matrix-spec/specification/modules/end_to_end_encryption.rst

.. Copyright 2016 OpenMarket Ltd
..
.. 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.

End-to-End Encryption
=====================

.. _module:e2e:

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.

Key Distribution
----------------
Encryption and Authentication in Matrix is based around public-key
cryptography. The Matrix protocol provides a basic mechanism for exchange of
public keys, though an out-of-band channel is required to exchange fingerprints
between users to build a web of trust.

Overview
~~~~~~~~

.. code::

    1) Bob publishes the public keys and supported algorithms for his
       device. This may include long-term identity keys, and/or one-time
       keys.

                                          +----------+  +--------------+
                                          | Bob's HS |  | Bob's Device |
                                          +----------+  +--------------+
                                                |              |
                                                |<=============|
                                                  /keys/upload

    2) Alice requests Bob's public identity keys and supported algorithms.

      +----------------+  +------------+  +----------+
      | Alice's Device |  | Alice's HS |  | Bob's HS |
      +----------------+  +------------+  +----------+
             |                  |               |
             |=================>|==============>|
               /keys/query        <federation>

    3) Alice selects an algorithm and claims any one-time keys needed.

      +----------------+  +------------+  +----------+
      | Alice's Device |  | Alice's HS |  | Bob's HS |
      +----------------+  +------------+  +----------+
             |                  |               |
             |=================>|==============>|
               /keys/claim         <federation>


Key algorithms
~~~~~~~~~~~~~~

The name ``ed25519`` corresponds to the `Ed25519`_ signature algorithm. The key
is a 32-byte Ed25519 public key, encoded using `unpadded Base64`_. Example:

.. code:: json

   "SogYyrkTldLz0BXP+GYWs0qaYacUI0RleEqNT8J3riQ"

The name ``curve25519`` corresponds to the `Curve25519`_ ECDH algorithm. The
key is a 32-byte Curve25519 public key, encoded using `unpadded
Base64`_. Example:

.. code:: json

  "JGLn/yafz74HB2AbPLYJWIVGnKAtqECOBf11yyXac2Y"

The name ``signed_curve25519`` also corresponds to the Curve25519 algorithm,
but keys using this algorithm are objects with the properties ``key`` (giving
the Base64-encoded 32-byte Curve25519 public key), and ``signatures`` (giving a
signature for the key object, as described in `Signing JSON`_). Example:

.. code:: json

  {
    "key":"06UzBknVHFMwgi7AVloY7ylC+xhOhEX4PkNge14Grl8",
    "signatures": {
      "@user:example.com": {
        "ed25519:EGURVBUNJP": "YbJva03ihSj5mPk+CHMJKUKlCXCPFXjXOK6VqBnN9nA2evksQcTGn6hwQfrgRHIDDXO2le49x7jnWJHMJrJoBQ"
      }
    }
  }

Device keys
~~~~~~~~~~~

Each device should have one Ed25519 signing key. This key should be generated
on the device from a cryptographically secure source, and the private part of
the key should never be exported from the device. This key is used as the
fingerprint for a device by other clients.

A device will generally need to generate a number of additional keys. Details
of these will vary depending on the messaging algorithm in use.

Algorithms generally require device identity keys as well as signing keys. Some
algorithms also require one-time keys to improve their secrecy and deniability.
These keys are used once during session establishment, and are then thrown
away.

For Olm version 1, each device requires a single Curve25519 identity key, and a
number of signed Curve25519 one-time keys.

Uploading keys
~~~~~~~~~~~~~~

A device uploads the public parts of identity keys to their homeserver as a
signed JSON object, using the |/keys/upload|_ API.
The JSON object must include the public part of the device's Ed25519 key, and
must be signed by that key, as described in `Signing JSON`_.

One-time keys are also uploaded to the homeserver using the |/keys/upload|_
API.

Devices must store the private part of each key they upload. They can
discard the private part of a one-time key when they receive a message using
that key. However it's possible that a one-time key given out by a homeserver
will never be used, so the device that generates the key will never know that
it can discard the key. Therefore a device could end up trying to store too
many private keys. A device that is trying to store too many private keys may
discard keys starting with the oldest.

Tracking the device list for a user
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Before Alice can send an encrypted message to Bob, she needs a list of each of
his devices and the associated identity keys, so that she can establish an
encryption session with each device. This list can be obtained by calling
|/keys/query|_, passing Bob's user ID in the ``device_keys`` parameter.

From time to time, Bob may add new devices, and Alice will need to know this so
that she can include his new devices for later encrypted messages. A naive
solution to this would be to call |/keys/query|_ before sending each message -
however, the number of users and devices may be large and this would be
inefficient.

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 suddenly gets a new
device).

Alice's client can maintain a list of Bob's devices via the following
process:

#. It first sets a flag to record that it is now tracking Bob's device list,
   and a separate flag to indicate that its list of Bob's devices is
   outdated. Both flags should be in storage which persists over client
   restarts.

#. It then makes a request to |/keys/query|_, passing Bob's user ID in the
   ``device_keys`` parameter. When the request completes, it stores the
   resulting list of devices in persistent storage, and clears the 'outdated'
   flag.

#. During its normal processing of responses to |/sync|_, Alice's client
   inspects the ``changed`` property of the |device_lists|_ field. If it is
   tracking the device lists of any of the listed users, then it marks the
   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
   can obtain a list of the users who have updated their device list while it
   was offline by calling |/keys/changes|_, passing the recorded ``next_batch``
   field as the ``from`` parameter. If the client is tracking the device list
   of any of the users listed in the response, it marks them as outdated. It
   combines this list with those already flagged as outdated, and initiates a
   |/keys/query|_ requests for all of them.

.. Warning::

   Bob may update one of his devices while Alice has a request to
   ``/keys/query`` in flight. Alice's client may therefore see Bob's user ID in
   the ``device_lists`` field of the ``/sync`` response while the first request
   is in flight, and initiate a second request to ``/keys/query``. This may
   lead to either of two related problems.

   The first problem is that, when the first request completes, the client will
   clear the 'outdated' flag for Bob's devices. If the second request fails, or
   the client is shut down before it completes, this could lead to Alice using
   an outdated list of Bob's devices.

   The second possibility is that, under certain conditions, the second request
   may complete *before* the first one. When the first request completes, the
   client could overwrite the later results from the second request with those
   from the first request.

   Clients MUST guard against these situations. For example, a client could
   ensure that only one request to ``/keys/query`` is in flight at a time for
   each user, by queuing additional requests until the first completes.
   Alternatively, the client could make a new request immediately, but ensure
   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`_

Claiming one-time keys
~~~~~~~~~~~~~~~~~~~~~~

A client wanting to set up a session with another device can claim a one-time
key for that device. This is done by making a request to the |/keys/claim|_
API.

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.

Key sharing
-----------

If Bob has an encrypted conversation with Alice on his computer, and then logs in
through his phone for the first time, he may want to have access to the previously
exchanged messages. To address this issue, events exist for requesting and sending
keys from device to device.

When a device is missing keys to decrypt messages, it can request the keys by
sending `m.room_key_request`_ to-device messages to other devices with
``action`` set to ``request``. If a device wishes to share the keys with that
device, it can forward the keys to the first device by sending an encrypted
`m.forwarded_room_key`_ to-device message. The first device should then send an
`m.room_key_request`_ to-device message with ``action`` set to
``cancel_request`` to the other devices that it had originally sent the key
request to; a device that receives a ``cancel_request`` should disregard any
previously-received ``request`` message with the same ``request_id`` and
``requesting_device_id``.

.. NOTE::

  Key sharing can be a big attack vector, thus it must be done very carefully.
  A reasonable stategy is for a user's client to only send keys requested by the
  verified devices of the same user.

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.

If a client has multiple sessions established with another device, it should
use the session from which it last received a message.  A client may expire old
sessions by defining a 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.

``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}}

{{m_room_key_request_event}}

{{m_forwarded_room_key_event}}

Key management API
~~~~~~~~~~~~~~~~~~

{{keys_cs_http_api}}


.. anchor for link from /sync api spec
.. |device_lists_sync| replace:: End-to-end encryption
.. _device_lists_sync:

Extensions to /sync
~~~~~~~~~~~~~~~~~~~

This module adds an optional ``device_lists`` property to the |/sync|_
response, as specified below. The server need only populate this property for
an incremental ``/sync`` (ie, one where the ``since`` parameter was
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", ... ]},

============ =========== =====================================================
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``

========= ========= =============================================
Parameter Type      Description
========= ========= =============================================
changed   [string]  List of users who have updated their device identity keys,
                    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:

.. code:: json

  {
    "next_batch": "s72595_4483_1934",
    "rooms": {"leave": {}, "join": {}, "invite": {}},
    "device_lists": {
      "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

.. |/keys/query| replace:: ``/keys/query``
.. _/keys/query: #post-matrix-client-%CLIENT_MAJOR_VERSION%-keys-query

.. |/keys/claim| replace:: ``/keys/claim``
.. _/keys/claim: #post-matrix-client-%CLIENT_MAJOR_VERSION%-keys-claim

.. |/keys/changes| replace:: ``/keys/changes``
.. _/keys/changes: #get-matrix-client-%CLIENT_MAJOR_VERSION%-keys-changes