What is used to provide mutual authentication for TLS/SSL?

X.509 certificates enable mutual authentication in TLS/SSL by requiring both parties to present a digital certificate signed by a trusted Certificate Authority, allowing each side to verify the other's identity using public-key cryptography. The certificate binds a public key to an identity and is validated through a chain of trust. This is the standard mechanism defined in TLS 1.2 and 1.3 for two-way certificate-based authentication.

Access lists filter network traffic based on IP addresses or ports and play no role in authenticating the identity of TLS/SSL endpoints.

A shared secret key is used in symmetric or pre-shared key scenarios, not in the certificate-based mutual authentication process that TLS/SSL requires for identity verification.

A mutually trusted issuer (Certificate Authority) is a prerequisite for certificate chain validation, but the mechanism that actually performs authentication is the X.509 certificate itself, not the concept of a trusted issuer alone.