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+# Changing Event IDs to be Hashes
+
+## Motivation
+
+Having event IDs separate from the hashes leads to issues when a server receives
+multiple events with the same event ID but different reference hashes. While
+APIs could be changed to better support dealing with this situation, it is
+easier and nicer to simply drop the idea of a separate event ID entirely, and
+instead use the reference hash of an event as its ID.
+
+## Identifier Format
+
+Currently hashes in our event format include the hash name, allowing servers to
+choose which hash functions to use. The idea here was to allow a gradual change
+between hash functions without the need to globally coordinate shifting from one
+hash function to another.
+
+However now that room versions exist, changing hash functions can be achieved by
+bumping the room version. Using this method would allow using a simple string as
+the event ID rather than a full structure, significantly easing their usage.
+
+One side effect of this would be that there would be no indication about which
+hash function was actually used, and it would need to be inferred from the room
+version. To aid debuggability it may be worth encoding the hash function into
+the ID format.
+
+**Conclusion:** Don't encode the hash function, since the hash will depend on
+the version specific redaction algorithm anyway.
+
+The proposal is therefore that the event IDs are a sha256 hash, encoded using
+[unpadded
+Base64](https://matrix.org/docs/spec/appendices.html#unpadded-base64), and
+prefixed with `$` (to aid distinguishing different types of identifiers). For
+example, an event ID might be: `$CD66HAED5npg6074c6pDtLKalHjVfYb2q4Q3LZgrW6o`.
+
+The hash is calculated in the same way as previous event reference hashes were,
+which is:
+
+1. Redact the event
+2. Remove `signatures` field from the event
+3. Serialize the event to canonical JSON
+4. Compute the hash of the JSON bytes
+
+Event IDs will no longer be included as part of the event, and so must be
+calculated by servers receiving the event.
+
+
+## Changes to Event Formats
+
+As well as changing the format of event IDs, we also change the format of the
+`auth_events` and `prev_events` keys in events to simply be lists of event IDs
+(rather than being lists of tuples).
+
+A full event would therefore look something like (note that this is just an
+illustrative example, and that the hashes are not correct):
+
+```json
+{
+  "auth_events": [
+    "$5hdALbO+xIhzcLTxCkspx5uqry9wO8322h/OI9ApnHE",
+    "$Ga0DBIICBsWIZbN292ATv8fTHIGGimwjb++w+zcHLRo",
+    "$zc4ip/DpPI9FZVLM1wN9RLqN19vuVBURmIqAohZ1HXg",
+  ],
+  "content": {
+    "body": "Here is the message content",
+    "msgtype": "m.message"
+  },
+  "depth": 6,
+  "hashes": {
+    "sha256": "M6/LmcMMJKc1AZnNHsuzmf0PfwladVGK2Xbz+sUTN9k"
+  },
+  "origin": "localhost:8800",
+  "origin_server_ts": 1548094046693,
+  "prev_events": [
+    "$MoOzCuB/sacqHAvgBNOLICiGLZqGT4zB16MSFOuiO0s",
+  ],
+  "room_id": "!eBrhCHJWOgqrOizwwW:localhost:8800",
+  "sender": "@anon-20190121_180719-33:localhost:8800",
+  "signatures": {
+    "localhost:8800": {
+      "ed25519:a_iIHH": "N7hwZjvHyH6r811ebZ4wwLzofKhJuIAtrQzaD3NZbf4WQNijXl5Z2BNB047aWIQCS1JyFOQKPVom4et0q9UOAA"
+    }
+  },
+  "type": "m.room.message"
+}
+```
+
+## Changes to existing APIs
+
+All APIs that accept event IDs must accept event IDs in the new format.
+
+For S2S API, whenever a server needs to parse an event from a request or
+response they must either already know the room version *or* be told the room
+version in the request/response. There are separate MSCs to update APIs where
+necessary.
+
+For C2S API, the only change clients will see is that the event IDs have changed
+format. Clients should already be treating event IDs as opaque strings, so no
+changes should be required. Servers must add the `event_id` when sending the
+event to clients, however.
+
+Note that the `auth_events` and `prev_events` fields aren't sent to clients, and
+so the changes proposed above won't affect clients.
+
+
+## Protocol Changes
+
+The `auth_events` and `prev_events` fields on an event need to be changed from a
+list of tuples to a list of strings, i.e. remove the old event ID and simply
+have the list of hashes.
+
+The auth rules also need to change:
+
+-   The event no longer needs to be signed by the domain of the event ID (but
+    still needs to be signed by the sender’s domain)
+
+-   We currently allow redactions if the domain of the redaction event ID
+    matches the domain of the event ID it is redacting; which allows self
+    redaction. This check is removed and redaction events are always accepted.
+    Instead, the redaction event only takes effect and is sent down to clients
+    if/when the original event is received, and the domain of the events'
+    senders match. (While this is clearly suboptimal, it is the only practical
+    suggestion)
+
+
+## Room Version
+
+There will be a new room version v3 that is the same as v2 except uses the new
+event format proposed above. v3 will be marked as 'stable' as defined in [MSC1804](https://github.com/matrix-org/matrix-doc/blob/travis/msc/room-version-client-advertising/proposals/1804-advertising-capable-room-versions.md)
+