Which of the following best describes symmetric encryption?

Symmetric encryption is characterized by the use of a single key for both the processes of encryption and decryption. This means that the same secret key is shared between the parties involved, enabling them to encrypt data before transmission and decrypt it upon receipt. The simplicity of using one key for both tasks makes symmetric encryption efficient and often faster than asymmetric methods.

When a sender encrypts a message, they employ this key to transform the plaintext into ciphertext. The recipient, possessing the same key, can reverse the process to convert the ciphertext back into plaintext. This shared-key approach is fundamental to many encryption protocols and systems, and it is widely used for securing data at rest as well as for data in transit.

The alternative options describe other types of encryption or concepts related to encryption but do not accurately define symmetric encryption. For instance, utilizing a public and a private key pertains specifically to asymmetric encryption, where two different keys are involved. Primarily addressing data in transit might also refer more to symmetric key exchange protocols but does not define the essence of symmetric encryption itself. Lastly, the reliance on a complex algorithm is true for many encryption methodologies, but complexity itself is not a defining feature of symmetric encryption; rather, it is the key usage that distinguishes it.