Symmetric Encryption

Symmetric encryption, also known as secret-key or shared-key cryptography, is the oldest and most straightforward form of encryption. Both the sender and recipient must possess the same secret key, which must be exchanged securely before encrypted communication can begin. This key management requirement is the primary challenge of symmetric encryption.

Modern symmetric ciphers are divided into two main categories: block ciphers and stream ciphers. Block ciphers like AES process data in fixed-size blocks (typically 128 bits), while stream ciphers like ChaCha20 encrypt data one bit or byte at a time. Block ciphers operate in various modes such as GCM, CBC, and CTR, each with different properties regarding parallelisation, error propagation, and authentication.

Symmetric encryption is significantly faster than asymmetric encryption, often by orders of magnitude, making it the preferred choice for encrypting bulk data. In practice, symmetric and asymmetric encryption are commonly used together in a hybrid approach: asymmetric encryption secures the initial key exchange, and symmetric encryption handles the actual data encryption. This is exactly how TLS works. When selecting a symmetric cipher, AES-256-GCM or ChaCha20-Poly1305 are recommended for new implementations, as both provide authenticated encryption with strong security margins and excellent performance.