matrix-doc/content/rooms/fragments/v4-event-explainer.md

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The event ID is the reference hash of the event encoded using a variation of Unpadded Base64 which replaces the 62nd and 63rd characters with - and _ instead of using + and /. This matches RFC4648's definition of URL-safe base64. Event IDs are still prefixed with $ and may result in looking like $Rqnc-F-dvnEYJTyHq_iKxU2bZ1CI92-kuZq3a5lr5Zg.

Just like in room version 3, event IDs should not be sent over federation to servers when the room uses this room version. On the receiving end of an event, the server should compute the relevant event ID for itself. Room version 3 also changes the format of auth_events and prev_events in a PDU.