cryptohack.org
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CryptoHack is an interactive learning platform that teaches cryptography through hands-on challenges and explanations, covering topics from classical ciphers to modern cryptographic protocols.
More resources on Cryptographic Protocols
Cryptography I
Cryptography is an indispensable tool for protecting information in computer systems. In this course you will learn the inner workings of cryptographic systems and how to correctly use them in real-world applications. The course begins with a detailed discussion of how two parties who have a shared secret key can communicate securely when a powerful adversary eavesdrops and tampers with traffic. We will examine many deployed protocols and analyze mistakes in existing systems. The second half of the course discusses public-key techniques that let two parties generate a shared secret key. Throughout the course participants will be exposed to many exciting open problems in the field and work on fun (optional) programming projects. In a second course (Crypto II) we will cover more advanced cryptographic tasks such as zero-knowledge, privacy mechanisms, and other forms of encryption.
crypto101.io
Crypto101 is a free online cryptography textbook that explains core concepts, algorithms, and cryptographic protocols with clear explanations and practical code examples.
Zero Knowledge Proofs Resources
The site of the ZKProof open standards effort, hosting its Community Reference document, workshop proceedings, and terminology. Readers get a grounding in zero-knowledge proof constructions, security definitions, and the interoperability questions the standardization process is trying to settle.
Applied Cryptography
Schneier's reference on cryptographic building blocks, covering protocols, algorithms, and source code in C. Readers gain a working map of ciphers, hashes, and key-exchange protocols, plus the practical failure modes that break real systems.
Introduction to Modern Cryptography
A rigorous textbook built on definitions, assumptions, and proofs rather than recipes. Working through it, a reader can state security definitions precisely, follow reduction-based proofs, and reason about why a construction is secure rather than merely how it works.