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.. Copyright 2016 OpenMarket Ltd
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..
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.. Licensed under the Apache License, Version 2.0 (the "License");
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.. you may not use this file except in compliance with the License.
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.. You may obtain a copy of the License at
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..
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.. http://www.apache.org/licenses/LICENSE-2.0
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..
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.. Unless required by applicable law or agreed to in writing, software
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.. distributed under the License is distributed on an "AS IS" BASIS,
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.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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.. See the License for the specific language governing permissions and
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.. limitations under the License.
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Third party invites
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===================
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.. _module:third-party-invites:
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This module adds in support for inviting new members to a room where their
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Matrix user ID is not known, instead addressing them by a third party identifier
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such as an email address.
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There are two flows here; one if a Matrix user ID is known for the third party
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identifier, and one if not. Either way, the client calls ``/invite`` with the
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details of the third party identifier.
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The homeserver asks the identity server whether a Matrix user ID is known for
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that identifier:
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- If it is, an invite is simply issued for that user.
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- If it is not, the homeserver asks the identity server to record the details of
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the invitation, and to notify the invitee's homeserver of this pending invitation if it gets
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a binding for this identifier in the future. The identity server returns a token
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and public key to the inviting homeserver.
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When the invitee's homeserver receives the notification of the binding, it
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should insert an ``m.room.member`` event into the room's graph for that user,
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with ``content.membership`` = ``invite``, as well as a
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``content.third_party_invite`` property which contains proof that the invitee
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does indeed own that third party identifier.
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Events
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------
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{{m_room_third_party_invite_event}}
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Client behaviour
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----------------
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A client asks a server to invite a user by their third party identifier.
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{{third_party_membership_cs_http_api}}
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Server behaviour
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----------------
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Upon receipt of an ``/invite``, the server is expected to look up the third party
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identifier with the provided identity server. If the lookup yields a result for
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a Matrix User ID then the normal invite process can be initiated. This process
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ends up looking like this:
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::
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+---------+ +-------------+ +-----------------+
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| Client | | Homeserver | | IdentityServer |
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+---------+ +-------------+ +-----------------+
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| | |
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| POST /invite | |
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|------------------------------------>| |
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| | |
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| | GET /lookup |
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| |--------------------------------------------------->|
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| | User ID result |
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| |<---------------------------------------------------|
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| | |
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| | Invite process for the discovered User ID |
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| |------------------------------------------ |
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| |<----------------------------------------- |
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| Complete the /invite request | |
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|<------------------------------------| |
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| | |
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However, if the lookup does not yield a bound User ID, the homeserver must store
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the invite on the identity server and emit a valid ``m.room.third_party_invite``
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event to the room. This process ends up looking like this:
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::
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+---------+ +-------------+ +-----------------+
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| Client | | Homeserver | | IdentityServer |
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+---------+ +-------------+ +-----------------+
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| POST /invite | |
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|------------------------------------>| |
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| | |
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| | GET /lookup |
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| |-------------------------------------------------------------->|
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| | |
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| | "no users" result |
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| |<--------------------------------------------------------------|
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| | POST /store-invite |
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| |-------------------------------------------------------------->|
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| | Information needed for the m.room.third_party_invite |
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| |<--------------------------------------------------------------|
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| | Emit m.room.third_party_invite to the room |
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| |------------------------------------------- |
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| |<------------------------------------------ |
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| Complete the /invite request | |
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|<------------------------------------| |
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All homeservers MUST verify the signature in the event's
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``content.third_party_invite.signed`` object.
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The third party user will then need to verify their identity, which results in
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a call from the identity server to the homeserver that bound the third party
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identifier to a user. The homeserver then exchanges the ``m.room.third_party_invite``
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event in the room for a complete ``m.room.member`` event for ``membership: invite``
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for the user that has bound the third party identifier.
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If a homeserver is joining a room for the first time because of an
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``m.room.third_party_invite``, the server which is already participating in the
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room (which is chosen as per the standard server-server specification) MUST
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validate that the public key used for signing is still valid, by checking
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``key_validity_url`` in the above described way.
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No other homeservers may reject the joining of the room on the basis of
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``key_validity_url``, this is so that all homeservers have a consistent view of
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the room. They may, however, indicate to their clients that a member's
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membership is questionable.
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For example, given H1, H2, and H3 as homeservers, UserA as a user of H1, and an
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identity server IS, the full sequence for a third party invite would look like
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the following. This diagram assumes H1 and H2 are residents of the room while
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H3 is attempting to join.
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::
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+-------+ +-----------------+ +-----+ +-----+ +-----+ +-----+
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| UserA | | ThirdPartyUser | | H1 | | H2 | | H3 | | IS |
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+-------+ +-----------------+ +-----+ +-----+ +-----+ +-----+
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| POST /invite for ThirdPartyUser | | | |
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|----------------------------------->| | | |
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| | | GET /lookup | | |
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| | |---------------------------------------------------------------------------------------------->|
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| | | | Lookup results (empty object) |
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| | |<----------------------------------------------------------------------------------------------|
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| | | POST /store-invite | | |
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| | |---------------------------------------------------------------------------------------------->|
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| | | | Token, keys, etc for third party invite |
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| | |<----------------------------------------------------------------------------------------------|
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| | | (Federation) Emit m.room.third_party_invite | | |
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| | |----------------------------------------------->| | |
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| Complete /invite request | | | |
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|<-----------------------------------| | | |
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| | Verify identity | | | |
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| |-------------------------------------------------------------------------------------------------------------------->|
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| | | | | POST /3pid/onbind |
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| | | | |<---------------------------|
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| | | PUT /exchange_third_party_invite/:roomId | |
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| | |<-----------------------------------------------------------------| |
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| | | Verify the request | | |
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| | |------------------- | | |
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| | |<------------------ | | |
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| | | (Federation) Emit m.room.member for invite | | |
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| | |----------------------------------------------->| | |
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| | | (Federation) Emit the m.room.member event sent to H2 | |
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| | |----------------------------------------------------------------->| |
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| | | Complete /exchange_third_party_invite/:roomId request | |
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| | |----------------------------------------------------------------->| |
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| | | | | Participate in the room |
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| | | | |------------------------ |
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| | | | |<----------------------- |
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Note that when H1 sends the ``m.room.member`` event to H2 and H3 it does not
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have to block on either server's receipt of the event. Likewise, H1 may complete
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the ``/exchange_third_party_invite/:roomId`` request at the same time as sending
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the ``m.room.member`` event to H2 and H3. Additionally, H3 may complete the
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``/3pid/onbind`` request it got from IS at any time - the completion is not shown
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in the diagram.
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H1 MUST verify the request from H3 to ensure the ``signed`` property is correct
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as well as the ``key_validity_url`` as still being valid. This is done by making
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a request to the `identity server /isvalid`_ endpoint, using the provided URL
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rather than constructing a new one. The query string and response for the provided
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URL must match the Identity Service Specification.
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The reason that no other homeserver may reject the event based on checking
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``key_validity_url`` is that we must ensure event acceptance is deterministic.
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If some other participating server doesn't have a network path to the keyserver,
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or if the keyserver were to go offline, or revoke its keys, that other server
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would reject the event and cause the participating servers' graphs to diverge.
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This relies on participating servers trusting each other, but that trust is
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already implied by the server-server protocol. Also, the public key signature
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verification must still be performed, so the attack surface here is minimized.
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Security considerations
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-----------------------
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There are a number of privacy and trust implications to this module.
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It is important for user privacy that leaking the mapping between a matrix user
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ID and a third party identifier is hard. In particular, being able to look up
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all third party identifiers from a matrix user ID (and accordingly, being able
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to link each third party identifier) should be avoided wherever possible.
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To this end, the third party identifier is not put in any event, rather an
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opaque display name provided by the identity server is put into the events.
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Clients should not remember or display third party identifiers from invites,
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other than for the use of the inviter themself.
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Homeservers are not required to trust any particular identity server(s). It is
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generally a client's responsibility to decide which identity servers it trusts,
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not a homeserver's. Accordingly, this API takes identity servers as input from
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end users, and doesn't have any specific trusted set. It is possible some
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homeservers may want to supply defaults, or reject some identity servers for
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*its* users, but no homeserver is allowed to dictate which identity servers
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*other* homeservers' users trust.
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There is some risk of denial of service attacks by flooding homeservers or
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identity servers with many requests, or much state to store. Defending against
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these is left to the implementer's discretion.
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.. _`identity server /isvalid`: ../identity_service/unstable.html#get-matrix-identity-api-v1-pubkey-isvalid
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