Cryptopals is a series of exercises designed to teach people about real-world cryptography by presenting problems to solve. The challenges are derived from weaknesses found in modern cryptographic constructions and systems. Participants are given enough information to learn the underlying crypto concepts themselves, making it unnecessary to know any crypto or math before starting. These sets of exercises are meant to be coded proficiently in any language, although the complexity of the sets varies, and become progressively harder. Cryptopals is an initiative designed to highlight the presence of potential weaknesses in current crypto systems, and aims to make more people aware of how to implement the best-known attacks.
https://cryptopals.com/