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

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 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 (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 MUST require User-Interactive Authentication (UIA) unless:

• there is no existing cross-signing master key uploaded to the Homeserver, OR
• there is an existing cross-signing master key and it exactly matches the cross-signing master key provided in the request body. If there are any additional keys provided in the request (self signing key, user signing key) they MUST also match the existing keys stored on the server. In other words, the request contains no new keys. If there are new keys, UIA MUST be performed.

In these scenarios, this endpoint is not protected by UIA. This means the client does not need to send an auth JSON object with their request.

This change allows clients to freely upload 1 set of keys, but not modify/overwrite keys if they already exist. By allowing clients to upload the same set of keys more than once, this makes this endpoint idempotent in the case where the response is lost over the network, which would otherwise cause a UIA challenge upon retry.

Potential issues

See security considerations below.

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.

Existing users without E2EE keys

Existing user accounts who do not already have cross-signing keys set up will now be able to upload keys without UIA. If such a user has their access token compromised, an attacker will be able to upload a master_key, self_signing_key and user_signing_key. This is a similar threat model to a malicious server admin replacing these keys in the homeserver database.

This does not mean:

• the attacker can "take over the account". It does not allow the attacker to login as they need to know the password to the account. Likewise, an attacker cannot logout all devices nor can they logout specific devices as these also go through UIA prompts.
• the device will appear as verified to other users. Other users need to verify the public key out-of-band. As the true owner of the account is not performing this verification, if an attacker physically met up with other users it would become obvious that this is not the true owner, and hence no verification would be performed.

The main usability issue around this endpoint is requiring UIA, so it is critical that we only require UIA when absolutely necessary for the security of the account. In practice, this means requiring UIA when keys are replaced. There have been suggestions to reintroduce a grace period (e.g after initial device login) or just mandate it entirely for these old existing accounts. This would negatively impact usability because:

• it introduces temporal variability which becomes difficult to debug. Not all users would be subject to these timers/mandates. As a result, it needs to be possible to detect in a bug report if the client is one of these special cases, and this is hard to do reliably, particularly when bug reports from other servers are involved. The kinds of bugs being debugged are typically around encryption, where there are complex interactions between different devices and continually changing server-side state. These kinds of bugs are incredibly time-sensitive as it is, and adding more temporal variability makes it even harder to debug correctly.
• it introduces configuration variability which becomes difficult to debug. It's not clear what the grace period should actually be. Anything less than 1 hour risks catching initial x-signing requests from users who are on particularly awful networks. However, even increasing this to 1 hour poses the risk that we incorrectly catch the initial upload (e.g the client logs in on a bad GSM connection, then give up waiting for it to login and close the app, only reopening it the next day). This becomes difficult to debug in bug reports, as they just report HTTP 401s and it is unknown what the HS configuration is for the time delay. This is seen already due to the use (or non-use) of ui_auth.session_timeout. A spec-mandated grace period would mitigate some of these concerns, which could be further improved by adding special error codes indicating that the grace period had expired. However, this adds extra API surface that will likely be poorly tested in clients, as it's unreasonable to wait 1+ hours in a test, hence some configuration would be likely included for testing purposes anyway.

For these reasons, this MSC does not specify a grace period or treat some user accounts without existing cross-signing keys as special.

Unstable prefix

Not applicable as client behaviour need not change.

Dependencies

None.