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Kegan DougalandClaude Opus 5
c692133ed4
An `m.room.member` event in a member key room carries two signatures: the member key named by `sender` signs the event, and the server named by `content.domain` co-signs the claim that the key in `state_key` belongs to `content.localpart` there. That co-signature was made under the signing server's own name, which leaves a reader unable to tell what is being asserted: an invite is sent by one member key and co-signed by the *invitee's* server, so a signature by `example.com` on it could plausibly stand behind either the sender or the target. Name the entity for the member the claim is about instead. The constant `state_key` joins the existing constant `sender`, and a signature under it is, by construction, an assertion about the member named by the event's `state_key` whoever sent the event. The key ID stays the signing server's real key ID, since that is what names the published key a verifier must check it against; only member keys use the constant `ed25519:1`. A signature under a server's name is now refused outright rather than treated as linkage information, so the only entities an event in such a room may carry are `sender`, `state_key` and the authorising member key of a restricted join. Nothing yet verifies a co-signature, so this changes only the events this server produces, and only in member key rooms. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>