MSC3967: Do not require UIA when first uploading cross signing keys

proposals/3967-device-signing-upload-uia.md

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+# MSC3967: Do not require UIA when first uploading cross signing keys
+
+When a user first sets up end-to-end encryption cross-signing, their client uploads their cross-signing keys to the server.
+
+This [upload operation](https://spec.matrix.org/v1.6/client-server-api/#post_matrixclientv3keysdevice_signingupload)
+requires a higher level of security by applying User-Interactive Auth (UIA) to the endpoint.
+
+This creates a usability issue at the point of user registration where a client will typically want to immediately set
+up cross-signing for a new user.
+
+The issue is that the client will immediately need the user to re-authenticate even though the user just authenticated.
+
+This usability issue has given rise to workarounds such as a 
+[configurable grace period](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#ui_auth)
+(`ui_auth`.`session_timeout`) in Synapse whereby UIA will not be required for uploading cross-signing keys where
+authentication has taken place recently.
+
+This proposal aims to provide for a standard way to address this UIA usability issue with respect to setting up cross-signing.
+
+## Proposal
+
+For the `POST /_matrix/client/v3/keys/device_signing/upload` endpoint the Homeserver should only require User-Interactive
+Authentication (UIA) if the user already has an existing cross-signing master key uploaded to the Homeserver.
+
+## Potential issues
+
+The Client-Server API [spec](https://spec.matrix.org/v1.6/client-server-api/#user-interactive-api-in-the-rest-api) states:
+
+> A request to an endpoint that uses User-Interactive Authentication never succeeds without auth. Homeservers may allow
+requests that don’t require auth by offering a stage with only the m.login.dummy auth type, but they must still give a
+401 response to requests with no auth data.
+
+Does this mean that the endpoint can't simply give back a `200` response in the case that `auth` was not given as an
+input? If this is the case then the existing Synapse behaviour of allowing a UIA grace period is probably also non-compliant.
+
+## Alternatives
+
+There has been some discussion around how to improve the usability of cross-signing more generally. It may be that an
+alternative solution is to provide a way to set up cross-signing in a single request.
+
+## Security considerations
+
+This change could be viewed as a degradation of security at the point of setting up cross-signing in that it requires
+less authentication to upload cross-signing keys on first use.
+
+However, this degradation needs to be weighed against the typical real world situation where a Homeserver will be
+applying a grace period and so allow a malicious actor to bypass UIA for a period of time after each authentication.
+
+## Unstable prefix
+
+Not applicable as client behaviour need not change.
+
+## Dependencies
+
+None.