Merge pull request #3122 from uhoreg/deprecate_verification_start

MSC3122: Deprecate starting verifications without requesting first

proposals/3122-deprecate-starting-verifications-without-request.md

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+# MSC3122: Deprecate starting key verifications without requesting first
+
+Currently, the [Key verification
+framework](https://spec.matrix.org/unstable/client-server-api/#key-verification-framework)
+allows a device to begin a verification via to-device messages by sending an
+`m.key.verification.start` event without first sending or receiving an
+`m.key.verification.request` message.  (The last sentence of the 5th paragraph
+of the Key verification framework in the unstable spec, as of the time of
+writing.)  However, doing so does not provide a good user experience, and
+allowing this adds unnecessary complexity to implementations.
+
+We propose to deprecate allowing this behaviour.
+
+Note that verifications in DMs do not allow this behaviour.  Currently, Element
+Web is the only client known to do this.
+
+## Proposal
+
+The ability to begin a key verification by sending an
+`m.key.verification.start` event as a to-device event without a prior
+`m.key.verification.request` is deprecated.  New clients should not begin
+verifications in this way, but will still need to accept verifications begun in
+this way, until it is removed from the spec.
+
+## Potential issues
+
+None.
+
+## Alternatives
+
+We could do nothing and leave it in the spec.  But we should clean up cruft when
+possible.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+No unstable prefix is required since we are simply deprecating behaviour.