Which of the following relies on the use of shared secrets to protect communication?

RADIUS protects its communications by requiring a shared secret that is pre-configured on both the RADIUS client (such as a VPN concentrator or wireless access point) and the RADIUS server. This shared secret is used to authenticate RADIUS messages and to encrypt the user password attribute within RADIUS packets, forming the basis of trust between the two parties.

Kerberos relies on symmetric key cryptography and tickets issued by a trusted Key Distribution Center (KDC), not on a simple pre-shared secret between communicating endpoints.

PKI relies on asymmetric cryptography using public and private key pairs and digital certificates issued by a Certificate Authority, not shared secrets.

LDAP is a directory access protocol used to query and modify directory services and does not inherently rely on shared secrets to protect its communications.