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The event ID is the [reference
hash](/server-server-api#calculating-the-reference-hash-for-an-event) of
the event encoded using a variation of [Unpadded
Base64](/appendices#unpadded-base64) which replaces the 62nd and
63rd characters with `-` and `_` instead of using `+` and `/`. This
matches [RFC4648's definition of URL-safe
base64](https://tools.ietf.org/html/rfc4648#section-5). 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.