Propose method of specifying custom images in reactions

Signed-off-by: Sumner Evans <sumner@beeper.com>

proposals/XXXX-custom-images-in-reactions.md

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+# MSCXXXX Custom Images in Reactions
+
+One of the most desired features within the Matrix ecosystem is the ability to
+react to messages with custom images. This feature is especially requested by
+users who come from Slack and Discord where this functionality is one of the
+main ways that the culture of a chat rooms develops.
+
+There is an existing proposal to
+[render image data in reactions (MSC3746)](https://github.com/matrix-org/matrix-spec-proposals/pull/3746/),
+but it has had little attention recently and has the flaw of not being conducive
+to deduplication (either on the client or server). Sorunome proposed a
+modification to that MSC to
+[use the MXC URI as the key](https://github.com/matrix-org/matrix-spec-proposals/pull/3746/files#r866285147)
+which this proposal adopts.
+
+This proposal is meant to replace
+[MSC3746](https://github.com/matrix-org/matrix-spec-proposals/pull/3746/) and is
+additionally intended to document the behaviour of existing clients and bridges.
+
+Like
+[MSC3746](https://github.com/matrix-org/matrix-spec-proposals/pull/3746/), this
+MSC does not propose a mechanism for providing a list of available images.
+
+## Proposal
+
+This proposal suggests two changes to events with the `m.annotation` relation.
+
+1. If the `key` of an `m.annotation` relation is an MXC URI of an image, clients
+   should render the referenced image instead of the key text.
+
+   Detecting if the `key` is an MXC URI can be as sophisticated as the client
+   wants, but this proposal recommends checking if the string starts with
+   `mxc://`.
+
+2. When the annotation's key is an MXC URI, a new (optional) `shortcode` key can
+   be added to the content of the event with a textual name for the image.
+
+   This shortcode should be shown by clients in situations such as hovering over
+   the annotation, as alt-text, or if the client does not support rendering
+   images.
+
+Example custom image reaction event content
+
+```json
+"content": {
+  "m.relates_to": {
+    "rel_type": "m.annotation",
+    "event_id": "$abcdefg",
+    "key": "mxc://matrix.org/VOczFYqjdGaUKNwkKsTjDwUa"
+  },
+  "shortcode": ":partyparrot:"
+}
+```
+
+## Potential issues
+
+### Clients rendering the MXC URI as text
+
+The biggest disadvantage is that clients that do not support rendering custom
+reactions will render the MXC URI as text. However, this is already problematic
+because many bridges and clients already support this MSC, and users likely
+already encounter this.
+
+### Un-renderable image referenced in the `key`
+
+The MXC URI could specify an asset that either does not exist, or is not a
+renderable image. Clients can opt to render the `shortcode` in these situations,
+or some placeholder/error image, or just opt to render the full key.
+
+## Alternatives
+
+### Use the shortcode as the `key`
+
+This is what was proposed by
+[MSC3746](https://github.com/matrix-org/matrix-spec-proposals/pull/3746/). The
+problem with this is that there could possibly be multiple distinct images with
+the same shortcode. Reactions are only deduplicated based on `key`, so clients
+and servers would group these distinct reactions together which is undesirable.
+
+### Put the `shortcode` as a key within `m.relates_to`
+
+Instead of being at the root of the `content` dictionary, the `shortcode` value
+could be included within `m.relates_to`. This is the wrong place to put this
+value because `m.relates_to` is meant to only contain information pertaining to
+the relationship between events, not information about the reaction event
+itself.
+
+## Security considerations
+
+### Image is unencrypted
+
+Reaction events are not encrypted, and so the MXC URI referenced by the key
+would have to be an unencrypted image. However, this is probably not a problem
+for the following reasons:
+
+- Custom reactions are most likely not sensitive information.
+
+- Users are already able to upload unencrypted content into encrypted rooms, so
+  this does not introduce any leakage that was not previously possible.
+
+- Clients can add UX to indicate to users that the reaction images are not
+  encrypted.
+
+## Unstable prefix
+
+Until this proposal is merged into the spec, the `shortcode` key should be
+prefixed with `com.beeper.mscXXXX.`.
+
+An unstable prefix for the `key` in `m.relates_to` is not necessary as the spec
+already allows arbitrary data to be used as the `key`. This MSC merely adds
+extra meaning to a specific class of key.