Multiple Twitter Proofs

As the subject says, we should be able to prove more than one Twitter account. For instance, I should be able to prove my personal account as well as any corporate account or bot that I own and operate.

Likewise, as others have suggested, we should be able to have multiple Keybase accounts prove the same Twitter account. Perhaps we can have a PRIMARY owner and SECONDARY owner. We could have the SECONDARY owner generate the proof and have the PRIMARY owner sign the proof, then Tweet it. This would allow Keybase to verify that the SECONDARY owner has access or ownership of the account and that the PRIMARY owner has approved their adding of it to their Keybase. Just a thought.

The issue names no files, tests, or entry points. Before implementation, clarify whether the goal is multiple Twitter accounts, multiple Keybase accounts for one Twitter account, or both, and define the PRIMARY/SECONDARY ownership and signing rules. Done should include agreed behavior and tests covering the selected ownership model.

Written by the indexing model from the issue text.