cryptohack.org
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CryptoHack is an interactive, browser-based platform for learning cryptography through hands-on challenges and puzzles. It offers a wide range of crypto problems, from basics to advanced techniques, with explanations and solutions to help you understand real-world cryptographic concepts.
More resources on Cryptography
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.
schneier.com
Bruce Schneier’s security blog offers in-depth analysis of cryptography, security, and privacy, with essays, commentary, and updates on current security issues. It’s a primary resource for understanding cryptographic concepts, threat landscapes, and cybersecurity policy debates.
crypto101.io
Crypto101 is a free, beginner-friendly guide to cryptography that explains core concepts and practical algorithms with code examples. It covers symmetric and asymmetric encryption, hashing, digital signatures, key exchange, and common security pitfalls.