Which protocol is commonly used for secure user authentication in PKI?

The correct answer focuses on SSL/TLS as the prominent protocol used for secure user authentication in Public Key Infrastructure (PKI). SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are cryptographic protocols that ensure secure communication over a computer network. They are widely used to secure connections—most notably in web traffic—by providing encryption, data integrity, and authentication.

In the context of PKI, SSL/TLS relies on digital certificates, which are issued by a trusted Certificate Authority (CA). These certificates contain a public key and the identity of the certificate holder, allowing clients and servers to authenticate each other securely. This mechanism is essential for establishing a secure channel where both parties can trust that they are communicating with the legitimate entities.

Other protocols mentioned have different purposes or contexts that do not primarily focus on secure user authentication in relation to PKI. For instance, SSH (Secure Shell) is mainly used for secure remote system access, while HTTP (Hypertext Transfer Protocol) and FTP (File Transfer Protocol) are primarily used for data transfer over the web and file transfers, respectively. They do not inherently provide secure authentication mechanisms as SSL/TLS does within PKI implementations.