RelMeAuth is a protocol for Web Sign-In. This protocol inspects your website for links to other identities of yours that could be used for authentication.
For example, these can be links to your profile on some platform like GitHub or social media like Twitter. If the service confirms such profile belongs to the same person that operates this website, the protocol knows that it can use this service’s authentication to confirm your identity.
RelMeAuth also supports different authentication delegates, including Email, SMS and a Public (PGP) Key challenge.