Merge pull request #1721 from matrix-org/rav/proposal/sso_login

MSC1721: Rename m.login.cas to m.login.sso

proposals/1721-rename-cas-to-sso.md

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+# MSC1721: Rename `m.login.cas` to `m.login.sso`
+
+The Matrix Client-Server spec includes a [section on client login using Central
+Authentication Service
+(CAS)](https://matrix.org/docs/spec/client_server/r0.4.0.html#cas-based-client-login).
+
+The spec currently fails to mention it, but this process is triggered when [`GET
+/login`](https://matrix.org/docs/spec/client_server/r0.4.0.html#get-matrix-client-r0-login)
+returns a flow type of `m.login.cas`.
+
+Nothing in this flow is specific to CAS - it is equally applicable for other
+web-based single-sign-on processes, such as SAML.
+
+Accordingly, we should rename `cas` to `sso`.
+
+## Proposal
+
+1. `m.login.sso` should be defined as a valid login type for return from `GET
+   /login`. (We should probably mention `m.login.cas` in the spec while we are
+   there.)
+
+2. When a client wishes to use the SSO login type, it should redirect to
+   `/_matrix/client/r0/login/sso/redirect` (instead of
+   `/_matrix/client/r0/login/cas/redirect`).
+
+3. Servers should treat `/_matrix/client/r0/login/sso/redirect` identically to
+   `/_matrix/client/r0/login/cas/redirect`: they should issue a redirect to
+   their configured single-sign-on system.
+
+4. Servers which support `m.login.sso` should make sure they update their [login
+   fallback page](https://matrix.org/docs/spec/client_server/r0.4.0.html#login-fallback)
+   to understand the new login type.