You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
1189 lines
51 KiB
ReStructuredText
1189 lines
51 KiB
ReStructuredText
.. Copyright 2016 OpenMarket Ltd
|
|
.. Copyright 2019 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.
|
|
|
|
End-to-End Encryption
|
|
=====================
|
|
|
|
.. _module:e2e:
|
|
|
|
Matrix optionally supports end-to-end encryption, allowing rooms to be created
|
|
whose conversation contents are 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|_ request 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`_
|
|
|
|
|
|
Sending encrypted attachments
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
When encryption is enabled in a room, files should be uploaded encrypted on
|
|
the homeserver.
|
|
|
|
In order to achieve this, a client should generate a single-use 256-bit AES
|
|
key, and encrypt the file using AES-CTR. The counter should be 64-bit long,
|
|
starting at 0 and prefixed by a random 64-bit Initialization Vector (IV), which
|
|
together form a 128-bit unique counter block.
|
|
|
|
.. Warning::
|
|
An IV must never be used multiple times with the same key. This implies that
|
|
if there are multiple files to encrypt in the same message, typically an
|
|
image and its thumbnail, the files must not share both the same key and IV.
|
|
|
|
Then, the encrypted file can be uploaded to the homeserver.
|
|
The key and the IV must be included in the room event along with the resulting
|
|
``mxc://`` in order to allow recipients to decrypt the file. As the event
|
|
containing those will be Megolm encrypted, the server will never have access to
|
|
the decrypted file.
|
|
|
|
A hash of the ciphertext must also be included, in order to prevent the homeserver from
|
|
changing the file content.
|
|
|
|
A client should send the data as an encrypted ``m.room.message`` event, using
|
|
either ``m.file`` as the msgtype, or the appropriate msgtype for the file
|
|
type. The key is sent using the `JSON Web Key`_ format, with a `W3C
|
|
extension`_.
|
|
|
|
.. anchor for link from m.message api spec
|
|
.. |encrypted_files| replace:: End-to-end encryption
|
|
.. _encrypted_files:
|
|
|
|
Extensions to ``m.message`` msgtypes
|
|
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
|
|
|
|
This module adds ``file`` and ``thumbnail_file`` properties, of type
|
|
``EncryptedFile``, to ``m.message`` msgtypes that reference files, such as
|
|
`m.file`_ and `m.image`_, replacing the ``url`` and ``thumbnail_url``
|
|
properties.
|
|
|
|
.. todo: generate this from a swagger definition?
|
|
|
|
``EncryptedFile``
|
|
|
|
========= ================ =====================================================
|
|
Parameter Type Description
|
|
========= ================ =====================================================
|
|
url string **Required.** The URL to the file.
|
|
key JWK **Required.** A `JSON Web Key`_ object.
|
|
iv string **Required.** The 128-bit unique counter block used by
|
|
AES-CTR, encoded as unpadded base64.
|
|
hashes {string: string} **Required.** A map from an algorithm name to a hash
|
|
of the ciphertext, encoded as unpadded base64. Clients
|
|
should support the SHA-256 hash, which uses the key
|
|
``sha256``.
|
|
v string **Required.** Version of the encrypted attachments
|
|
protocol. Must be ``v2``.
|
|
========= ================ =====================================================
|
|
|
|
``JWK``
|
|
|
|
========= ========= ============================================================
|
|
Parameter Type Description
|
|
========= ========= ============================================================
|
|
kty string **Required.** Key type. Must be ``oct``.
|
|
key_ops [string] **Required.** Key operations. Must at least contain
|
|
``encrypt`` and ``decrypt``.
|
|
alg string **Required.** Algorithm. Must be ``A256CTR``.
|
|
k string **Required.** The key, encoded as urlsafe unpadded base64.
|
|
ext boolean **Required.** Extractable. Must be ``true``. This is a
|
|
`W3C extension`_.
|
|
========= ========= ============================================================
|
|
|
|
Example:
|
|
|
|
.. code :: json
|
|
|
|
{
|
|
"content": {
|
|
"body": "something-important.jpg",
|
|
"file": {
|
|
"url": "mxc://example.org/FHyPlCeYUSFFxlgbQYZmoEoe",
|
|
"mimetype": "image/jpeg",
|
|
"v": "v2",
|
|
"key": {
|
|
"alg": "A256CTR",
|
|
"ext": true,
|
|
"k": "aWF6-32KGYaC3A_FEUCk1Bt0JA37zP0wrStgmdCaW-0",
|
|
"key_ops": ["encrypt","decrypt"],
|
|
"kty": "oct"
|
|
},
|
|
"iv": "w+sE15fzSc0AAAAAAAAAAA",
|
|
"hashes": {
|
|
"sha256": "fdSLu/YkRx3Wyh3KQabP3rd6+SFiKg5lsJZQHtkSAYA"
|
|
}
|
|
},
|
|
"info": {
|
|
"mimetype": "image/jpeg",
|
|
"h": 1536,
|
|
"size": 422018,
|
|
"thumbnail_file": {
|
|
"hashes": {
|
|
"sha256": "/NogKqW5bz/m8xHgFiH5haFGjCNVmUIPLzfvOhHdrxY"
|
|
},
|
|
"iv": "U+k7PfwLr6UAAAAAAAAAAA",
|
|
"key": {
|
|
"alg": "A256CTR",
|
|
"ext": true,
|
|
"k": "RMyd6zhlbifsACM1DXkCbioZ2u0SywGljTH8JmGcylg",
|
|
"key_ops": ["encrypt", "decrypt"],
|
|
"kty": "oct"
|
|
},
|
|
"mimetype": "image/jpeg",
|
|
"url": "mxc://example.org/pmVJxyxGlmxHposwVSlOaEOv",
|
|
"v": "v2"
|
|
},
|
|
"thumbnail_info": {
|
|
"h": 768,
|
|
"mimetype": "image/jpeg",
|
|
"size": 211009,
|
|
"w": 432
|
|
},
|
|
"w": 864
|
|
},
|
|
"msgtype": "m.image"
|
|
},
|
|
"event_id": "$143273582443PhrSn:example.org",
|
|
"origin_server_ts": 1432735824653,
|
|
"room_id": "!jEsUZKDJdhlrceRyVU:example.org",
|
|
"sender": "@example:example.org",
|
|
"type": "m.room.message",
|
|
"unsigned": {
|
|
"age": 1234
|
|
}
|
|
}
|
|
|
|
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, verification works by Alice meeting Bob in person, or contacting him
|
|
via some other trusted medium, and use `SAS Verification`_ to interactively
|
|
verify Bob's devices. Alice and Bob may also read aloud their unpadded base64
|
|
encoded Ed25519 public key, as returned by ``/keys/query``.
|
|
|
|
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/docs/olm_signing.html.
|
|
|
|
|
|
Key verification framework
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Verifying keys manually by reading out the Ed25519 key is not very user friendly,
|
|
and can lead to errors. In order to help mitigate errors, and to make the process
|
|
easier for users, some verification methods are supported by the specification.
|
|
The methods all use a common framework for negotiating the key verification.
|
|
|
|
To use this framework, Alice's client would send ``m.key.verification.request``
|
|
events to Bob's devices. All of the ``to_device`` messages sent to Bob MUST have
|
|
the same ``transaction_id`` to indicate they are part of the same request. This
|
|
allows Bob to reject the request on one device, and have it apply to all of his
|
|
devices. Similarly, it allows Bob to process the verification on one device without
|
|
having to involve all of his devices.
|
|
|
|
When Bob's device receives a ``m.key.verification.request``, it should prompt Bob
|
|
to verify keys with Alice using one of the supported methods in the request. If
|
|
Bob's device does not understand any of the methods, it should not cancel the request
|
|
as one of his other devices may support the request. Instead, Bob's device should
|
|
tell Bob that an unsupported method was used for starting key verification. The
|
|
prompt for Bob to accept/reject Alice's request (or the unsupported method prompt)
|
|
should be automatically dismissed 10 minutes after the ``timestamp`` field or 2
|
|
minutes after Bob's client receives the message, whichever comes first, if Bob
|
|
does not interact with the prompt. The prompt should additionally be hidden if
|
|
an appropriate ``m.key.verification.cancel`` message is received.
|
|
|
|
If Bob rejects the request, Bob's client must send a ``m.key.verification.cancel``
|
|
message to Alice's device. Upon receipt, Alice's device should tell her that Bob
|
|
does not want to verify her device and send ``m.key.verification.cancel`` messages
|
|
to all of Bob's devices to notify them that the request was rejected.
|
|
|
|
If Bob accepts the request, Bob's device starts the key verification process by
|
|
sending a ``m.key.verification.start`` message to Alice's device. Upon receipt
|
|
of this message, Alice's device should send a ``m.key.verification.cancel`` message
|
|
to all of Bob's other devices to indicate the process has been started. The start
|
|
message must use the same ``transaction_id`` from the original key verification
|
|
request if it is in response to the request. The start message can be sent indepdently
|
|
of any request.
|
|
|
|
Individual verification methods may add additional steps, events, and properties to
|
|
the verification messages. Event types for methods defined in this specification must
|
|
be under the ``m.key.verification`` namespace and any other event types must be namespaced
|
|
according to the Java package naming convention.
|
|
|
|
Any of Alice's or Bob's devices can cancel the key verification request or process
|
|
at any time with a ``m.key.verification.cancel`` message to all applicable devices.
|
|
|
|
This framework yields the following handshake, assuming both Alice and Bob each have
|
|
2 devices, Bob's first device accepts the key verification request, and Alice's second
|
|
device initiates the request. Note how Alice's first device is not involved in the
|
|
request or verification process.
|
|
|
|
::
|
|
|
|
+---------------+ +---------------+ +-------------+ +-------------+
|
|
| AliceDevice1 | | AliceDevice2 | | BobDevice1 | | BobDevice2 |
|
|
+---------------+ +---------------+ +-------------+ +-------------+
|
|
| | | |
|
|
| | m.key.verification.request | |
|
|
| |---------------------------------->| |
|
|
| | | |
|
|
| | m.key.verification.request | |
|
|
| |-------------------------------------------------->|
|
|
| | | |
|
|
| | m.key.verification.start | |
|
|
| |<----------------------------------| |
|
|
| | | |
|
|
| | m.key.verification.cancel | |
|
|
| |-------------------------------------------------->|
|
|
| | | |
|
|
|
|
|
|
After the handshake, the verification process begins.
|
|
|
|
{{m_key_verification_request_event}}
|
|
|
|
{{m_key_verification_start_event}}
|
|
|
|
{{m_key_verification_cancel_event}}
|
|
|
|
|
|
.. _`SAS Verification`:
|
|
|
|
Short Authentication String (SAS) verification
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
SAS verification is a user-friendly key verification process built off the common
|
|
framework outlined above. SAS verification is intended to be a highly interactive
|
|
process for users, and as such exposes verfiication methods which are easier for
|
|
users to use.
|
|
|
|
The verification process is heavily inspired by Phil Zimmermann's ZRTP key agreement
|
|
handshake. A key part of key agreement in ZRTP is the hash commitment: the party that
|
|
begins the Diffie-Hellman key sharing sends a hash of their part of the Diffie-Hellman
|
|
exchange, and does not send their part of the Diffie-Hellman exchange until they have
|
|
received the other party's part. Thus an attacker essentially only has one attempt to
|
|
attack the Diffie-Hellman exchange, and hence we can verify fewer bits while still
|
|
achieving a high degree of security: if we verify n bits, then an attacker has a 1 in
|
|
2\ :sup:`n` chance of success. For example, if we verify 40 bits, then an attacker has
|
|
a 1 in 1,099,511,627,776 chance (or less than 1 in 1012 chance) of success. A failed
|
|
attack would result in a mismatched Short Authentication String, alerting users to the
|
|
attack.
|
|
|
|
The verification process takes place over `to-device`_ messages in two phases:
|
|
|
|
1. Key agreement phase (based on `ZRTP key agreement <https://tools.ietf.org/html/rfc6189#section-4.4.1>`_).
|
|
#. Key verification phase (based on HMAC).
|
|
|
|
The process between Alice and Bob verifying each other would be:
|
|
|
|
.. |AlicePublicKey| replace:: :math:`K_{A}^{public}`
|
|
.. |AlicePrivateKey| replace:: :math:`K_{A}^{private}`
|
|
.. |AliceCurve25519| replace:: :math:`K_{A}^{private},K_{A}^{public}`
|
|
.. |BobPublicKey| replace:: :math:`K_{B}^{public}`
|
|
.. |BobPrivateKey| replace:: :math:`K_{B}^{private}`
|
|
.. |BobCurve25519| replace:: :math:`K_{B}^{private},K_{B}^{public}`
|
|
.. |BobAliceCurve25519| replace:: :math:`K_{B}^{private}K_{A}^{public}`
|
|
.. |AliceBobECDH| replace:: :math:`ECDH(K_{A}^{private},K_{B}^{public})`
|
|
|
|
1. Alice and Bob establish a secure out-of-band connection, such as meeting
|
|
in-person or a video call. "Secure" here means that either party cannot be
|
|
impersonated, not explicit secrecy.
|
|
#. Alice and Bob communicate which devices they'd like to verify with each other.
|
|
#. Alice selects Bob's device from the device list and begins verification.
|
|
#. Alice's client ensures it has a copy of Bob's device key.
|
|
#. Alice's device sends Bob's device a ``m.key.verification.start`` message.
|
|
#. Bob's device receives the message and selects a key agreement protocol, hash
|
|
algorithm, message authentication code, and SAS method supported by Alice's
|
|
device.
|
|
#. Bob's device ensures it has a copy of Alice's device key.
|
|
#. Bob's device creates an ephemeral Curve25519 key pair (|BobCurve25519|), and
|
|
calculates the hash (using the chosen algorithm) of the public key |BobPublicKey|.
|
|
#. Bob's device replies to Alice's device with a ``m.key.verification.accept`` message.
|
|
#. Alice's device receives Bob's message and stores the commitment hash for later use.
|
|
#. Alice's device creates an ephemeral Curve25519 key pair (|AliceCurve25519|) and
|
|
replies to Bob's device with a ``m.key.verification.key``, sending only the public
|
|
key |AlicePublicKey|.
|
|
#. Bob's device receives Alice's message and replies with its own ``m.key.verification.key``
|
|
message containing its public key |BobPublicKey|.
|
|
#. Alice's device receives Bob's message and verifies the commitment hash from earlier
|
|
matches the hash of the key Bob's device just sent and the content of Alice's
|
|
``m.key.verification.start`` message.
|
|
#. Both Alice and Bob's devices perform an Elliptic-curve Diffie-Hellman (|AliceBobECDH|),
|
|
using the result as the shared secret.
|
|
#. Both Alice and Bob's devices display a SAS to their users, which is derived
|
|
from the shared key using one of the methods in this section. If multiple SAS
|
|
methods are available, clients should allow the users to select a method.
|
|
#. Alice and Bob compare the strings shown by their devices, and tell their devices if
|
|
they match or not.
|
|
#. Assuming they match, Alice and Bob's devices calculate the HMAC of their own device keys
|
|
and a comma-separated sorted list of of the key IDs that they wish the other user
|
|
to verify, using SHA-256 as the hash function. HMAC is defined in `RFC 2104 <https://tools.ietf.org/html/rfc2104>`_.
|
|
The key for the HMAC is different for each item and is calculated by generating
|
|
32 bytes (256 bits) using `the key verification HKDF <#sas-hkdf>`_.
|
|
#. Alice's device sends Bob's device a ``m.key.verification.mac`` message containing the
|
|
MAC of Alice's device keys and the MAC of her key IDs to be verified. Bob's device does
|
|
the same for Bob's device keys and key IDs concurrently with Alice.
|
|
#. When the other device receives the ``m.key.verification.mac`` message, the device
|
|
calculates the HMAC of its copies of the other device's keys given in the message,
|
|
as well as the HMAC of the comma-seperated, sorted, list of key IDs in the message.
|
|
The device compares these with the HMAC values given in the message, and if everything
|
|
matches then the device keys are verified.
|
|
|
|
The wire protocol looks like the following between Alice and Bob's devices::
|
|
|
|
+-------------+ +-----------+
|
|
| AliceDevice | | BobDevice |
|
|
+-------------+ +-----------+
|
|
| |
|
|
| m.key.verification.start |
|
|
|-------------------------------->|
|
|
| |
|
|
| m.key.verification.accept |
|
|
|<--------------------------------|
|
|
| |
|
|
| m.key.verification.key |
|
|
|-------------------------------->|
|
|
| |
|
|
| m.key.verification.key |
|
|
|<--------------------------------|
|
|
| |
|
|
| m.key.verification.mac |
|
|
|-------------------------------->|
|
|
| |
|
|
| m.key.verification.mac |
|
|
|<--------------------------------|
|
|
| |
|
|
|
|
Error and exception handling
|
|
<<<<<<<<<<<<<<<<<<<<<<<<<<<<
|
|
|
|
At any point the interactive verfication can go wrong. The following describes what
|
|
to do when an error happens:
|
|
|
|
* Alice or Bob can cancel the verification at any time. A ``m.key.verification.cancel``
|
|
message must be sent to signify the cancellation.
|
|
* The verification can time out. Clients should time out a verification that does not
|
|
complete within 10 minutes. Additionally, clients should expire a ``transaction_id``
|
|
which goes unused for 10 minutes after having last sent/received it. The client should
|
|
inform the user that the verification timed out, and send an appropriate
|
|
``m.key.verification.cancel`` message to the other device.
|
|
* When the same device attempts to intiate multiple verification attempts, the receipient
|
|
should cancel all attempts with that device.
|
|
* When a device receives an unknown ``transaction_id``, it should send an appropriate
|
|
``m.key.verfication.cancel`` message to the other device indicating as such. This
|
|
does not apply for inbound ``m.key.verification.start`` or ``m.key.verification.cancel``
|
|
messages.
|
|
* If the two devices do not share a common key share, hash, HMAC, or SAS method then
|
|
the device should notify the other device with an appropriate ``m.key.verification.cancel``
|
|
message.
|
|
* If the user claims the Short Authentication Strings do not match, the device should
|
|
send an appropriate ``m.key.verification.cancel`` message to the other device.
|
|
* If the device receives a message out of sequence or that it was not expecting, it should
|
|
notify the other device with an appropriate ``m.key.verification.cancel`` message.
|
|
|
|
|
|
Verification messages specific to SAS
|
|
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
|
|
|
|
Building off the common framework, the following events are involved in SAS verification.
|
|
|
|
The ``m.key.verification.cancel`` event is unchanged, however the following error codes
|
|
are used in addition to those already specified:
|
|
|
|
* ``m.unknown_method``: The devices are unable to agree on the key agreement, hash, MAC,
|
|
or SAS method.
|
|
* ``m.mismatched_commitment``: The hash commitment did not match.
|
|
* ``m.mismatched_sas``: The SAS did not match.
|
|
|
|
|
|
{{m_key_verification_start_m_sas_v1_event}}
|
|
|
|
{{m_key_verification_accept_event}}
|
|
|
|
{{m_key_verification_key_event}}
|
|
|
|
{{m_key_verification_mac_event}}
|
|
|
|
|
|
.. _sas-hkdf:
|
|
|
|
HKDF calculation
|
|
<<<<<<<<<<<<<<<<
|
|
|
|
In all of the SAS methods, HKDF is as defined in `RFC 5869 <https://tools.ietf.org/html/rfc5869>`_
|
|
and uses the previously agreed-upon hash function for the hash function. The shared
|
|
secret is supplied as the input keying material. No salt is used, and the info
|
|
parameter is the concatenation of:
|
|
|
|
* The string ``MATRIX_KEY_VERIFICATION_SAS``.
|
|
* The Matrix ID of the user who sent the ``m.key.verification.start`` message.
|
|
* The Device ID of the device which sent the ``m.key.verification.start`` message.
|
|
* The Matrix ID of the user who sent the ``m.key.verification.accept`` message.
|
|
* The Device ID of the device which sent the ``m.key.verification.accept`` message.
|
|
* The ``transaction_id`` being used.
|
|
|
|
.. admonition:: Rationale
|
|
|
|
HKDF is used over the plain shared secret as it results in a harder attack
|
|
as well as more uniform data to work with.
|
|
|
|
For verification of each party's device keys, HKDF is as defined in RFC 5869 and
|
|
uses SHA-256 as the hash function. The shared secret is supplied as the input keying
|
|
material. No salt is used, and in the info parameter is the concatenation of:
|
|
|
|
* The string ``MATRIX_KEY_VERIFICATION_MAC``.
|
|
* The Matrix ID of the user whose key is being MAC-ed.
|
|
* The Device ID of the device sending the MAC.
|
|
* The Matrix ID of the other user.
|
|
* The Device ID of the device receiving the MAC.
|
|
* The ``transaction_id`` being used.
|
|
* The Key ID of the key being MAC-ed, or the string ``KEY_IDS`` if the item
|
|
being MAC-ed is the list of key IDs.
|
|
|
|
SAS method: ``decimal``
|
|
<<<<<<<<<<<<<<<<<<<<<<<
|
|
|
|
Generate 5 bytes using `HKDF <#sas-hkdf>`_ then take sequences of 13 bits to
|
|
convert to decimal numbers (resulting in 3 numbers between 0 and 8191 inclusive
|
|
each). Add 1000 to each calculated number.
|
|
|
|
The bitwise operations to get the numbers given the 5 bytes
|
|
:math:`B_{0}, B_{1}, B_{2}, B_{3}, B_{4}` would be:
|
|
|
|
* First: :math:`(B_{0} \ll 5 | B_{1} \gg 3) + 1000`
|
|
* Second: :math:`((B_{1} \& 0x7) \ll 10 | B_{2} \ll 2 | B_{3} \gg 6) + 1000`
|
|
* Third: :math:`((B_{3} \& 0x3F) \ll 7 | B_{4} \gg 1) + 1000`
|
|
|
|
The digits are displayed to the user either with an appropriate separator,
|
|
such as dashes, or with the numbers on individual lines.
|
|
|
|
SAS method: ``emoji``
|
|
<<<<<<<<<<<<<<<<<<<<<
|
|
|
|
Generate 6 bytes using `HKDF <#sas-hkdf>`_ then split the first 42 bits into
|
|
7 groups of 6 bits, similar to how one would base64 encode something. Convert
|
|
each group of 6 bits to a number and use the following table to get the corresponding
|
|
emoji:
|
|
|
|
{{sas_emoji_table}}
|
|
|
|
.. Note::
|
|
This table is available as JSON at
|
|
https://github.com/matrix-org/matrix-doc/blob/master/data-definitions/sas-emoji.json
|
|
|
|
.. admonition:: Rationale
|
|
|
|
The emoji above were chosen to:
|
|
|
|
* Be recognisable without colour.
|
|
* Be recognisable at a small size.
|
|
* Be recognisable by most cultures.
|
|
* Be distinguishable from each other.
|
|
* Easily described by a few words.
|
|
* Avoid symbols with negative connotations.
|
|
* Be likely similar across multiple platforms.
|
|
|
|
Clients SHOULD show the emoji with the descriptions from the table, or appropriate
|
|
translation of those descriptions. Client authors SHOULD collaborate to create a
|
|
common set of translations for all languages.
|
|
|
|
|
|
.. section name changed, so make sure that old links keep working
|
|
.. _key-sharing:
|
|
|
|
Sharing keys between devices
|
|
----------------------------
|
|
|
|
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, several methods are provided to
|
|
allow users to transfer keys from one device to another.
|
|
|
|
Key requests
|
|
~~~~~~~~~~~~
|
|
|
|
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
|
|
``request_cancellation`` to the other devices that it had originally sent the key
|
|
request to; a device that receives a ``request_cancellation`` 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 strategy is for a user's client to only send keys requested by the
|
|
verified devices of the same user.
|
|
|
|
Key exports
|
|
~~~~~~~~~~~
|
|
|
|
Keys can be manually exported from one device to an encrypted file, copied to
|
|
another device, and imported. The file is encrypted using a user-supplied
|
|
passphrase, and is created as follows:
|
|
|
|
1. Encode the sessions as a JSON object, formatted as described in `Key export
|
|
format`_.
|
|
2. Generate a 512-bit key from the user-entered passphrase by computing
|
|
`PBKDF2`_\(HMAC-SHA-512, passphrase, S, N, 512), where S is a 128-bit
|
|
cryptographically-random salt and N is the number of rounds. N should be at
|
|
least 100,000. The keys K and K' are set to the first and last 256 bits of
|
|
this generated key, respectively. K is used as an AES-256 key, and K' is
|
|
used as an HMAC-SHA-256 key.
|
|
3. Serialize the JSON object as a UTF-8 string, and encrypt it using
|
|
AES-CTR-256 with the key K generated above, and with a 128-bit
|
|
cryptographically-random initialization vector, IV, that has bit 63 set to
|
|
zero. (Setting bit 63 to zero in IV is needed to work around differences in
|
|
implementations of AES-CTR.)
|
|
4. Concatenate the following data:
|
|
|
|
============ ===============================================================
|
|
Size (bytes) Description
|
|
============ ===============================================================
|
|
1 Export format version, which must be ``0x01``.
|
|
16 The salt S.
|
|
16 The initialization vector IV.
|
|
4 The number of rounds N, as a big-endian unsigned 32-bit integer.
|
|
variable The encrypted JSON object.
|
|
32 The HMAC-SHA-256 of all the above string concatenated together,
|
|
using K' as the key.
|
|
============ ===============================================================
|
|
|
|
5. Base64-encode the string above. Newlines may be added to avoid overly long
|
|
lines.
|
|
6. Prepend the resulting string with ``-----BEGIN MEGOLM SESSION DATA-----``,
|
|
with a trailing newline, and append ``-----END MEGOLM SESSION DATA-----``,
|
|
with a leading and trailing newline.
|
|
|
|
Key export format
|
|
<<<<<<<<<<<<<<<<<
|
|
|
|
The exported sessions are formatted as a JSON array of ``SessionData`` objects
|
|
described as follows:
|
|
|
|
``SessionData``
|
|
|
|
.. table::
|
|
:widths: auto
|
|
|
|
=============================== =========== ====================================
|
|
Parameter Type Description
|
|
=============================== =========== ====================================
|
|
algorithm string Required. The encryption algorithm
|
|
that the session uses. Must be
|
|
``m.megolm.v1.aes-sha2``.
|
|
forwarding_curve25519_key_chain [string] Required. Chain of Curve25519 keys
|
|
through which this session was
|
|
forwarded, via
|
|
`m.forwarded_room_key`_ events.
|
|
room_id string Required. The room where the
|
|
session is used.
|
|
sender_key string Required. The Curve25519 key of the
|
|
device which initiated the session
|
|
originally.
|
|
sender_claimed_keys {string: Required. The Ed25519 key of the
|
|
string} device which initiated the session
|
|
originally.
|
|
session_id string Required. The ID of the session.
|
|
session_key string Required. The key for the session.
|
|
=============================== =========== ====================================
|
|
|
|
Example:
|
|
|
|
.. code:: json
|
|
|
|
[
|
|
{
|
|
"algorithm": "m.megolm.v1.aes-sha2",
|
|
"forwarding_curve25519_key_chain": [
|
|
"hPQNcabIABgGnx3/ACv/jmMmiQHoeFfuLB17tzWp6Hw"
|
|
],
|
|
"room_id": "!Cuyf34gef24t:localhost",
|
|
"sender_key": "RF3s+E7RkTQTGF2d8Deol0FkQvgII2aJDf3/Jp5mxVU",
|
|
"sender_claimed_keys": {
|
|
"ed25519": "<device ed25519 identity key>",
|
|
},
|
|
"session_id": "X3lUlvLELLYxeTx4yOVu6UDpasGEVO0Jbu+QFnm0cKQ",
|
|
"session_key": "AgAAAADxKHa9uFxcXzwYoNueL5Xqi69IkD4sni8Llf..."
|
|
},
|
|
...
|
|
]
|
|
|
|
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 and successfully decrypted a
|
|
message. For these purposes, a session that has not received any messages
|
|
should use its creation time as the time that 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.
|
|
|
|
Recovering from undecryptable messages
|
|
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
|
|
|
|
Occasionally messages may be undecryptable by clients due to a variety of reasons.
|
|
When this happens to an Olm-encrypted message, the client should assume that the Olm
|
|
session has become corrupted and create a new one to replace it.
|
|
|
|
.. Note::
|
|
Megolm-encrypted messages generally do not have the same problem. Usually the key
|
|
for an undecryptable Megolm-encrypted message will come later, allowing the client
|
|
to decrypt it successfully. Olm does not have a way to recover from the failure,
|
|
making this session replacement process required.
|
|
|
|
To establish a new session, the client sends a `m.dummy <#m-dummy>`_ to-device event
|
|
to the other party to notify them of the new session details.
|
|
|
|
Clients should rate-limit the number of sessions it creates per device that it receives
|
|
a message from. Clients should not create a new session with another device if it has
|
|
already created one for that given device in the past 1 hour.
|
|
|
|
Clients should attempt to mitigate loss of the undecryptable messages. For example,
|
|
Megolm sessions that were sent using the old session would have been lost. The client
|
|
can attempt to retrieve the lost sessions through ``m.room_key_request`` messages.
|
|
|
|
|
|
``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": "<encrypted_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}}
|
|
|
|
{{m_dummy_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
|
|
.. _`JSON Web Key`: https://tools.ietf.org/html/rfc7517#appendix-A.3
|
|
.. _`W3C extension`: https://w3c.github.io/webcrypto/#iana-section-jwk
|
|
.. _`PBKDF2`: https://tools.ietf.org/html/rfc2898#section-5.2
|
|
|
|
.. _`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
|