MSC2285: Private read receipts (#2285)

* What if we let people hide their read receipts?

* Add unstable prefix

* Update proposals/2285-hidden-read-receipts.md

Co-authored-by: Šimon Brandner <[email protected]>

* Use a separate `receiptType` for hidden read receipts  (#3750)

* Update the way MSC2285 works

Signed-off-by: Šimon Brandner <[email protected]>

* Update proposals/2285-hidden-read-receipts.md

Co-authored-by: Travis Ralston <[email protected]>

* Clarify notification behaviour and other things

Co-authored-by: Patrick Cloke <[email protected]>

* Mention adding `m.fully_read` to `/receipt`

* Rename MSC to `Private read receipts`

Signed-off-by: Šimon Brandner <[email protected]>

* Apply suggestions from review

Co-authored-by: Richard van der Hoff <[email protected]>

* Apply suggestions from review

Co-authored-by: Richard van der Hoff <[email protected]>

* Remove trailing comma

Co-authored-by: Richard van der Hoff <[email protected]>

* Apply suggestions from review

Signed-off-by: Šimon Brandner <[email protected]>

* Apply suggestions from review

Signed-off-by: Šimon Brandner <[email protected]>

* `r0` -> `v3`

Signed-off-by: Šimon Brandner <[email protected]>

* Describe how to determine server support while stable

Signed-off-by: Šimon Brandner <[email protected]>

* Be more precise about the failure mode

Signed-off-by: Šimon Brandner <[email protected]>

* Improve wording

Co-authored-by: Richard van der Hoff <[email protected]>

* Improve wording

Co-authored-by: Richard van der Hoff <[email protected]>

* Improve wording

Signed-off-by: Šimon Brandner <[email protected]>

* Be more explicit

Signed-off-by: Šimon Brandner <[email protected]>

* Be more explicit around server support

Signed-off-by: Šimon Brandner <[email protected]>

* Simplify text

Signed-off-by: Šimon Brandner <[email protected]>

* Be more explicit about detecting server support

Signed-off-by: Šimon Brandner <[email protected]>

* Add note about consistency

Signed-off-by: Šimon Brandner <[email protected]>

* Clarify how exactly to detect support

* Try to fix clarity around precedence

* Be clearer about alternatives

Signed-off-by: Šimon Brandner <[email protected]>

* Add sentence about `/receipt` federation

Signed-off-by: Šimon Brandner <[email protected]>

* Be more explicit about adding `m.read.private` to `/receipt`

Signed-off-by: Šimon Brandner <[email protected]>

* Fix wrong RR type

Co-authored-by: Kévin Commaille <[email protected]>

Co-authored-by: Šimon Brandner <[email protected]>
Co-authored-by: Patrick Cloke <[email protected]>
Co-authored-by: Richard van der Hoff <[email protected]>
Co-authored-by: Kévin Commaille <[email protected]>

proposals/2285-hidden-read-receipts.md

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+# MSC2285: Private read receipts
+
+Currently users must send read receipts in order to affect their notification
+counts, which alerts other people that the user has read their message. For
+primarily privacy reasons, it may be desirable to users to not advertise to
+others that they've read a message.
+
+## Proposal
+
+This MSC proposes adding a new `receiptType` (see [the receipts
+spec](https://spec.matrix.org/v1.3/client-server-api/#receipts)) of
+`m.read.private`. This `receiptType` is used when the user wants to affect their
+notification count but doesn't want other users to see their read receipt.
+
+To move the user's private read receipt to `$123` the client can make a POST
+request to the [`/receipt`
+endpoint](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidreceiptreceipttypeeventid).
+For example:
+
+```HTTP
+POST /_matrix/client/v3/rooms/!a:example.org/receipt/m.read.private/$123
+{}
+```
+
+The MSC also proposes adding `m.fully_read` and `m.read.private` as a possible
+`receiptType` for `/receipt` to make this endpoint consistent with
+`/read_markers`. (we have two endpoints that do essentially the same thing, so
+it would make sense for them to be consistent)
+
+Alternatively, the client can move the user's `m.fully_read` marker and/or
+`m.read` receipt at the same time as `m.read.private` by making a POST request
+to the [`/read_markers`
+endpoint](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidread_markers).
+For example:
+
+```HTTP
+POST /_matrix/client/r0/rooms/!a:example.org/read_markers
+{
+    "m.fully_read": "$123",
+    "m.read": "$123",
+    "m.read.private": "$123"
+}
+```
+
+Both `m.read` and `m.read.private` clear notifications in the same way. If the
+user sent two receipts into a room, the later one should be the one that decides
+the notification count.
+
+The receipt that is more "ahead" of the other takes precedence when considering
+notifications and a client's rendering of read receipts. This means that given
+an ordered set of events A, B, C, and D the public read receipt could be at
+point C, private at point A. If the user moves the private receipt from A to B
+then the user's notification count is still considered from point C as the public
+receipt is further ahead, still. Other users would also see the user's public read
+receipt as not having moved. The user can then move the private read receipt
+to point D, hopping over the public receipt, to change their notification count.
+
+For clarity, if the public receipt is "fast forwarded" to be at the same position
+as the private receipt then the public receipt is broadcast to other users, even
+if previously considered private.
+
+Note that like regular read receipts today, neither receipt can cause a backwards
+movement: both receipts can only move forwards, but do not have to be ahead of
+each other. It's valid to, for example, update a public read receipt which lags
+20 messages behind the private one.
+
+The `m.fully_read` property is now optional for the [`/read_markers`
+endpoint](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidread_markers)
+as sometimes we only want to send `m.read.private`.
+
+The MSC proposes that from now on, not all things sent over `/receipt` are
+federated. Servers MUST NOT send receipts of `receiptType` `m.read.private` to
+any other user than the sender. Servers also MUST NOT send receipts of
+`receiptType` `m.read.private` to any server over federation.
+
+## Security considerations
+
+Servers could act as if `m.read.private` is the same as `m.read` so the user
+must already trust the homeserver to a degree however, and the methods of
+notifying the user to the problem are difficult to implement. Users can always
+run their own homeservers to ensure it behaves correctly.
+
+## Potential issues
+
+Clients which support read receipts would end up rendering the user's receipt as
+jumping down when they send a message. This is no different from how IRC and
+similarly bridged users are perceived today.
+
+## Alternatives
+
+It has been suggested to use account data to store the setting that controls
+whether read receipts should be private on a per-account/per-room basis. While
+this might have some benefits, it is much less flexible.
+
+Previous iterations of this MSC additionally suggested that having an `m.hidden`
+flag on existing read receipts could work, however this feels like assigning too
+much responsibility to an existing structure.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should use
+`org.matrix.msc2285` as a namespace.
+
+|Stable (post-FCP)|Unstable                         |
+|-----------------|---------------------------------|
+|`m.read.private` |`org.matrix.msc2285.read.private`|
+
+Clients should check for server support before sending private read receipts:
+if the server does not support them, then a private read receipt will not clear
+any notifications for the user.
+
+The presence of `org.matrix.msc2285` or `org.matrix.msc2285.stable` in
+`unstable_features` is a reliable indication that a server supports private read
+receipts; however the converse is not true: their absence does not necessarily
+mean that the server does *not* support private read receipts. In particular,
+the server may have been updated to a future spec version which includes
+private read receipts, and hence removed the `unstable_features` entry.
+
+Therefore, if a client has this feature enabled, but the server does not advertise
+support for this MSC in `unstable_features`, the client should either keep sending
+private read receipts with the risk that notifications will not be clearing, or it
+should warn the user and start sending public read receipts instead.
+
+To mitigate this problem, once this MSC gets merged and once it becomes a part of a
+spec version, clients should update their implementations as fast as possible to
+accommodate the fact that the way of detecting server support will change: clients
+will now be looking for that spec version in `/versions`.
+
+### While the MSC is unstable
+
+During this period, to detect server support clients should check for the
+presence of the `org.matrix.msc2285` flag in `unstable_features` on `/versions`.
+Clients are also required to use the unstable prefixes (see [unstable
+prefix](#unstable-prefix)) during this time.
+
+### Once the MSC is merged but not in a spec version
+
+Once this MSC is merged, but is not yet part of the spec, clients should rely on
+the presence of the `org.matrix.msc2285.stable` flag in `unstable_features` to
+determine server support. If the flag is present, clients are required to use
+stable prefixes (see [unstable prefix](#unstable-prefix)).
+
+### Once the MSC is in a spec version
+
+Once this MSC becomes a part of a spec version, clients should rely on the
+presence of the spec version, that supports the MSC, in `versions` on
+`/versions`, to determine support. Servers are encouraged to keep the
+`org.matrix.msc2285.stable` flag around for a reasonable amount of time
+to help smooth over the transition for clients. "Reasonable" is intentionally
+left as an implementation detail, however the MSC process currently recommends
+*at most* 2 months from the date of spec release.