This will probably never be implemented because the OpenID URL of a users is somewhat secret, and would definitely be considered private.On dev.meta.stackoverflow, someone asked this question, and the dev team was very quick to say that the OpenID URL would never be revealed.

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There might, however, be a more secure method to validate the User/OpenID connection.

Instead of returning the user's OpenID in the API, another API call would be made to verify it. You would have to send a url encoded version of the OpenID URL to StackOverflow, and they would compare it with the one they have, and if it matches they will return a success method.

This would have to be a limited call (as to prevent guessing), and it would work very similar to a Username/Password system, where that the only way you would get a success is if you gave the correct Username/OpenID combination.