The fundamental difference between symmetric and asymmetric key cryptographic systems is that symmetric key cryptography uses which of the following?

Non-repudiation is a property of asymmetric (public key) cryptography achieved through digital signatures, not a feature of symmetric key systems.

Using different keys on both ends describes asymmetric cryptography, which is the opposite of how symmetric systems work.

The transmission medium (fiber) is irrelevant to the definition of symmetric cryptography; the key relationship, not the medium, is the distinguishing factor.

Symmetric key cryptography requires both the sender and the receiver to possess the exact same secret key, which is used for both encryption and decryption. This contrasts with asymmetric cryptography, where a public key encrypts and a separate private key decrypts. The shared-key model makes symmetric algorithms fast and suitable for bulk encryption, but introduces the challenge of securely distributing that single key to all parties.