Proposal for per-message profiles

Signed-off-by: Tulir Asokan <tulir@beeper.com>

proposals/4144-per-message-profile.md

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+# MSC4144: Per-message profiles
+Currently profiles in Matrix are defined by `m.room.member` state events, and
+there is no easy way to have different profiles per message.
+
+## Proposal
+The proposed solution is a new field called `m.per_message_profile`, which
+contains a displayname and/or avatar URL to override the default profile,
+plus an ID to identify different profiles within the same Matrix user.
+
+```json
+{
+  "msgtype": "m.text",
+  "body": "Hello, World!",
+  "m.per_message_profile": {
+    "id": "meow",
+    "displayname": "cat",
+    "avatar_url": "mxc://maunium.net/hgXsKqlmRfpKvCZdUoWDkFQo"
+  }
+}
+```
+
+The `id` field is required and is an opaque string. Clients may use it to group
+messages with the same ID like they would group messages from the same sender.
+For example, bridges would likely set it to the immutable remote user ID.
+
+### Encrypted avatars
+Because the profile is inside the ciphertext in encrypted events, the entire
+profile can be hidden the server, as long as the avatar is also encrypted.
+Encrypted avatars are placed under `avatar_file` instead of `avatar_url`.
+The `avatar_file` field has the same schema as the `file` field in
+`m.room.message` events.
+
+<details>
+<summary>Encrypted avatar example</summary>
+
+```json
+{
+  "msgtype": "m.text",
+  "body": "Hello, World!",
+  "m.per_message_profile": {
+    "id": "meow",
+    "displayname": "cat",
+    "avatar_file": {
+      "v": "v2",
+      "key": {
+        "alg": "A256CTR",
+        "ext": true,
+        "k": "8dXeBMBMthuXGY5zmUh9Mi0aqC1kndMZ4NCa-0RhELc",
+        "key_ops": [
+          "encrypt",
+          "decrypt"
+        ],
+        "kty": "oct"
+      },
+      "iv": "L6zup2cR570AAAAAAAAAAA",
+      "hashes": {
+        "sha256": "/cTs+PajUcznbV3h1w5gh1AHnLjrKQVl2jU3xLCqoBI"
+      },
+      "url": "mxc://maunium.net/eKLhozQduElYSgBkWjtwSXoi"
+    }
+  }
+}
+```
+
+</details>
+
+### Behavior of omitted and empty fields
+If the `displayname` field is omitted, null, or an empty string, the
+displayname from the member event should be used instead. Setting an empty
+displayname using a per-message profile is not supported, as there aren't any
+clear use cases for it.
+
+However, there are use cases for setting an empty avatar, so `avatar_url` being
+an empty string should be treated as clearing the avatar and falling back to
+the client's default blank avatar behavior (e.g. generating one based on the
+displayname). If both `avatar_url` and `avatar_file` are omitted or null, the
+avatar from the member event should be used instead.
+
+### Extensible profiles
+This MSC is not related to extensible profiles and does not attempt to
+implement them. However, in case extensible profiles are implemented as
+something that can be referenced (e.g. room IDs), the MSC adding them could
+allow per-message profiles to specify which extensible profile is used.
+
+## Use cases
+
+### Bridging
+Per-message profiles will allow making "lighter-weight" bridges that don't need
+appservice access. Currently the only option for such bridges is to prepend the
+displayname to the message, which is extremely ugly.
+
+Such bridges would obviously have downsides, like not being able to start chats
+via standard mechanisms, and not being able to see the member list on Matrix.
+However, those may be acceptable compromises for non-puppeting bridges that
+only operate in specific predetermined rooms.
+
+### Feature-parity with other platforms
+Other chat applications such as Slack and Discord have "webhooks" which allow
+per-message profile overrides. This MSC effectively enables the same on Matrix.
+
+### Roleplaying, plural users, etc
+Some users want to be able to switch between profiles quickly, which would be
+much easier using this MSC. Currently easiest way is to have multiple accounts,
+which has other benefits, but is much more cumbersome to manage.
+
+## Potential issues
+Implementing encrypted avatars could cause difficulty for clients that assume
+that avatars are always unencrypted mxc URIs.
+
+## Alternatives
+None.
+
+## Security considerations
+
+### Preventing impersonation
+To prevent impersonation using per-message profiles, clients should somehow
+indicate to the user that the message has a per-message profile with an easy
+way to see the user's MXID or default profile. For example, a client could have
+a small `via @user:example.com` text next to the per-message displayname.
+
+## Unstable prefix
+`com.beeper.per_message_profile` should be used instead of `m.per_message_profile`
+until this MSC is accepted.