Cryptography 101: A High-Level Introduction

This repository provides a high-level introduction to cryptography, referencing classic and modern resources. The goal is to offer a structured overview of the field for students, engineers, and enthusiasts, before diving into more advanced topics like constant-time implementations, side-channel resistance, and post-quantum cryptography (PQC).

References

• Handbook of Applied Cryptography (HAC) – A foundational reference by Menezes, van Oorschot, and Vanstone: HAC PDF

• Cryptography 101 – An accessible introduction to the basics of cryptography:

  • Deployments
  • Building Blocks

Overview

Cryptography is the science of secure communication in the presence of adversaries. It provides the tools to ensure confidentiality, integrity, authenticity, and non-repudiation in digital systems.

Core Goals

1. Confidentiality – Preventing unauthorized access to information.
2. Integrity – Ensuring information is not altered in transit or storage.
3. Authentication – Verifying the identity of communicating parties.
4. Non-repudiation – Preventing denial of participation in a communication.

Building Blocks

• Symmetric cryptography – Encryption and decryption with the same secret key.
• Asymmetric cryptography (Public-Key) – Using mathematically related key pairs.
• Cryptographic hash functions – One-way functions mapping arbitrary input to fixed-length output.
• Digital signatures – Providing authenticity and integrity using asymmetric keys.
• Key exchange protocols – Establishing shared secrets over insecure channels.

Applications

• Secure communication protocols (TLS, SSH, IPsec)
• Authentication systems (password hashing, digital certificates)
• Data protection (disk encryption, file integrity)
• Blockchain and cryptocurrencies

Structure of This Repository

This repository is organized into multiple sections:

1. Introduction (this document)
2. Constant-Time Implementations – Why timing leaks matter, and coding practices to avoid them.
3. Side-Channel Attacks – Power, EM, timing, and cache-based attacks.
4. Post-Quantum Cryptography (PQC) – Algorithms designed to resist quantum adversaries.

Each section will include explanations, references, and where relevant, practical examples.

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This repository is a work in progress. Contributions, corrections, and discussions are welcome.