Symmetric encryption uses a single shared secret key for both operations: the sender encrypts the plaintext with the key, and the receiver decrypts the ciphertext with the same key. Anyone who holds the key can both lock and unlock the data, so the key must be kept secret between the communicating parties. This is the oldest and most direct model of encryption, and Shannon’s 1949 analysis of secrecy systems was built around exactly this kind of keyed cipher.
Because the operations are simple and fast, symmetric encryption is the workhorse for bulk data. The U.S. government standardized the Data Encryption Standard (DES) in the 1970s and later replaced it with the Advanced Encryption Standard (AES); NIST publishes these as Federal Information Processing Standards in its cryptographic standards program. AES protects the vast majority of encrypted disk storage and network traffic precisely because it is efficient enough to run on everything from servers to phones.
The catch is key distribution. Before two parties can communicate, they must somehow agree on the same secret key without an eavesdropper learning it. For people who have never met, or for the open internet where any message can be observed, this is a genuinely hard problem. Mailing a key, meeting in person, or trusting a courier does not scale.
This key-distribution problem is the gap that public-key cryptography was invented to close. Modern systems typically use public-key methods to establish a shared secret and then switch to fast symmetric encryption for the actual data, combining the security of one with the speed of the other.