m.third_party_invite: Allow multiple public keys

event-schemas/examples/m.room.third_party_invite

@@ -3,7 +3,11 @@
   "content": {
     "display_name": "Alice Margatroid",
     "key_validity_url": "https://magic.forest/verifykey",
-    "public_key": "abc123"
+    "public_key": "abc123",
+    "public_keys": [{
+      "public_key": "def456",
+      "key_validity_url": "https://magic.forest/verifykey"
+    }]
   },
   "state_key": "pc98",
   "origin_server_ts": 1431961217939,

event-schemas/schema/m.room.third_party_invite

@@ -20,7 +20,25 @@
                 },
                 "public_key": {
                     "type": "string",
-            "description": "A base64-encoded ed25519 key with which token must be signed."
+                    "description": "A base64-encoded ed25519 key with which token must be signed (though a signature from any entry in public_keys is also sufficient). This exists for backwards compatibility."
+                },
+                "public_keys": {
+                    "type": "array",
+                    "description": "Keys with which the token may be signed.",
+                    "items": {
+                        "type": "object",
+                        "properties": {
+                            "public_key": {
+                                "type": "string",
+                                "description": "A base-64 encoded ed25519 key with which token may be signed."
+                            },
+                            "key_validity_url": {
+                                "type": "string",
+                                "description": "An optional URL which can be fetched, with querystring public_key=public_key, to validate whether the key has been revoked. The URL must return a JSON object containing a boolean property named 'valid'. If this URL is absent, the key must be considered valid indefinitely."
+                            },
+                        },
+                        "required": ["public_key"]
+                    }
                 }
             },
             "required": ["display_name", "key_validity_url", "public_key"]