Practical Cryptography for Developers Book

Digital signatures are cryptographic tools used to sign messages and verify signatures to provide proof of authenticity. They provide three core security properties:

• Message authentication: Proof that a specific known sender (the secret key owner) created and signed the message.
• Message integrity: Proof that the message has not been altered after being signed.
• Non-repudiation: The signer cannot deny having signed the document once the signature is created.

Note: Digital signatures bind messages to public keys, not directly to digital identities. To bind a public key to a specific identity (person, organization, or website), they must be used in combination with a digital certificate.

Symmetric key ciphers use the same secret key (or a passphrase used to derive a key) for both encryption and decryption. They are categorized into two main types:

• Block Ciphers: Encrypt data in fixed-size blocks (e.g., AES, Twofish, CAST, RC6).
• Stream Ciphers: Encrypt data as a continuous sequence of bytes (e.g., ChaCha20).

Block ciphers can be converted into stream ciphers using block cipher modes of operation (such as CBC or CTR). Symmetric ciphers are considered quantum-resistant provided sufficiently large key lengths are utilized.

This repository contains the source material for the book Practical Cryptography for Developers by Svetlin Nakov. It is a practical guide designed for developers, providing code examples (primarily in Python) to implement core cryptographic concepts.

Key topics covered include:

• Hashes: SHA-3, BLAKE2, etc.
• MAC codes: HMAC, GMAC.
• Key Derivation Functions (KDF): Scrypt, Argon2.
• Key Agreement: DHKE, ECDH.
• Symmetric Ciphers: AES, ChaCha20, AES-GCM, ChaCha20-Poly1305 (AEAD).
• Asymmetric Ciphers & Public-Key Cryptosystems: RSA, ECC, ECIES.
• Elliptic Curve Cryptography (ECC): secp256k1, curve25519.
• Digital Signatures: ECDSA, EdDSA.
• Secure Random Numbers: PRNG, CSRNG.
• Quantum-safe cryptography.

Note: The book is currently a work in progress and is not yet finished.

Asymmetric key cryptosystems (also known as public-key cryptosystems) use a mathematically linked pair of keys: a public key (used for encryption or signature verification) and a private key (used for decryption or signing).

Key capabilities provided by these systems include:

• Key-pair generation: Creating a random private key and its corresponding public key.
• Encryption/Decryption: Encrypting data with a public key so that only the holder of the private key can decrypt it.
• Digital Signatures: Signing messages with a private key to provide authentication, integrity, and non-repudiation, which can then be verified by anyone with the public key.
• Key Exchange: Securely exchanging cryptographic keys between parties over an insecure channel (e.g., DHKE, ECDH).

The Diffie-Hellman Key Exchange (DHKE) is an anonymous key agreement protocol that allows two parties to establish a shared secret key over an insecure (public) channel. Once established, this shared secret is typically used as a key for symmetric ciphers like AES.

Security Properties

• Resistant to: Sniffing attacks (data interception). An eavesdropper seeing the public exchange cannot efficiently calculate the shared secret.
• Vulnerable to: Man-in-the-middle (MITM) attacks. An attacker can intercept and alter the communication between the two parties. Authentication is required to prevent this.

Implementation Variants

• Classical DHKE: Uses modular exponentiations and the Discrete Logarithm Problem (DLP).
• ECDH (Elliptic-Curve Diffie-Hellman): Uses elliptic-curve calculations instead of modular exponentiation for improved security and efficiency.

Proof-of-Work (PoW) mining algorithms utilize a specific class of hash functions designed to be both computationally-intensive and memory-intensive. These are referred to as ASIC-resistant hash functions.

Key Characteristics

• Resource Consumption: They are designed to consume significant computational power and large amounts of RAM.
• Hardware Resistance: They are intentionally difficult to implement efficiently on specialized hardware like FPGAs or ASICs, favoring general-purpose hardware like GPUs (e.g., NVIDIA GTX 1080) or powerful CPUs (e.g., Intel Core i7-8700K) paired with fast RAM (e.g., DDR4).
• Goal: The primary objective is to minimize mining centralization by making it economically viable for small players (home users) to participate, thereby limiting the dominance of large-scale mining corporations using ASIC miners.

EdDSA (Edwards-curve Digital Signature Algorithm) is a modern, secure digital signature algorithm based on performance-optimized elliptic curves. It is based on the Schnorr signature algorithm and relies on the difficulty of the ECDLP problem.

Key variants include:

• Ed25519: Uses the edwards25519 curve (255-bit).
• Ed448: Uses the edwards448 curve (448-bit).

EdDSA is generally recommended for modern applications as it is simpler to implement and often faster than ECDSA.

Symmetric encryption uses a single secret key (also known as an encryption key or shared key) to both encrypt and decrypt data.

In a complete symmetric encryption scheme, the process typically involves several components:

1. Password-to-key-derivation algorithm: Extracts a binary secret key from a user-friendly password.
2. Symmetric cipher algorithm: The core encryption algorithm (e.g., AES).
3. Cipher block mode algorithm: Determines how the cipher handles data blocks (e.g., CBC, CTR).
4. Message authentication (MAC) algorithm: Provides integrity and authenticity, a combination known as authenticated encryption.

When designing a system for user authentication, choose a password storage method based on the required security level.