Proposal to remove `m.login.token` ui auth type

proposals/2611-remove-login-auth-type.md

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+# MSC2611: Remove `m.login.token` User-Interactive Authentication type from the specification
+
+The client-server API specification
+[defines](https://matrix.org/docs/spec/client_server/r0.6.1#authentication-types)
+a number of "authentication types" for use with the User-Interactive
+Authentication protocol.
+
+Of these, `m.login.token` is unused and confusing. This MSC proposes removing it.
+
+## Proposal
+
+The definition of
+[token-based](https://matrix.org/docs/spec/client_server/r0.6.1#token-based)
+authentication is unclear about how this authentication type should be used. It
+suggests "via some external service, such as email or SMS", but in practice
+those validation mechanisms have their own token-submission mechanisms (for
+example, the
+`submit_url` field of the responses from
+[`/account/password/email/requestToken`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-account-password-email-requesttoken)
+and
+[`/account/password/msisdn/requestToken`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-account-password-msisdn-requesttoken)
+respectively). Additionally, the specification requires that "the server must
+encode the user ID in the token", which seems at odds with any token which can
+be sent to a user over SMS.
+
+Additional confusion stems from the presence of an `m.login.token` [login
+type](https://matrix.org/docs/spec/client_server/r0.6.1#login), which is used
+quite differently: it forms part of the single-sign-on login flow. For clarity:
+this proposal does not suggest making any changes to the `m.login.token` login
+type.
+
+In practice, we are not aware of any implementations of the `m.login.token`
+authentication type, and the inconsistency adds unnecessary confusion to the
+specification.
+
+## Potential Issues
+
+It's possible that somebody has found a use for this mechanism. However, that
+would necessarily entail some custom development of clients and servers, so is
+not materially affected by the removal from the specification.