Symmetric Encryption

Symmetric encryption, also called shared-key or secret-key cryptography, uses the same cryptographic key for both encrypting and decrypting a message. Its central practical challenge is key distribution: both sender and receiver must possess the identical secret key before they can communicate securely, and that key must itself be exchanged through some channel resistant to interception, a chicken-and-egg problem that public-key cryptography would later help resolve. Symmetric algorithms are generally far faster than public-key algorithms for the same level of security, which is why virtually all bulk data encryption, from disk encryption to the payload of a TLS connection, relies on symmetric ciphers even in systems that also use public-key techniques for authentication and key exchange. A foundational principle, formulated by the Dutch cryptographer Auguste Kerckhoffs in 1883 and known as Kerckhoffs's principle, holds that a cryptographic system should remain secure even if everything about it except the key is public knowledge; security should reside in the secrecy of the key, not in the secrecy of the algorithm. This principle underlies the modern practice of publishing cipher designs openly for public scrutiny, on the reasoning that algorithms subjected to sustained cryptanalytic attack by the global research community are more trustworthy than those kept secret.