Make CAS a subset of SSO
Travis Ralston
6 years ago
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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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CAS-based client login
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======================
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.. _module:cas_login:
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`Central Authentication Service
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<https://github.com/apereo/cas/blob/master/docs/cas-server-documentation/protocol/CAS-Protocol-Specification.md>`_
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(CAS) is a web-based single sign-on protocol.
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An overview of the process, as used in Matrix, is as follows:
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1. The Matrix client instructs the user's browser to navigate to the
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|/login/cas/redirect|_ endpoint on the user's homeserver.
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2. The homeserver responds with an HTTP redirect to the CAS user interface,
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which the browser follows.
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3. The CAS system authenticates the user.
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4. The CAS server responds to the user's browser with a redirect back to the
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|/login/cas/ticket|_ endpoint on the homeserver, which the browser
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follows. A 'ticket' identifier is passed as a query parameter in the
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redirect.
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5. The homeserver receives the ticket ID from the user's browser, and makes a
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request to the CAS server to validate the ticket.
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6. Having validated the ticket, the homeserver responds to the browser with a
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third HTTP redirect, back to the Matrix client application. A login token
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is passed as a query parameter in the redirect.
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7. The Matrix client receives the login token and passes it to the |/login|_
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API.
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Client behaviour
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----------------
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The client starts the process by instructing the browser to navigate to
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|/login/cas/redirect|_ with an appropriate ``redirectUrl``. Once authentication
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is successful, the browser will be redirected to that ``redirectUrl``.
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.. TODO-spec
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Should we recommend some sort of CSRF protection here (specifically, we
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should guard against people accidentally logging in by sending them a link
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to ``/login/cas/redirect``.
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Maybe we should recommend that the ``redirectUrl`` should contain a CSRF
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token which the client should then check before sending the login token to
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``/login``?
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{{cas_login_redirect_cs_http_api}}
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{{cas_login_ticket_cs_http_api}}
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Server behaviour
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----------------
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The URI for the CAS system to be used should be configured on the server by the
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server administrator.
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Handling the redirect endpoint
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When responding to the ``/login/cas/redirect`` endpoint, the server must
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generate a URI for the CAS login page. The server should take the base CAS URI
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described above, and add a ``service`` query parameter. This parameter MUST be
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the URI of the ``/login/cas/ticket`` endpoint, including the ``redirectUrl``
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query parameter. Because the homeserver may not know its base URI, this may
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also require manual configuration.
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.. TODO-spec:
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It might be nice if the server did some validation of the ``redirectUrl``
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parameter, so that we could check that aren't going to redirect to a non-TLS
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endpoint, and to give more meaningful errors in the case of
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faulty/poorly-configured clients.
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Handling the authentication endpoint
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When responding to the ``/login/cas/ticket`` endpoint, the server MUST make a
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request to the CAS server to validate the provided ticket. The server MAY also
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check for certain user attributes in the response. Any required attributes
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should be configured by the server administrator.
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Once the ticket has been validated, the server MUST map the CAS ``user_id``
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to a valid `Matrix user identifier <../index.html#user-identifiers>`_. The
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guidance in `Mapping from other character sets
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<../index.html#mapping-from-other-character-sets>`_ may be useful.
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If the generated user identifier represents a new user, it should be registered
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as a new user.
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Finally, the server should generate a short-term login token. The generated
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token should be a macaroon, suitable for use with the ``m.login.token`` type of
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the |/login|_ API, and `token-based interactive login <#token-based>`_. The
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lifetime of this token SHOULD be limited to around five seconds.
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.. |/login| replace:: ``/login``
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.. _/login: #post-matrix-client-%CLIENT_MAJOR_VERSION%-login
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.. |/login/cas/redirect| replace:: ``/login/cas/redirect``
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.. _/login/cas/redirect: #get-matrix-client-%CLIENT_MAJOR_VERSION%-login-cas-redirect
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.. |/login/cas/ticket| replace:: ``/login/cas/ticket``
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.. _/login/cas/ticket: #get-matrix-client-%CLIENT_MAJOR_VERSION%-login-cas-ticket
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