matrix-spec/proposals/1884-replace-slashes-in-event_ids.md

MSC1884: Proposal to replace slashes in event IDs

MSC1659 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:

• The act of escaping and unescaping slash characters when manually calling the API during devops work becomes an constant and annoying chore which is entirely avoidable. Whenever using tools like curl and grep or manipulating SQL, developers will have to constantly keep in mind whether they are dealing with escaped or unescaped IDs, and manually convert between the two as needed. This will only get worse with further keys-as-IDs landing with MSC1228.

• 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 remember 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 than 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.

• Cosmetically, URL-escaping base64 in otherwise-constant-length IDs results in variable length IDs, making it harder to visually scan lists of IDs and manipulate them in columnar form when doing devops work.

• Those developing against the CS API might reasonably expect us to use URL-safe identifiers in URLs where available, rather than deliberately choosing non-URL-safe IDs, which could be seen as developer-unfriendly.

Proposal

This MSC proposes that we should introduce a new room version, in which event IDs are encoded using the URL-safe Base64 encoding (which uses - and _ as the 62nd and 63rd characters instead of + and /).

We will then aim to use URL-safe Base64 encoding across Matrix in future, such that typical CS API developers should be able to safely assume that for all common cases (including upcoming MSC1228 identifiers) they should use URL-safe Base64 when decoding base64 strings.

The exception would be for E2EE data (device keys and signatures etc) which currently use normal Base64 with no easy mechanism to migrate to a new encoding. Given E2EE development is rare and requires expert skills, it seems acceptable to expect E2EE developers to be able to use the right encoding without tripping up significantly.

Similarly, the S2S API could continue to use standard base64-encoded hashes and signatures in the places it does today, given they are only exposed to S2S API developers who are necessarily expert and should be able to correctly pick the right encoding.

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.

   However, the current uses of standard Base64 encodings are not exposed to common CS API developers, and so whilst this might be slightly confusing for the minority of expert homeserver developers, the confusion does not exist today for client developers (except those implementing E2EE). Therefore it seems safe to standardise on URL-safe Base64 for identifiers exposed to the client developers, who form by far the majority of the Matrix ecosystem today, and expect as simple an API as possible.

   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.

   The counter-counterargument to this is that it is of course best practice for implementations is to URL-escape any IDs used in URLs, and human-selected IDs such as Room aliases, Group IDs, Matrix user IDs etc apply an adequate forcing function already to remind developers to do this. However, it doesn't follow that we should then also deliberately pick URL-unsafe encodings for machine-selected IDs - the argument that it is better for software to fail 50% of the time to force a fix is irrelevant when the possibility exists for the software to fail 0% of the time in the first place by picking an identifier format which cannot fail.

[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 (though other solutions to that are also possible).

Conclusion

There are two main questions here:

1. Whether it's worth forcing CS API developers to juggle escaping of machine-selected IDs during manual use of the API in order to remind them to escape all variables in their URIs correctly when writing code.

2. Whether it's a significant problem for E2EE & SS API developers to have to handle strings which are a mix of standard Base64 and URL-safe Base64 encodings.

Both of these are a subjective judgement call.

Given we wish the CS API particularly to be as easy as possible for manual use, it feels that we should find another way to encourage developers to escape variables in their URLs in general - e.g. by recommending that developers test their clients against a 'torture room' full of exotic IDs and data, or by improving warnings in the spec... rather than (ab)using machine-selected IDs as a reminder.

Meanwhile, given we have many more people manually invoking the CS API than developing on the SS or E2EE APIs, and we wish to make the CS API particularly easy for developers to manually invoke, it feels we should not prioritise consistency of encodings for SS/E2EE developers over the usability of the CS API.

Therefore, on balance, it seems plausible that changing the format of event IDs does solve sufficient problems to make it desirable.