Google provides unique, domain-dependent OpenIDs. E.g. one.domain.com and two.domain.com will not have the same unique identifier. How can different Stack Exchange websites map my Google OpenID across each other?

I would like to implement a similar functionality for my websites. (E.g. same Google user across all websites — Google does not provide the same identifier across domains for security reasons, so it is not possible the easy way — how is this possible?)

What other famous providers use different unique identifiers across domains instead of the same unique ID?

  • possible duplicate of How does SO's new auto-login feature work? -- See Kevin Montrose's answer.
    – Jon Seigel
    Sep 30, 2010 at 17:13
  • I know how open ID works, I am concerned about Google replying with different unique userid for each domain for same user. in that case, is stackoverflow using any alternate technology to identify user ? may be email id etc ?
    – user145901
    Oct 2, 2010 at 17:53

1 Answer 1


This is covered in


That’s a major bummer for site networks like us with multiple domains. We use the OpenID string as your user “fingerprint”, so if your “fingerprint” changes, we can’t tell who you are any more. It’s a frustrating problem, but we think we’ve finally come up with a fix: we demand email from Google GMail OpenIDs!

If we have an email address from a verified OpenID email provider (that is, an OpenID from a large email service we trust, like Google or Yahoo), then it’s guaranteed to be a globally unique string. We treat this as part of the identifying user token, attached only at login time, that is not editable by the user.

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