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# Allowing 3PID Owners to Rebind
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```
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3PID
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noun
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A "third-party identifier" such as an email address or phone number, that
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can be tied to your Matrix ID in order for your contacts outside of
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Matrix to find you, typically with the help of an identity server.
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Identity server
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noun
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A queryable server that holds mappings between 3PIDs and Matrix IDs.
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Bind
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verb
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Create a mapping between a 3PID and a Matrix ID. Useful for people to
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find a user based on their existing third-party contact information.
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```
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As part of the on-going privacy work, Matrix client applications are
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attempting to make the concept of an identity server clearer to the user, as
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well as allowing a user to interact with multiple identity servers while
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logged in. In facilitating this, Matrix clients should be able to allow
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logged-in users the ability to pick an identity server, see what 3PIDs they
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currently have bound to their Matrix ID, and bind/unbind addresses as they
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desire.
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When implementing this functionality, a technicality in the spec was found
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to prevent certain abilities for a user. A user could not add a 3PID to their
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homeserver before binding it to an identity server. It also prevents users
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from binding the same 3PID to multiple identity servers. The line "The
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homeserver must check that the given email address is **not** already
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associated with an account on this homeserver." appears under the [POST
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/_matrix/client/r0/account/3pid/email/requestToken](https://matrix.org/docs/spec/client_server/r0.5.0#post-matrix-client-r0-account-3pid-email-requesttoken)
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endpoint description. The same goes for the [equivalent msisdn (phone)
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endpoint](https://matrix.org/docs/spec/client_server/r0.5.0#post-matrix-client-r0-account-3pid-msisdn-requesttoken).
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When a user adds an email to their account on their homeserver, they can
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choose to bind that email to an identity server at the same time. This is
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specified through a `bind` boolean. If the user first adds the 3PID with
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`bind: false`, then decides they want to bind that 3PID to an identity server
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to make themselves discoverable by it, by making another request with `bind:
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true`, the homeserver will reject the second request, because this 3PID is
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already tied to the user's account.
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Similarly, when a user initially sends their 3PID with `bind: true` through a
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homeserver to identity server A, the homeserver keeps a record and attaches
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the address to the local account. If the user then switches to identity
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server B to try and do the same, the homeserver will reject the second
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request as this address has already been bound.
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## Proposal
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This proposal calls for allowing 3PID owners to rebind their 3PIDs using the
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[`POST
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/_matrix/client/r0/account/3pid/email/requestToken`](https://matrix.org/docs/spec/client_server/r0.5.0#post-matrix-client-r0-account-3pid-email-requesttoken)
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and [`POST
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/_matrix/client/r0/account/3pid/msisdn/requestToken`](https://matrix.org/docs/spec/client_server/r0.5.0#post-matrix-client-r0-account-3pid-msisdn-requesttoken)
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endpoints by extending the definition of what homeservers should check before
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rejecting a bind.
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Homeservers should reject the binding of a 3PID if it has already been bound,
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**unless** the requesting user is the one who originally bound that 3PID. If
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so, then they should be able to bind it again and again if they so choose.
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In doing so, users would be able to rebind their 3PIDs, even if the
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homeserver has already been made aware of it.
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## Tradeoffs
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Identity servers will still let 3PIDs be rebound to another Matrix ID, while
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a single homeserver won't let a 3PID transition between two users. If one
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thinks about typical internet services however, you aren't allowed to simply
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take an email address from another account even if you have control of it, so
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this shouldn't be too unintuitive.
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## Potential issues
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Newer clients will expect homeservers to allow them to switch between
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identity servers and bind/rebind emails as they please. If dealing with an
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older homeserver, clients will receive an `HTTP 400 M_THREEPID_IN_USE`.
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Clients should be prepared to understand that this may just mean they are
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dealing with an old homeserver, versus the 3PID already being bound on this
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homeserver by another user.
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## Security considerations
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None.
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## Conclusion
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By lifting the restriction of not allowing a user to bind a 3PID multiple
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times, we allow the basic ability of publishing a 3PID after associating it
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with an account, as well as allow users to interact with multiple identity
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servers on the same account with the same 3PIDs. This not only allows the
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user to play around and gain a better understanding of the purpose of an
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identity server, but it is also one step towards further decentralisation in
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the identity server space.
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