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.