You can implement decentralized logins using Handshake names. There are many potential implementations, but here's one: say I have tieshun/ on Handshake (I do). I can generate a pgp key and pin the fingerprint to tieshun/ on-chain as a TXT record while storing the private key in a chrome extension. When I visit a website the website can authenticate I am tieshun/ by asking the chrome extension to sign an arbitrary message, which the website can then trustlessly verify against the pinned key.
There are a lot of amazing use-cases this tool can enable. Outside of sovereign identity, it would also enable shared login across decentralized applications, such as a decentralized Reddit where each subreddit is a separate server controlled by the subreddit's maintainers.