Merge pull request #1284 from Zil0/e2e_doc

Document missing parts of E2E

api/client-server/keys.yaml

@@ -194,7 +194,7 @@ paths:
                       "user_id": "@alice:example.com",
                       "device_id": "JLAFKJWSCS",
                       "algorithms": [
-                        "m.olm.curve25519-aes-sha256",
+                        "m.olm.v1.curve25519-aes-sha256",
                         "m.megolm.v1.aes-sha"
                       ],
                       "keys": {
@@ -247,7 +247,7 @@ paths:
                     description: algorithm
                     example: "signed_curve25519"
                 example:
-                  "@alice:example.com": { "JLAFKJWSCS": "curve25519" }
+                  "@alice:example.com": { "JLAFKJWSCS": "signed_curve25519" }
             required:
               - one_time_keys
       responses:

api/client-server/sync.yaml

@@ -253,6 +253,14 @@ paths:
                 description: |-
                   Information on end-to-end device updates, as specified in
                   |device_lists_sync|_.
+              device_one_time_keys_count:
+                title: One-time keys count
+                type: object
+                additionalProperties:
+                  type: integer
+                description: |-
+                  Information on end-to-end encryption keys, as specified
+                  in |device_lists_sync|_.
           examples:
             application/json: {
                 "next_batch": "s72595_4483_1934",

changelogs/client_server/newsfragments/1284.clarification

@@ -0,0 +1 @@
+Clarify ``changed`` field behaviour in device tracking process

changelogs/client_server/newsfragments/1284.feature

@@ -0,0 +1,7 @@
+End-to-end encryption for group chats:
+
+  - Olm and Megolm messaging algorithms.
+  - ``m.room.encrypted``, ``m.room.encryption``, ``m.room_key`` events.
+  - Device verification process.
+  - ``device_one_time_keys_count`` sync parameter.
+  - ``device_lists:left`` sync parameter.

event-schemas/examples/m.room.encrypted#megolm

@@ -0,0 +1,14 @@
+{
+  "content": {
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "ciphertext": "AwgAEnACgAkLmt6qF84IK++J7UDH2Za1YVchHyprqTqsg...",
+    "device_id": "RJYKSTBOIE",
+    "sender_key": "IlRMeOPX2e0MurIyfWEucYBRVOEEUMrOHqn/8mLqMjA",
+    "session_id": "X3lUlvLELLYxeTx4yOVu6UDpasGEVO0Jbu+QFnm0cKQ"
+  },
+  "event_id": "$WLGTSEFSEF:localhost",
+  "room_id": "!Cuyf34gef24t:localhost",
+  "origin_server_ts": 1476648761524,
+  "sender": "@example:localhost",
+  "type": "m.room.encrypted"
+}

event-schemas/examples/m.room.encrypted#olm

@@ -0,0 +1,14 @@
+{
+  "type": "m.room.encrypted",
+  "sender": "@example:localhost",
+  "content": {
+    "algorithm": "m.olm.v1.curve25519-aes-sha2",
+    "sender_key": "Szl29ksW/L8yZGWAX+8dY1XyFi+i5wm+DRhTGkbMiwU",
+    "ciphertext": {
+      "7qZcfnBmbEGzxxaWfBjElJuvn7BZx+lSz/SvFrDF/z8": {
+        "type": 0,
+        "body": "AwogGJJzMhf/S3GQFXAOrCZ3iKyGU5ZScVtjI0KypTYrW..."
+      }
+    }
+  }
+}

event-schemas/examples/m.room.encryption

@@ -0,0 +1,13 @@
+{
+  "content": {
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "rotation_period_ms": 604800000,
+    "rotation_period_msgs": 100
+  },
+  "event_id": "$WLGTSEFJJKJ:localhost",
+  "origin_server_ts": 1476648761524,
+  "sender": "@example:localhost",
+  "room_id": "!Cuyf34gef24t:localhost",
+  "state_key": "",
+  "type": "m.room.encryption"
+}

event-schemas/examples/m.room_key

@@ -0,0 +1,9 @@
+{
+  "content": {
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "room_id": "!Cuyf34gef24t:localhost",
+    "session_id": "X3lUlvLELLYxeTx4yOVu6UDpasGEVO0Jbu+QFnm0cKQ",
+    "session_key": "AgAAAADxKHa9uFxcXzwYoNueL5Xqi69IkD4sni8LlfJL7qNBEY..."
+  },
+  "type": "m.room_key"
+}

event-schemas/schema/m.room.encrypted

@@ -0,0 +1,61 @@
+---
+allOf:
+  - $ref: core-event-schema/event.yaml
+
+description: |-
+  This event type is used when sending encrypted events. It can be used either
+  within a room (in which case it will have all of the `Room Event fields`_), or
+  as a `to-device`_ event.
+
+properties:
+  content:
+    properties:
+      algorithm:
+        type: string
+        enum:
+          - m.olm.curve25519-aes-sha256
+          - m.megolm.v1.aes-sha
+        description: |-
+          The encryption algorithm used to encrypt this event. The
+          value of this field determines which other properties will be
+          present.
+      ciphertext:
+        oneOf:
+          - type: string
+          - type: object
+            additionalProperties:
+              type: object
+              title: CiphertextInfo
+              properties:
+                body:
+                  type: string
+                  description: The encrypted payload.
+                type:
+                  type: integer
+                  description: The Olm message type.
+        description: |-
+          The encrypted content of the event. Either the encrypted payload
+          itself, in the case of a Megolm event, or a map from the recipient
+          Curve25519 identity key to ciphertext information, in the case of an
+          Olm event. For more details, see `Messaging Algorithms`_.
+      sender_key:
+        type: string
+        description: The Curve25519 key of the sender.
+      device_id:
+        type: string
+        description: The ID of the sending device. Required with Megolm.
+      session_id:
+        type: string
+        description: |-
+          The ID of the session used to encrypt the message. Required with
+          Megolm.
+    required:
+      - algorithm
+      - sender_key
+      - ciphertext
+    type: object
+  type:
+    enum:
+      - m.room.encrypted
+    type: string
+type: object

event-schemas/schema/m.room.encryption

@@ -0,0 +1,36 @@
+---
+allOf:
+  - $ref: core-event-schema/state_event.yaml
+description: Defines how messages sent in this room should be encrypted.
+properties:
+  content:
+    properties:
+      algorithm:
+        type: string
+        enum:
+          - "m.megolm.v1.aes-sha2"
+        description: |-
+          The encryption algorithm to be used to encrypt messages sent in this
+          room.
+      rotation_period_ms:
+        type: integer
+        description: |-
+          How long the session should be used before changing it. ``604800000``
+          (a week) is the recommended default.
+      rotation_period_msgs:
+        type: integer
+        description: |-
+          How many messages should be sent before changing the session. ``100`` is the
+          recommended default.
+    required:
+      - algorithm
+    type: object
+  state_key:
+    description: A zero-length string.
+    pattern: '^$'
+    type: string
+  type:
+    enum:
+      - m.room.encryption
+    type: string
+type: object

event-schemas/schema/m.room_key

@@ -0,0 +1,35 @@
+---
+allOf:
+  - $ref: core-event-schema/event.yaml
+
+description: |-
+  This event type is used to exchange keys for end-to-end encryption. Typically
+  it is encrypted as an ``m.room.encrypted`` event, then sent as a `to-device`_ event.
+properties:
+  content:
+    properties:
+      algorithm:
+        type: string
+        enum: ["m.megolm.v1.aes-sha2"]
+        description: |-
+          The encryption algorithm the key in this event is to be used with.
+      room_id:
+        type: string
+        description: The room where the key is used.
+      session_id:
+        type: string
+        description: The ID of the session that the key is for.
+      session_key:
+        type: string
+        description: The key to be exchanged.
+    required:
+      - algorithm
+      - room_id
+      - session_id
+      - session_key
+    type: object
+  type:
+    enum:
+      - m.room_key
+    type: string
+type: object

specification/modules/end_to_end_encryption.rst

@@ -21,12 +21,6 @@ Matrix optionally supports end-to-end encryption, allowing rooms to be created
 whose conversation contents is not decryptable or interceptable on any of the
 participating homeservers.
 
-.. WARNING::
-
-   End to end encryption is being worked on and will be coming soon. This
-   section is incomplete. You can read more about what's underway at
-   http://matrix.org/speculator/spec/drafts%2Fe2e/client_server/unstable.html#end-to-end-encryption.
-
 Key Distribution
 ----------------
 Encryption and Authentication in Matrix is based around public-key
@@ -159,7 +153,7 @@ It is therefore expected that each client will maintain a list of devices for a
 number of users (in practice, typically each user with whom we share an
 encrypted room). Furthermore, it is likely that this list will need to be
 persisted between invocations of the client application (to preserve device
-verification data and to alert Alice if Bob suddently gets a new
+verification data and to alert Alice if Bob suddenly gets a new
 device).
 
 Alice's client can maintain a list of Bob's devices via the following
@@ -176,9 +170,10 @@ process:
    flag.
 
 #. During its normal processing of responses to |/sync|_, Alice's client
-   inspects the |device_lists|_ field. If it is tracking the device lists of
+   inspects the ``changed`` property of the |device_lists|_ field. If it is
-   any of the listed users, then it marks the device lists for those users
+   tracking the device lists of any of the listed users, then it marks the
-   outdated, and initiates another request to |/keys/query|_ for them.
+   device lists for those users outdated, and initiates another request to
+   |/keys/query|_ for them.
 
 #. Periodically, Alice's client stores the ``next_batch`` field of the result
    from |/sync|_ in persistent storage. If Alice later restarts her client, it
@@ -214,6 +209,18 @@ process:
    that the first request's results are ignored (possibly by cancelling the
    request).
 
+.. Note::
+
+  When Bob and Alice share a room, with Bob tracking Alice's devices, she may leave
+  the room and then add a new device. Bob will not be notified of this change,
+  as he doesn't share a room anymore with Alice. When they start sharing a
+  room again, Bob has an out-of-date list of Alice's devices. In order to address
+  this issue, Bob's homeserver will add Alice's user ID to the ``changed`` property of
+  the ``device_lists`` field, thus Bob will update his list of Alice's devices as part
+  of his normal processing. Note that Bob can also be notified when he stops sharing
+  any room with Alice by inspecting the ``left`` property of the ``device_lists``
+  field, and as a result should remove her from its list of tracked users.
+
 .. |device_lists| replace:: ``device_lists``
 .. _`device_lists`: `device_lists_sync`_
 
@@ -228,10 +235,238 @@ A homeserver should rate-limit the number of one-time keys that a given user or
 remote server can claim. A homeserver should discard the public part of a one
 time key once it has given that key to another user.
 
+Device verification
+-------------------
+
+Before Alice sends Bob encrypted data, or trusts data received from him, she
+may want to verify that she is actually communicating with him, rather than a
+man-in-the-middle. This verification process requires an out-of-band channel:
+there is no way to do it within Matrix without trusting the administrators of
+the homeservers.
+
+In Matrix, the basic process for device verification is for Alice to verify
+that the public Ed25519 signing key she received via ``/keys/query`` for Bob's
+device corresponds to the private key in use by Bob's device. For now, it is
+recommended that clients provide mechanisms by which the user can see:
+
+1. The public part of their device's Ed25519 signing key, encoded using
+   `unpadded Base64`_.
+
+2. The list of devices in use for each user in a room, along with the public
+   Ed25519 signing key for each device, again encoded using unpadded Base64.
+
+Alice can then meet Bob in person, or contact him via some other trusted
+medium, and ask him to read out the Ed25519 key shown on his device. She
+compares this with the value shown for his device on her client.
+
+Device verification may reach one of several conclusions. For example:
+
+* Alice may "accept" the device. This means that she is satisfied that the
+  device belongs to Bob. She can then encrypt sensitive material for that
+  device, and knows that messages received were sent from that device.
+
+* Alice may "reject" the device. She will do this if she knows or suspects
+  that Bob does not control that device (or equivalently, does not trust
+  Bob). She will not send sensitive material to that device, and cannot trust
+  messages apparently received from it.
+
+* Alice may choose to skip the device verification process. She is not able
+  to verify that the device actually belongs to Bob, but has no reason to
+  suspect otherwise. The encryption protocol continues to protect against
+  passive eavesdroppers.
+
+.. NOTE::
+
+   Once the signing key has been verified, it is then up to the encryption
+   protocol to verify that a given message was sent from a device holding that
+   Ed25519 private key, or to encrypt a message so that it may only be
+   decrypted by such a device. For the Olm protocol, this is documented at
+   https://matrix.org/git/olm/about/docs/signing.rst.
+
+Messaging Algorithms
+--------------------
+
+Messaging Algorithm Names
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Messaging algorithm names use the extensible naming scheme used throughout this
+specification. Algorithm names that start with ``m.`` are reserved for
+algorithms defined by this specification. Implementations wanting to experiment
+with new algorithms must be uniquely globally namespaced following Java's package
+naming conventions.
+
+Algorithm names should be short and meaningful, and should list the primitives
+used by the algorithm so that it is easier to see if the algorithm is using a
+broken primitive.
+
+A name of ``m.olm.v1`` is too short: it gives no information about the primitives
+in use, and is difficult to extend for different primitives. However a name of
+``m.olm.v1.ecdh-curve25519-hdkfsha256.hmacsha256.hkdfsha256-aes256-cbc-hmac64sha256``
+is too long despite giving a more precise description of the algorithm: it adds
+to the data transfer overhead and sacrifices clarity for human readers without
+adding any useful extra information.
+
+``m.olm.v1.curve25519-aes-sha2``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The name ``m.olm.v1.curve25519-aes-sha2`` corresponds to version 1 of the Olm
+ratchet, as defined by the `Olm specification`_. This uses:
+
+* Curve25519 for the initial key agreement.
+* HKDF-SHA-256 for ratchet key derivation.
+* Curve25519 for the root key ratchet.
+* HMAC-SHA-256 for the chain key ratchet.
+* HKDF-SHA-256, AES-256 in CBC mode, and 8 byte truncated HMAC-SHA-256 for authenticated encryption.
+
+Devices that support Olm must include "m.olm.v1.curve25519-aes-sha2" in their
+list of supported messaging algorithms, must list a Curve25519 device key, and
+must publish Curve25519 one-time keys.
+
+An event encrypted using Olm has the following format:
+
+.. code:: json
+
+    {
+      "type": "m.room.encrypted",
+      "content": {
+        "algorithm": "m.olm.v1.curve25519-aes-sha2",
+        "sender_key": "<sender_curve25519_key>",
+        "ciphertext": {
+          "<device_curve25519_key>": {
+            "type": 0,
+            "body": "<encrypted_payload_base_64>"
+          }
+        }
+      }
+    }
+
+``ciphertext`` is a mapping from device Curve25519 key to an encrypted payload
+for that device. ``body`` is a Base64-encoded Olm message body. ``type`` is an
+integer indicating the type of the message body: 0 for the initial pre-key
+message, 1 for ordinary messages.
+
+Olm sessions will generate messages with a type of 0 until they receive a
+message. Once a session has decrypted a message it will produce messages with
+a type of 1.
+
+When a client receives a message with a type of 0 it must first check if it
+already has a matching session. If it does then it will use that session to
+try to decrypt the message. If there is no existing session then the client
+must create a new session and use the new session to decrypt the message. A
+client must not persist a session or remove one-time keys used by a session
+until it has successfully decrypted a message using that session.
+
+Messages with type 1 can only be decrypted with an existing session. If there
+is no matching session, the client must treat this as an invalid message.
+
+The plaintext payload is of the form:
+
+.. code:: json
+
+   {
+     "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`` and the ``ed25519`` field value
+under the ``keys`` property match the keys returned by |/keys/query|_ for
+the given user, and must also verify the signature of the payload. 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.
+
+``m.megolm.v1.aes-sha2``
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The name ``m.megolm.v1.aes-sha2`` corresponds to version 1 of the Megolm
+ratchet, as defined by the `Megolm specification`_. This uses:
+
+* HMAC-SHA-256 for the hash ratchet.
+* HKDF-SHA-256, AES-256 in CBC mode, and 8 byte truncated HMAC-SHA-256 for authenticated encryption.
+* Ed25519 for message authenticity.
+
+Devices that support Megolm must support Olm, and include "m.megolm.v1.aes-sha2" in
+their list of supported messaging algorithms.
+
+An event encrypted using Megolm has the following format:
+
+.. code:: json
+
+    {
+      "type": "m.room.encrypted",
+      "content": {
+        "algorithm": "m.megolm.v1.aes-sha2",
+        "sender_key": "<sender_curve25519_key>",
+        "device_id": "<sender_device_id>",
+        "session_id": "<outbound_group_session_id>",
+        "ciphertext": "<encypted_payload_base_64>"
+      }
+    }
+
+The encrypted payload can contain any message event. The plaintext is of the form:
+
+.. code:: json
+
+    {
+      "type": "<event_type>",
+      "content": "<event_content>",
+      "room_id": "<the room_id>"
+    }
+
+We include the room ID in the payload, because otherwise the homeserver would
+be able to change the room a message was sent in.
+
+Clients must guard against replay attacks by keeping track of the ratchet indices
+of Megolm sessions. They should reject messages with a ratchet index that they
+have already decrypted. Care should be taken in order to avoid false positives, as a
+client may decrypt the same event twice as part of its normal processing.
+
+As with Olm events, clients must confirm that the ``sender_key`` belongs to the user
+who sent the message. The same reasoning applies, but the sender ed25519 key has to be
+inferred from the ``keys.ed25519`` property of the event which established the Megolm
+session.
+
+In order to enable end-to-end encryption in a room, clients can send a
+``m.room.encryption`` state event specifying ``m.megolm.v1.aes-sha2`` as its
+``algorithm`` property.
+
+When creating a Megolm session in a room, clients must share the corresponding session
+key using Olm with the intended recipients, so that they can decrypt future messages
+encrypted using this session. A ``m.room_key`` event is used to do this. Clients
+must also handle ``m.room_key`` events sent by other devices in order to decrypt their
+messages.
 
 Protocol definitions
 --------------------
 
+Events
+~~~~~~
+
+{{m_room_encryption_event}}
+
+{{m_room_encrypted_event}}
+
+{{m_room_key_event}}
+
+Key management API
+~~~~~~~~~~~~~~~~~~
+
 {{keys_cs_http_api}}
 
 
@@ -249,6 +484,9 @@ specified). The client is expected to use |/keys/query|_ or |/keys/changes|_
 for the equivalent functionality after an initial sync, as documented in
 `Tracking the device list for a user`_.
 
+It also adds a ``one_time_keys_count`` property. Note the spelling difference
+with the ``one_time_key_counts`` property in the |/keys/upload|_ response.
+
 .. todo: generate this from a swagger definition?
 
 .. device_lists: { changed: ["@user:server", ... ]},
@@ -258,6 +496,9 @@ Parameter    Type        Description
 ============ =========== =====================================================
 device_lists DeviceLists Optional. Information on e2e device updates. Note:
                          only present on an incremental sync.
+|device_otk| {string:    Optional. For each key algorithm, the number of
+             integer}    unclaimed one-time keys currently held on the server
+                         for this device.
 ============ =========== =====================================================
 
 ``DeviceLists``
@@ -265,10 +506,20 @@ device_lists DeviceLists Optional. Information on e2e device updates. Note:
 ========= ========= =============================================
 Parameter Type      Description
 ========= ========= =============================================
-changed   [string]  List of users who have updated their device identity keys
+changed   [string]  List of users who have updated their device identity keys,
-                    since the previous sync response.
+                    or who now share an encrypted room with the client since
+                    the previous sync response.
+left      [string]  List of users with whom we do not share any encrypted rooms
+                    anymore since the previous sync response.
 ========= ========= =============================================
 
+.. NOTE::
+
+  For optimal performance, Alice should be added to ``changed`` in Bob's sync only
+  when she adds a new device, or when Alice and Bob now share a room but didn't
+  share any room previously. However, for the sake of simpler logic, a server
+  may add Alice to ``changed`` when Alice and Bob share a new room, even if they
+  previously already shared a room.
 
 Example response:
 
@@ -281,17 +532,27 @@ Example response:
       "changed": [
          "@alice:example.com",
       ],
+      "left": [
+         "@bob:example.com",
+      ],
     },
+    "device_one_time_keys_count": {
+      "curve25519": 10,
+      "signed_curve25519": 20
+    }
   }
 
 .. References
 
 .. _ed25519: http://ed25519.cr.yp.to/
 .. _curve25519: https://cr.yp.to/ecdh.html
+.. _`Olm specification`: http://matrix.org/docs/spec/olm.html
+.. _`Megolm specification`: http://matrix.org/docs/spec/megolm.html
 
 .. _`Signing JSON`: ../appendices.html#signing-json
 
 .. |m.olm.v1.curve25519-aes-sha2| replace:: ``m.olm.v1.curve25519-aes-sha2``
+.. |device_otk| replace:: device_one_time_keys_count
 
 .. |/keys/upload| replace:: ``/keys/upload``
 .. _/keys/upload: #post-matrix-client-%CLIENT_MAJOR_VERSION%-keys-upload