Symmetric cryptography

Symmetric cryptography is a branch of cryptography in which the same key is used for both encryption and decryption of data. Because the two operations share one secret, the sender and receiver must first agree on that key through some channel they both trust, before any protected communication can take place.

That shared-key requirement is symmetric cryptography’s main practical weakness. The key itself has to be distributed and kept secret from everyone but the intended parties, which becomes harder as the number of communicating parties grows.

This is the problem that asymmetric cryptography was designed to solve, by splitting the single shared key into a public and a private half. In practice the two approaches are often combined. Asymmetric cryptography establishes a shared key safely, and symmetric cryptography — generally much faster — does the bulk of the encryption work afterward.