknesses, XOR/multi-time-pad oracles, and homemade ciphers with math tooling (z3, sympy, sage). Load when the handout is .py/.sage math, n/e/c values, or an "encryption service" on nc. Signals: "crypto" category, chall.py with Crypto.Util.number, e=3 or e=65537, "encrypted_flag", XOR with a key, nc service that encrypts your input.

skills/ctf/ctf-crypto/SKILL.md

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Last checked: Nov 11, 2026

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Repository stars (not skill ratings): 27
Repository last updated: Oct 14, 2026
License: MIT

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# CTF crypto challenges

## When it applies
The handout is a short encryption script (`chall.py`, rarely sage), a set of numbers
(`n, e, c` / intercepted ciphertexts), or an `nc` service that encrypts/signs things for you.
This skill covers the CTF shapes — reading the script for the flaw, XOR games, homemade ciphers,
and math tooling. For the deep attack mechanics it defers to `crypto-rsa-attacks` and
`crypto-oracle-attacks`; run the RSA battery there first.

## Why it works
CTF crypto is a *code review against math*: the script is short, self-contained, and contains
exactly one mistake the author planted — a reused nonce, a factorable `n`, an invertible
homegrown round function, or an oracle the service hands you. Find the one line that looks
"clever" and the challenge is usually done; the rest is algebra.