cryptopals.com is a collection of 48 exercises that demonstrate attacks on real-world crypto. They give you problems to solve. They’re derived from weaknesses in real-world systems and modern cryptographic constructions. We give you enough info to learn about the underlying crypto concepts yourself. When you’re finished, you’ll not only have learned a good deal about how cryptosystems are built, but you’ll also understand how they’re attacked.
CryptoHack is a fun platform for learning cryptography. Their emphasis is on breaking bad implementations of “modern” crypto, such as AES, RSA, and Elliptic-curve. The format is a series of puzzles that teach small lessons and motivate further research.
They took inspiration from Capture The Flag contests but wanted to build something that’s permanent, focusing exclusively on cryptography, and is easier than CTFs which can be tough.