crypto101.io
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Crypto101 is a free, beginner-friendly online cryptography textbook that teaches core concepts through clear explanations and code-driven examples. It covers symmetric and asymmetric encryption, hash functions, and common cryptographic protocols with practical, real-world examples.
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.
cryptohack.org
CryptoHack is an interactive learning platform for cryptography that teaches core concepts through hands-on challenges. It offers in-browser puzzles (RSA, AES, hashes, padding) with tutorials and explanations to learn by doing.
schneier.com
Bruce Schneier's site hosts his security blog, offering in-depth analysis of cryptography, security, privacy, and technology policy. It features regular posts, essays, and commentary on current security issues and practical tips.
The Code Book
The Code Book - A book resource