CryptoLearn
A free, interactive course on cryptography — from the Caesar cipher to post-quantum algorithms. Live encoder widgets, math playgrounds, quizzes, and hands-on challenges. No account, no build tools, just open and learn.
Course Map
Introduction
What cryptography is, its 4 goals, 4000 years of history, and the vocabulary you'll need.
SECTION 02Math Foundations
Modular arithmetic, primes, GCD, Euler's totient, entropy, and XOR — with live calculators.
SECTION 03Classical Cryptography
Caesar, Vigenère, Playfair, transposition — and the frequency analysis that breaks them all.
SECTION 04Symmetric Cryptography
Stream vs block ciphers, DES, AES round by round, modes of operation, and why ECB leaks.
SECTION 05Hashing
Hash properties, why MD5/SHA-1 died, SHA-256, HMAC, and proper password hashing.
SECTION 06Asymmetric Cryptography
RSA with full math walkthrough, Diffie-Hellman, elliptic curves, signatures, and PKI.
SECTION 07Applied Cryptography
TLS handshakes step by step, PGP, the Signal protocol, and blockchain crypto.
SECTION 08Attacks & Vulnerabilities
Padding oracles, timing attacks, MITM, nonce reuse, side channels, and real-world breaches.
SECTION 09Advanced Topics
Zero-knowledge proofs, homomorphic encryption, post-quantum crypto, MPC, threshold schemes.
SECTION 10Practice & Challenges
Break-this-cipher puzzles, implementation exercises, and the best external CTF platforms.
Why learn cryptography?
Cryptography is the invisible infrastructure of the digital world. Every HTTPS page load, every chat message, every card payment, every software update relies on it. Understanding it makes you a better developer, a better security engineer, and a harder target. Misunderstanding it causes real breaches — Section 8 is full of examples.