Key backup: add PUT /room_keys/version/{version}

[manuroe]

to allow matrix clients to add signatures to an existing backup or update any information in the auth_data field.

[uhoreg]

A couple of minor issues, but overall looks pretty good. I'm wondering, though, if we should say that the server will ensure that the public key in the auth_data is the same as the previous value, though that means that the server must understand the different backup formats.

[uhoreg]

AIUI, in REST, the contents of PUT are supposed to be the full object that is to be stored, so I think algorithm should be required.

[uhoreg]

Having the {version} optional seems like it makes it a bit too easy for a client to accidentally overwrite a new backup version with an old backup version. I think we should just make the version required.

[uhoreg]

There's a race condition in this API for use with updating signatures. If the user is setting up backup on two devices A and B, they both fetch the backup version info, and then both sign the backup info, and then both send their updates to the server, then the server will only end up with one of the signatures. This might not be too bad in practice, but clients should do what they can to avoid this situation (e.g. re-fetching the backup info just before signing and sending, so that they have as much up-to-date information as they can get).

[manuroe]

I'm wondering, though, if we should say that the server will ensure that the public key in the auth_data is the same as the previous value, though that means that the server must understand the different backup formats.

Given we specify m.megolm_backup.v1.curve25519-aes-sha2 in this spec, we could require the server to make additional check when encountering that format.

There's a race condition in this API for use with updating signatures

You are right. Client should manage it.