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+# MSC1884: Proposal to replace slashes in event IDs
+
+[MSC1659](https://github.com/matrix-org/matrix-doc/pull/1659) mandated that,
+starting in version 3 rooms, event IDs must be calculated as a base64-encoding
+of a hash. This implies that event IDs may contain any character in the
+standard Base64 alphabet, which notably includes the slash character, `/`.
+
+Event IDs are often embedded in URI paths, and since the slash character is
+used as a separator in URI paths, this presents a problem. The immediate
+solution is to ensure that event IDs are URL-encoded, so that `/` is instead
+represented as `%2F`. However, this is not entirely satisfactory for a number
+of reasons:
+
+ * There exist a number of client (and possibly server) implementations which
+   do not currently URL-encode such parameters; these are therefore broken by
+   such event IDs and must be updated. Furthermore, all future client
+   implementers must remember to do the encoding correctly.
+
+ * Even if client implementations do rembember to URL-encode their parameters,
+   they may not do it correctly: many URL-encoding implementations may be
+   intended to encode parameters in the query-string (which can of course
+   contain literal slashes) rather tha the path component.
+
+ * Some proxy software may treat `%2F` specially: for instance, Apache, when
+   configured as a reverse-proxy, will reject requests for a path containing
+   `%2F` unless it is also configured with `nocanon`. Again this means that
+   existing setups will be broken by this change, and it is a trap for new
+   users of the software.
+
+## Proposal
+
+This MSC proposes that we should introduce a new room version, in which event
+IDs are encoded using the [URL-safe Base64
+encoding](https://tools.ietf.org/html/rfc4648#section-5) (which uses `-` and
+`_` as the 62nd and 63rd characters instead of `+` and `/`).
+
+## Counterarguments
+
+1. Inconsistency. Base64 encoding is used heavily elsewhere in the matrix
+   protocol and in all cases the standard encoding is used (though with some
+   variation as to the inclusion of padding characters). Further, SHA256 hashes
+   are used in a number of places and are universally included with standard,
+   unpadded Base64.
+
+   Changing event IDs alone would therefore leave us with a confusing mix of
+   encodings.
+
+   A potential extension would be to change *all* Base64 encodings to be
+   URL-safe. This would address the inconsistency. However, it feels like a
+   large job which would span the entire matrix ecosystem (far larger than
+   updating clients to URL-encode their URL prarameters), and again the
+   situation would be confusing while the transition was in progress.
+
+2. Incompleteness. Event IDs are certainly not the only identifier which can
+   contain slashes - Room aliases, Room IDs, Group IDs, User IDs [1], and state
+   keys can all contain slashes, as well as a number of identifiers whose
+   grammars are currently underspecified (eg transaction ids, event types,
+   device IDs). (Indeed, there was nothing preventing Event IDs from containing
+   slashes before room v3 - it just happened that Synapse used an algorithm
+   which didn't generate them).
+
+   All of these other identifiers can appear in URLs in either or both the
+   client-server or server-server APIs, and all have the potential to cause
+   misbehaviour if software does not correctly URL-encode them.
+
+   It can be argued that it is better for software to fail 50% of the time [2]
+   so that it can be fixed than it is to fail only on edge-cases or, worse,
+   when deliberately provoked by a malicious or "curious" actor.
+
+   Of course, an alternative is to modify the grammars of all of these
+   identifiers to forbid slashes.
+
+[1] Discussion remains open as to whether allowing slashes in User IDs was a
+good idea.
+
+[2] 48% of random 32-byte sequences will contain a slash when Base64-encoded.
+
+## Alternatives
+
+An alternative would be to modify all REST endpoints to use query or body
+parameters instead of path parameters.  This would of course be a significant
+and incompatible change, but it would also bring the benefit of solving a
+common problem where forgetting to use `nocanon` in a reverse-proxy
+configuration [breaks
+federation](https://github.com/matrix-org/synapse/issues/3294) (though other
+solutions to that are also possible).
+
+## Conclusion
+
+It's unclear to me that changing the format of event IDs alone solves any
+problems.