|
|
|
# Remove prev_content from the essential keys list
|
|
|
|
|
|
|
|
Matrix supports the concept of event redaction. The ability to redact rather
|
|
|
|
than delete is necessary because some events e.g. membership events are
|
|
|
|
essential to the protocol and _cannot_ be deleted. Therefore we do not delete
|
|
|
|
events outright and instead redact them. This involves removing all keys from
|
|
|
|
an event that are not required by the protocol. The stripped down event is
|
|
|
|
thereafter returned anytime a client or remote server requests it.
|
|
|
|
|
|
|
|
|
|
|
|
## Proposal
|
|
|
|
|
|
|
|
[The redaction algorithm](https://matrix.org/docs/spec/client_server/r0.4.0.html#redactions)
|
|
|
|
defines which keys must be retained through a redaction. Currently it lists
|
|
|
|
```prev_content``` as a key to retain, though in practice there is no need to
|
|
|
|
do so at the protocol level.
|
|
|
|
|
|
|
|
The proposal is simply to remove ```prev_content``` from the essential keys
|
|
|
|
list.
|
|
|
|
|
|
|
|
Note: the inclusion of ```prev_content``` in the essential keys list was
|
|
|
|
unintentional and should be considered a spec bug. Synapse (and other server
|
|
|
|
implementations) have not implemented the bug and already omit
|
|
|
|
```prev_content``` from redacted events.
|
|
|
|
|
|
|
|
|
|
|
|
## Tradeoffs
|
|
|
|
|
|
|
|
When sending events over federation the events are [hashed and
|
|
|
|
signed](https://matrix.org/docs/spec/server_server/unstable.html#adding-hashes-and-signatures-to-outgoing-events),
|
|
|
|
this involves operating not only on the original event but also the redacted
|
|
|
|
form of the event. The redacted hash and redacted signed event are necessary if
|
|
|
|
the event is ever redacted in future. As a result, any change of the essential
|
|
|
|
keys list must be managed carefully. If disparate servers implement different
|
|
|
|
versions of the redaction algorithm (for a given event) attempts to send the
|
|
|
|
event over federation will fail.
|
|
|
|
|
|
|
|
We _could_ manage this change via room versioning and create a new room
|
|
|
|
version that implements this MSC. However, because the federation already
|
|
|
|
omits the ```prev_content``` key by convention, implementing this MSC only in
|
|
|
|
the new room version would mean that the entire existing federation would not
|
|
|
|
be spec compliant.
|
|
|
|
|
|
|
|
As a result it seems pragmatic to have the spec reflect reality, acknowledge
|
|
|
|
that the spec and federation have deviated and instead update the spec
|
|
|
|
retrospectively to describe the de-facto redaction algorithm.
|
|
|
|
|
|
|
|
## Potential issues
|
|
|
|
|
|
|
|
It is theoretically possible that a closed federation could exist whose servers
|
|
|
|
do follow the spec as is. This MSC would render those servers non-compliant with
|
|
|
|
the spec. On balance this seems unlikely and in the worst case those
|
|
|
|
implementors could add the change to a subsequent room version, eventually
|
|
|
|
reaching spec consistency as older room versions are deprecated.
|
|
|
|
|
|
|
|
Another scenario is that a client may redact events according to the spec as is
|
|
|
|
and persist prev_content through the redaction, thereby diverting from that on
|
|
|
|
the server(s). Client authors will have to update their code to drop
|
|
|
|
```prev_content``` - however, given that prev_content should not be used in
|
|
|
|
important calculations and/or visualisations, this ought to be a relatively
|
|
|
|
non-invasive change.
|
|
|
|
|
|
|
|
|
|
|
|
## Security considerations
|
|
|
|
|
|
|
|
A further reason to support removal of ```prev_content``` is the case where a
|
|
|
|
malicious user adds illegal or abusive content into a state event and then
|
|
|
|
overwrites that state event. The content would then be preserved through the
|
|
|
|
redaction.
|
|
|
|
|
|
|
|
Additionally, there are plenty of reasons to have security concerns over a
|
|
|
|
precedent that the federation can deviate from the spec.
|
|
|
|
|
|
|
|
## Conclusions
|
|
|
|
Removing ```prev_content``` is pragmatic response to the current situation. It
|
|
|
|
alligns the federation and the spec, and does so in a way that removes
|
|
|
|
unecessary overhead.
|