Spec PR - MSC4147: Including device keys with Olm-encrypted events (#2122)

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>
Co-authored-by: Richard van der Hoff <richard@matrix.org>

changelogs/client_server/newsfragments/2122.feature

@@ -0,0 +1 @@
+Include device keys with Olm-encrypted events as per [MSC4147](https://github.com/matrix-org/matrix-spec-proposals/pull/4147).

content/client-server-api/modules/end_to_end_encryption.md

@@ -1512,40 +1512,11 @@ message.
 
 The plaintext payload is of the form:
 
-```json
+{{% definition path="api/client-server/definitions/olm_payload" %}}
-{
-  "type": "<type of the plaintext event>",
-  "content": "<content for the plaintext event>",
-  "sender": "<sender_user_id>",
-  "recipient": "<recipient_user_id>",
-  "recipient_keys": {
-    "ed25519": "<our_ed25519_key>"
-  },
-  "keys": {
-    "ed25519": "<sender_ed25519_key>"
-  }
-}
-```
 
 The type and content of the plaintext message event are given in the
 payload.
 
-Other properties are included in order to prevent an attacker from
-publishing someone else's curve25519 keys as their own and subsequently
-claiming to have sent messages which they didn't. `sender` must
-correspond to the user who sent the event, `recipient` to the local
-user, and `recipient_keys` to the local ed25519 key.
-
-Clients must confirm that the `sender_key` property in the cleartext
-`m.room.encrypted` event body, and the `keys.ed25519` property in the
-decrypted plaintext, match the keys returned by
-[`/keys/query`](#post_matrixclientv3keysquery) for
-the given user. Clients must also verify the signature of the keys from the
-`/keys/query` response. Without this check, a client cannot be sure that
-the sender device owns the private part of the ed25519 key it claims to
-have in the Olm payload. This is crucial when the ed25519 key corresponds
-to a verified device.
-
 If a client has multiple sessions established with another device, it
 should use the session from which it last received and successfully
 decrypted a message. For these purposes, a session that has not received
@@ -1555,6 +1526,68 @@ maximum number of olm sessions that it will maintain for each device,
 and expiring sessions on a Least Recently Used basis. The maximum number
 of olm sessions maintained per device should be at least 4.
 
+###### Validation of incoming decrypted events
+
+{{% changed-in v="1.15" %}} Existing checks made more explicit, and checks for `sender_device_keys` added.
+
+After decrypting an incoming encrypted event, clients MUST apply the
+following checks:
+
+1.  The `sender` property in the decrypted content must match the
+    `sender` of the event.
+2.  The `keys.ed25519` property in the decrypted content must match
+    the `sender_key` property in the cleartext `m.room.encrypted`
+    event body.
+3.  The `recipient` property in the decrypted content must match
+    the user ID of the local user.
+4.  The `recipient_keys.ed25519` property in the decrypted content
+    must match the client device's [Ed25519 signing key](#device-keys).
+5.  Where `sender_device_keys` is present in the decrypted content:
+    1.  `sender_device_keys.user_id` must also match the `sender`
+        of the event.
+    2.  `sender_device_keys.keys.ed25519:<device_id>` must also match
+        the `sender_key` property in the cleartext `m.room.encrypted`
+        event body.
+    3.  `sender_device_keys.keys.curve25519:<device_id>` must match
+        the Curve25519 key used to establish the Olm session.
+    4.  The `sender_device_keys` structure must have a valid signature
+        from the key with ID `ed25519:<device_id>` (i.e., the sending
+        device's Ed25519 key).
+
+Any event that does not comply with these checks MUST be discarded.
+
+###### Verification of the sending user for incoming events
+
+{{% added-in v="1.15" %}}
+
+In addition, for each Olm session, clients MUST verify that the
+Curve25519 key used to establish the Olm session does indeed belong
+to the claimed `sender`. This requires a signed "device keys" structure
+for that Curve25519 key, which can be obtained in one of two ways:
+
+1.  An Olm message may be received with a `sender_device_keys` property
+    in the decrypted content.
+2.  The keys are returned via a [`/keys/query`](#post_matrixclientv3keysquery)
+    request. Note that both the Curve25519 key **and** the Ed25519 key in
+    the returned device keys structure must match those used in an
+    Olm-encrypted event as above. (In particular, the Ed25519 key must
+    be present in the **encrypted** content of an Olm-encrypted event
+    to prevent an attacker from claiming another user's Curve25519 key
+    as their own.)
+
+Ownership of the Curve25519 key is then established in one of two ways:
+
+1.  Via [cross-signing](#cross-signing). For this to be sufficient, the
+    device keys structure must be signed by the sender's self-signing key,
+    and that self-signing key must itself have been validated (either via
+    [explicit verification](#device-verification) or a "trust on first use" (TOFU) mechanism).
+2.  Via explicit verification of the device's Ed25519 signing key, as
+    contained in the device keys structure. This is no longer recommended.
+
+A failure to complete these verifications does not necessarily mean that
+the session is bogus; however it is the case that there is no proof that
+the claimed sender is accurate, and the user should be warned accordingly.
+
 ###### Recovering from undecryptable messages
 
 Occasionally messages may be undecryptable by clients due to a variety

data/api/client-server/definitions/olm_payload.yaml

@@ -0,0 +1,88 @@
+# Copyright 2025 The Matrix.org Foundation C.I.C
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+type: object
+title: OlmPayload
+description: |-
+  The plaintext payload of Olm message events.
+properties:
+  type:
+    type: string
+    description: The type of the event.
+  content:
+    type: object
+    description: The event content.
+  sender:
+    type: string
+    description: The user ID of the event sender.
+  recipient:
+    type: string
+    description: The user ID of the intended event recipient.
+  recipient_keys:
+    description: The recipient's signing keys of the encrypted event.
+    $ref: "#/components/schemas/SigningKeys"
+  keys:
+    $ref: "#/components/schemas/SigningKeys"
+    description: The sender's signing keys of the encrypted event.
+  sender_device_keys:
+    $ref: device_keys.yaml 
+    description: The sender's device keys.
+    x-addedInMatrixVersion: "1.15"
+required:
+  - type
+  - content
+  - sender
+  - recipient
+  - recipient_keys
+  - keys
+components:
+  schemas:
+    SigningKeys:
+      type: object
+      title: SigningKeys
+      description: Public keys used for an `m.olm.v1.curve25519-aes-sha2` event.
+      properties:
+        ed25519:
+          type: string
+          description: The Ed25519 public key encoded using unpadded base64.
+      required:
+        - ed25519
+example: {
+  "type": "<type of the plaintext event>",
+  "content": "<content for the plaintext event>",
+  "sender": "<sender_user_id>",
+  "recipient": "<recipient_user_id>",
+  "recipient_keys": {
+    "ed25519": "<our_ed25519_key>"
+  },
+  "keys": {
+    "ed25519": "<sender_ed25519_key>"
+  },
+  "sender_device_keys": {
+    "algorithms": ["<supported>", "<algorithms>"],
+    "user_id": "<user_id>",
+    "device_id": "<device_id>",
+    "keys": {
+      "ed25519:<device_id>": "<sender_ed25519_key>",
+      "curve25519:<device_id>": "<sender_curve25519_key>"
+    },
+    "signatures": {
+      "<user_id>": {
+        "ed25519:<device_id>": "<device_signature>",
+        "ed25519:<ssk_id>": "<ssk_signature>",
+      }
+    }
+  }
+}