Merge pull request #1284 from Zil0/e2e_doc

Document missing parts of E2E
pull/977/head
Richard van der Hoff 6 years ago committed by GitHub
commit fe3f3b6aae
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@ -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:

@ -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",

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

@ -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.

@ -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"
}

@ -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..."
}
}
}
}

@ -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"
}

@ -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"
}

@ -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

@ -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

@ -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

@ -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
any of the listed users, then it marks the device lists for those users
outdated, and initiates another request to |/keys/query|_ for them.
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
@ -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
since the previous sync response.
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:
@ -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

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