A symmetric cipher is one that uses the same key for encryption and decryption.
Ciphers or algorithms can be either symmetric or asymmetric. Symmetric ones use the same key (called a secret key or private key) for transforming the original message, called plaintext, into ciphertext and vice versa. Symmetric ciphers are generally faster than their asymmetric counterparts, which use a closely-held private key as well as a public key shared between the two parties (hence public-key cryptography, or PKC). Examples of symmetric ciphers are Advanced Encryption Standard (AES), Data Encryption Standard (DES), Blowfish, and International Data Encryption Algorithm (IDEA).
The use of a symmetric cipher presents the familiar challenge of how to share the secret key between the parties securely, as an unauthorized party to the conversation may intercept it and eavesdrop on future conversations. As a solution, an asymmetric cipher is typically used for the key exchange. Examples of widely-used key-exchange asymmetric ciphers include the Diffie–Hellman key exchange protocol, the Digital Signature Standard (DSS, which incorporates the Digital Signature Algorithm or DSA), various elliptic curve techniques, and the RSA encryption algorithm (PKCS#1).
“Symmetric ciphers such as AES are fast and efficient, but since they use the same key for encryption and decryption, their private keys must be distributed to the parties in a way that safeguards their secrecy. For that reason, PKC such as Diffie-Hellman is used.”