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.

.. 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
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 suddently 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 |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).

.. |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.


Protocol definitions
--------------------

{{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`_.

.. 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.
============ =========== =====================================================

``DeviceLists``

========= ========= =============================================
Parameter Type      Description
========= ========= =============================================
changed   [string]  List of users who have updated their device identity keys
                    since the previous sync response.
========= ========= =============================================


Example response:

.. code:: json

  {
    "next_batch": "s72595_4483_1934",
    "rooms": {"leave": {}, "join": {}, "invite": {}},
    "device_lists": {
      "changed": [
         "@alice:example.com",
      ],
    },
  }

.. References

.. _ed25519: http://ed25519.cr.yp.to/
.. _curve25519: https://cr.yp.to/ecdh.html

.. _`Signing JSON`: ../appendices.html#signing-json

.. |m.olm.v1.curve25519-aes-sha2| replace:: ``m.olm.v1.curve25519-aes-sha2``

.. |/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