initial proposal of `m.key.verification.accept`

proposals/xxxx-key-verification-accept.md

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+# Key verification flow addition: `m.key.verification.accept`
+
+The current key verification framework is asymmetrical in that the user who
+requests the verification is unable to select the key verification method.
+This makes it harder for more experienced users who wish to guide less
+experienced users through the verification process, especially if they are not
+verifying in-person, but are using a trusted but remote channel of verification
+(such as telephone or video conference).
+
+## Proposal
+
+A new event type is added to the key verification framework:
+`m.key.verification.accept`, which may be sent by the target of the
+`m.key.verification.request` message, upon receipt of the
+`m.key.verification.request` event.  It has the fields:
+
+- `from_device`: the ID of the device that sent the `m.key.verification.accept`
+  message
+- `methods`: an array of verification methods that the device supports
+
+It also has the usual `transaction_id` or `m.relates_to` fields for key
+verification events, depending on whether it is sent as a to-device event
+or an in-room event.
+
+After the `m.key.verification.accept` event is sent, either party can send an
+`m.key.verification.start` event to begin the verification.  If both parties
+send an `m.key.verification.start` event, and they both specify the same
+verification method, then the event sent by the user whose ID is the smallest
+is used, and the event sent by the user whose ID is the largest is ignored.  In
+the case of a single user verifying two of their devices, the device ID is
+compared instead.  If both parties send an `m.key.verification.start` event,
+but they specify different verification methods, the verification should be
+cancelled with a `code` of `m.unexpected_message`.
+
+The `m.key.verification.accept` event is optional; the recipient of the
+`m.key.verification.request` event may respond directly with a
+`m.key.verification.start` event instead.  This is for compatibility with the
+current version of the spec.
+
+## Potential issues
+
+There are now three possible ways that a key verification can be performed:
+
+1. A device begins a verification by sending an `m.key.verification.start`
+   event.  This is only possible for to-device verification.
+2. A device sends an `m.key.verification.request` event and the recipient
+   replies with an `m.key.verification.start` event.
+3. A device sends an `m.key.verification.request` event and the recipient
+   replies with an `m.key.verification.accept` event, and which point, either
+   device can send an `m.key.verification.start` event to begin the
+   verification.
+
+This increases the complexity of implementations.