What is the primary function of a digital certificate?

The primary function of a digital certificate is to prove ownership of a public key. Digital certificates are part of a public key infrastructure (PKI) and serve as a means to establish trust among parties in digital communications. When a digital certificate is issued by a trusted Certificate Authority (CA), it binds a public key to the identity of the individual or organization that owns it. This binding allows other parties to verify that the public key indeed belongs to the claimed owner, thereby enhancing security in communications and transactions, such as SSL/TLS in web browsing.

In contrast, the other options do not align with the core purpose of digital certificates. Storing user passwords securely pertains to different security mechanisms, while enabling public access to private networks and restricting connections based on IP address involve network access control, not the authentication or identification capabilities provided by digital certificates.