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