Which method allows users to authenticate with certain websites using a single account, enabling them to retain a single login for all participating sites?

OpenID is a decentralized authentication protocol that allows users to log in to multiple websites using a single set of credentials. This method streamlines the authentication process, letting users avoid the hassle of remembering numerous usernames and passwords for different sites. When a user logs into a site using OpenID, they are redirected to their OpenID provider (the account they registered with), which confirms their identity and sends an assertion back to the website they originally wanted to access. This creates a seamless experience where users can engage with various participating websites without needing to create and manage separate accounts for each one.

The other options serve different purposes. Shibboleth primarily focuses on federated identity management and is often used in academic and research institution contexts for single sign-on (SSO). Transitive trust generally refers to the concept where trust relationships can extend across multiple systems but is not a direct method for user authentication. SAML is an XML-based protocol that facilitates SSO across different systems but is more complex than OpenID and typically requires more configuration and infrastructure. Thus, OpenID stands out as the simplest and most user-friendly option for single account authentication across multiple websites.