In symmetric encryption, what is required by both sender and receiver?

In symmetric encryption, both the sender and the receiver must use the same encryption key to encrypt and decrypt messages. This shared encryption key allows both parties to maintain confidentiality and ensures that the information can only be accessed by those who possess the key. The essence of symmetric encryption relies on the single key that is both secret and shared between the communicating entities, which distinguishes it from asymmetric encryption, where different keys are utilized.

The other options do not apply to symmetric encryption. A public encryption key is specific to asymmetric encryption models, where public and private keys are used. Having two different encryption keys contradicts the premise of symmetric encryption, as it requires a uniform key for both encryption and decryption by the same user. A digital signature is utilized for authenticity and non-repudiation but does not provide the encryption process essential to symmetric encryption. Therefore, the correct answer centers on the necessity of a shared encryption key to facilitate secure communication between sender and receiver.