Symmetric encryption simply means the sender (encryptor) and the receiver (decryptor) use an identical cryptographic key. The algorithm, which is able to both encrypt and decrypt—depending on the mode—is a reversible operation, as shown in the following diagram:
Symmetric-encryption.graffle
In many protocols, a different symmetric key is used for each direction of travel. So, for example, Device A may encrypt to Device B using key X. Both parties have key X. The opposite direction (B to A) may use key Y, which both parties also have.
Symmetric algorithms consist of a ciphering operation using plaintext or ciphertext input, ...
