OpenID

OpenID is an open standard and decentralized authentication protocol that enables single sign-on across participating websites and applications. It allows users to sign in to multiple sites using a single identity provided by an identity provider (IdP), reducing the need to manage separate credentials for each site and giving users a central point for authentication and account management.

In typical deployments, a relying party (the site the user wants to access) redirects the user to their identity provider for authentication. After successful login, the IdP returns an assertion that the user has been authenticated, allowing the relying party to log the user in without handling passwords. OpenID defines the messages and formats used to perform this exchange. The ecosystem historically included OpenID 2.0, which used a decentralized trust model with URL-based identifiers, and OpenID Connect, a modern identity layer built on OAuth 2.0 that provides standardized user information via an ID token and a userinfo endpoint. OpenID Connect is the predominant choice in contemporary deployments.

OpenID was created by Brad Fitzpatrick in 2005. OpenID 2.0 became widely used, but over time many providers migrated to OpenID Connect; Google retired support for OpenID 2.0 in 2015. Governance and specification work is coordinated by the OpenID Foundation. OpenID remains a foundational approach in federated identity, enabling cross-site sign-in while emphasizing the importance of trusting the identity provider and managing privacy and consent.