matrix-nio/tests/data/events/room_key_request_cancel.json

{
    "sender": "@alice:example.org",
    "content": {
        "action": "request_cancellation",
        "request_id": "1495474790150.19",
        "requesting_device_id": "RJYKSTBOIE"
    },
    "type": "m.room_key_request"
}