What is Cipher?

A cipher is the mathematical recipe for scrambling and unscrambling data. From ancient substitution ciphers to modern AES, the goal remains the same: make data unreadable without the key.

Types of Modern Ciphers

Block Ciphers

• Process fixed-size blocks (e.g., 128 bits)
• Same key encrypts all blocks
• Need modes of operation for larger data
• Examples: AES, Blowfish, Twofish

Stream Ciphers

• Process one bit/byte at a time
• Generate keystream XORed with plaintext
• Often faster, simpler
• Examples: ChaCha20, RC4 (deprecated)

Block Cipher Modes

Block ciphers need modes to handle data larger than one block:

ECB (Electronic Codebook)

• Each block encrypted independently
• Never use: Patterns visible in ciphertext

CBC (Cipher Block Chaining)

• Each block XORed with previous ciphertext
• Requires initialization vector (IV)
• Vulnerable to padding oracle attacks

CTR (Counter Mode)

• Turns block cipher into stream cipher
• Parallelizable, random access
• Nonce must never repeat

GCM (Galois/Counter Mode)

• CTR + authentication
• Detects tampering
• Recommended for most uses

Cipher Suites

A cipher suite is a complete set of algorithms for secure communication:

TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
│    │     │        │    │     │
│    │     │        │    │     └─ Hash for integrity
│    │     │        │    └─ Mode of operation
│    │     │        └─ Symmetric cipher
│    │     └─ Authentication
│    └─ Key exchange
└─ Protocol

Historical vs Modern

Historical Ciphers

• Caesar cipher (shift letters)
• Vigenère (polyalphabetic)
• Enigma (mechanical rotor)
• All broken by modern standards

Modern Requirements

• Mathematically proven security properties
• Resistance to known-plaintext attacks
• No practical key recovery attacks
• Fast in software and hardware