The Complete Cryptography Cheatsheet: Essential Concepts and Techniques

Introduction: What is Cryptography and Why It Matters

Cryptography is the science of securing information by transforming it into formats that are unreadable to unauthorized parties. It enables confidentiality, integrity, authentication, and non-repudiation in our digital communications and data storage. In today’s interconnected world, cryptography forms the backbone of digital security, protecting everything from personal messages to financial transactions, national security communications, and critical infrastructure.

Core Cryptographic Principles

Fundamental Security Properties

Key Cryptographic Concepts

• Plaintext: The original, readable message
• Ciphertext: The encrypted, unreadable message
• Encryption: Process of converting plaintext to ciphertext
• Decryption: Process of converting ciphertext back to plaintext
• Key: A piece of information that determines the output of cryptographic algorithms
• Entropy: Measure of randomness or unpredictability in a system
• Cryptanalysis: The study of breaking cryptographic systems

Cryptographic Algorithms and Methods

Symmetric Key Cryptography

Uses the same key for both encryption and decryption

Modes of Operation for Block Ciphers:

• ECB (Electronic Codebook): Simple but insecure for most applications
• CBC (Cipher Block Chaining): Each block XORed with previous ciphertext
• CTR (Counter): Converts block cipher to stream cipher
• GCM (Galois/Counter Mode): Provides both confidentiality and authentication
• XTS: Specialized for disk encryption

Asymmetric (Public Key) Cryptography

Uses separate keys for encryption and decryption

Hash Functions

One-way functions that map data of arbitrary size to fixed-size values

Message Authentication Codes (MACs)

Provide both authentication and integrity

• HMAC: Hash-based MAC, combines a cryptographic hash with a secret key
• CMAC: Cipher-based MAC, based on block cipher
• Poly1305: High-speed MAC often used with ChaCha20

Key Derivation Functions (KDFs)

Derive cryptographic keys from master keys or passwords

Step-by-Step Cryptographic Processes

Secure Communication Setup (TLS Handshake)

1. Client Hello: Client sends supported cipher suites and random number
2. Server Hello: Server selects cipher suite and sends certificate and random number
3. Key Exchange: Using asymmetric cryptography (e.g., RSA or ECDHE)
4. Verification: Client verifies server’s certificate against trusted CAs
5. Session Key Derivation: Both parties derive the same symmetric session key
6. Secure Communication: Data encrypted with symmetric algorithm using session key

Digital Signature Process

1. Hash Calculation: Create hash of the message to be signed
2. Signature Creation: Encrypt hash with signer’s private key
3. Signature Attachment: Attach signature to message
4. Verification: Recipient decrypts signature with signer’s public key and compares to independently calculated hash

Public Key Infrastructure (PKI) Process

1. Key Pair Generation: Entity creates public and private key pair
2. Certificate Signing Request: Entity submits public key and identity information to CA
3. Verification: CA verifies entity’s identity
4. Certificate Issuance: CA issues signed certificate binding identity to public key
5. Certificate Distribution: Entity distributes certificate with public key
6. Certificate Verification: Relying parties verify certificate against trusted CAs

Cryptographic Protocols and Applications

Secure Communication Protocols

Authentication Methods

• Password-based: Combined with KDFs for secure storage
• Certificate-based: Using PKI and digital signatures
• Multi-factor: Combining multiple authentication methods
• Zero-knowledge proofs: Proving knowledge without revealing it

Blockchain and Cryptocurrency

• Digital signatures: Authenticate transactions
• Hash functions: Create transaction links and proof-of-work
• Merkle trees: Efficiently verify transaction inclusion
• Zero-knowledge proofs: Enable privacy features in some cryptocurrencies

Comparison of Cryptographic Approaches

Symmetric vs. Asymmetric Encryption

Hash Function Comparison

Common Cryptographic Challenges and Solutions

Challenge: Secure Key Management

Solutions:

• Use hardware security modules (HSMs) for critical keys
• Implement principle of least privilege for key access
• Establish key rotation policies
• Use key derivation functions rather than storing multiple keys
• Separate storage of key components (key splitting)

Challenge: Random Number Generation

Solutions:

• Use cryptographically secure random number generators (CSPRNGs)
• Combine multiple entropy sources
• Use hardware random number generators when available
• Regularly test randomness quality
• Avoid using standard random functions from programming languages

Challenge: Side-Channel Attacks

Solutions:

• Implement constant-time operations for cryptographic functions
• Add random delays or noise to operations
• Use hardware with side-channel protections
• Avoid branching based on secret data
• Monitor for unusual access patterns or timing anomalies

Challenge: Implementation Vulnerabilities

Solutions:

• Use vetted cryptographic libraries instead of custom implementations
• Keep cryptographic software updated
• Conduct regular security audits and penetration testing
• Implement defense in depth
• Follow coding standards specific to cryptographic implementations

Best Practices and Practical Tips

General Cryptographic Best Practices

• Don’t roll your own crypto: Use established, peer-reviewed algorithms and libraries
• Keep it simple: Complexity increases the risk of implementation errors
• Defense in depth: Never rely on a single cryptographic control
• Future-proof: Design systems to allow algorithm and key size updates
• Assume compromise: Design with the assumption that some parts will be compromised
• Kerckhoffs’s principle: Security should depend on the key, not the secrecy of the algorithm

Algorithm Selection Guidelines

• Choose algorithms based on:
  • Security requirements and threat model
  • Performance constraints
  • Regulatory requirements
  • Industry standards
  • Implementation availability
  • Future-proofing (quantum resistance if needed)

Implementation Checklist

• [ ] Use up-to-date, well-maintained cryptographic libraries
• [ ] Implement proper key management procedures
• [ ] Use appropriate key sizes for the protection period needed
• [ ] Add entropy collection for random number generation
• [ ] Implement secure error handling that doesn’t leak information
• [ ] Validate all cryptographic inputs
• [ ] Consider side-channel attack mitigations
• [ ] Implement logging and monitoring for cryptographic operations

Emerging Trends in Cryptography

Post-Quantum Cryptography

• Lattice-based: CRYSTALS-Kyber, NTRU
• Hash-based: SPHINCS+
• Code-based: Classic McEliece
• Multivariate: Rainbow (broken), GeMSS
• Isogeny-based: SIKE (broken)

Homomorphic Encryption

• Allows computation on encrypted data without decrypting it
• Types: Fully, Somewhat, and Partially Homomorphic Encryption
• Applications: privacy-preserving computation, secure cloud computing

Secure Multi-party Computation (MPC)

• Enables multiple parties to jointly compute a function over inputs while keeping those inputs private
• Applications: privacy-preserving analytics, threshold cryptography

Zero-Knowledge Proofs

• Proves knowledge of a value without revealing the value itself
• Types: Interactive, Non-interactive (zk-SNARKs, zk-STARKs)
• Applications: privacy-focused cryptocurrencies, authentication, anonymous credentials

Resources for Further Learning

Books

• “Cryptography Engineering” by Ferguson, Schneier, and Kohno
• “Real-World Cryptography” by David Wong
• “Serious Cryptography” by Jean-Philippe Aumasson
• “The Code Book” by Simon Singh (historical perspective)
• “Applied Cryptography” by Bruce Schneier (classic reference)

Online Courses

• Cryptography I & II by Dan Boneh (Stanford) on Coursera
• Introduction to Cryptography by Christof Paar on YouTube
• Practical Cryptography for Developers (free online book)

Standards and Guidelines

• NIST Cryptographic Standards and Guidelines
• IETF RFCs for cryptographic protocols
• BSI Cryptographic Mechanisms Recommendations
• ENISA Algorithms, Key Sizes and Parameters Report

Tools and Libraries

• OpenSSL: Comprehensive crypto library and toolkit
• Libsodium: Easy-to-use, high-level cryptographic library
• Bouncy Castle: Java and C# implementations
• Crypto++: C++ class library of cryptographic schemes
• PyCA/cryptography: Python cryptography library
• CrypTool: Educational tool for cryptography and cryptanalysis

Websites and Communities

• Cryptography Stack Exchange
• /r/cryptography subreddit
• The International Association for Cryptologic Research (IACR)
• Bruce Schneier’s blog “Schneier on Security”

Disclaimer

This cheatsheet provides general information about cryptography concepts and techniques. When implementing cryptographic systems, always consult current standards, guidelines, and expert advice. Cryptography is a rapidly evolving field, and what is secure today may not be secure tomorrow. Always follow the latest security best practices and keep cryptographic systems updated.

Last Updated: May 2025