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---
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type: module
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weight: 100
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---
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### End-to-End Encryption
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Matrix optionally supports end-to-end encryption, allowing rooms to be
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created whose conversation contents are not decryptable or interceptable
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on any of the participating homeservers.
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#### Key Distribution
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Encryption and Authentication in Matrix is based around public-key
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cryptography. The Matrix protocol provides a basic mechanism for
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exchange of public keys, though an out-of-band channel is required to
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exchange fingerprints between users to build a web of trust.
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##### Overview
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1) Bob publishes the public keys and supported algorithms for his
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device. This may include long-term identity keys, and/or one-time
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keys.
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```
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+----------+ +--------------+
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| Bob's HS | | Bob's Device |
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+----------+ +--------------+
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|<=============|
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/keys/upload
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```
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2) Alice requests Bob's public identity keys and supported algorithms.
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```
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+----------------+ +------------+ +----------+
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| Alice's Device | | Alice's HS | | Bob's HS |
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+----------------+ +------------+ +----------+
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| | |
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|=================>|==============>|
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/keys/query <federation>
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```
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3) Alice selects an algorithm and claims any one-time keys needed.
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```
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+----------------+ +------------+ +----------+
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| Alice's Device | | Alice's HS | | Bob's HS |
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+----------------+ +------------+ +----------+
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| | |
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|=================>|==============>|
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/keys/claim <federation>
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```
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##### Key algorithms
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The name `ed25519` corresponds to the
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[Ed25519](http://ed25519.cr.yp.to/) signature algorithm. The key is a
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32-byte Ed25519 public key, encoded using [unpadded Base64](/appendices/#unpadded-base64). Example:
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"SogYyrkTldLz0BXP+GYWs0qaYacUI0RleEqNT8J3riQ"
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The name `curve25519` corresponds to the
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[Curve25519](https://cr.yp.to/ecdh.html) ECDH algorithm. The key is a
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32-byte Curve25519 public key, encoded using [unpadded Base64](/appendices/#unpadded-base64).
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Example:
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"JGLn/yafz74HB2AbPLYJWIVGnKAtqECOBf11yyXac2Y"
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The name `signed_curve25519` also corresponds to the Curve25519
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algorithm, but a key using this algorithm is represented by an object
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with the following properties:
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`KeyObject`
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| Parameter | Type | Description |
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|------------|------------|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| key | string | **Required.** The unpadded Base64-encoded 32-byte Curve25519 public key. |
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| signatures | Signatures | **Required.** Signatures of the key object. The signature is calculated using the process described at [Signing JSON](/appendices/#signing-json). |
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Example:
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```json
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{
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"key":"06UzBknVHFMwgi7AVloY7ylC+xhOhEX4PkNge14Grl8",
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"signatures": {
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"@user:example.com": {
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"ed25519:EGURVBUNJP": "YbJva03ihSj5mPk+CHMJKUKlCXCPFXjXOK6VqBnN9nA2evksQcTGn6hwQfrgRHIDDXO2le49x7jnWJHMJrJoBQ"
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}
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}
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}
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```
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##### Device keys
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Each device should have one Ed25519 signing key. This key should be
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generated on the device from a cryptographically secure source, and the
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private part of the key should never be exported from the device. This
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key is used as the fingerprint for a device by other clients.
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A device will generally need to generate a number of additional keys.
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Details of these will vary depending on the messaging algorithm in use.
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Algorithms generally require device identity keys as well as signing
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keys. Some algorithms also require one-time keys to improve their
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secrecy and deniability. These keys are used once during session
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establishment, and are then thrown away.
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For Olm version 1, each device requires a single Curve25519 identity
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key, and a number of signed Curve25519 one-time keys.
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##### Uploading keys
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A device uploads the public parts of identity keys to their homeserver
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as a signed JSON object, using the [`/keys/upload`](/client-server-api/#post_matrixclientr0keysupload) API. The JSON object
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must include the public part of the device's Ed25519 key, and must be
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signed by that key, as described in [Signing
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JSON](/appendices/#signing-json).
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One-time keys are also uploaded to the homeserver using the
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[`/keys/upload`](/client-server-api/#post_matrixclientr0keysupload) API.
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Devices must store the private part of each key they upload. They can
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discard the private part of a one-time key when they receive a message
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using that key. However it's possible that a one-time key given out by a
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homeserver will never be used, so the device that generates the key will
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never know that it can discard the key. Therefore a device could end up
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trying to store too many private keys. A device that is trying to store
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too many private keys may discard keys starting with the oldest.
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##### Tracking the device list for a user
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Before Alice can send an encrypted message to Bob, she needs a list of
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each of his devices and the associated identity keys, so that she can
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establish an encryption session with each device. This list can be
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obtained by calling [`/keys/query`](/client-server-api/#post_matrixclientr0keysquery), passing Bob's user ID in the
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`device_keys` parameter.
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From time to time, Bob may add new devices, and Alice will need to know
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this so that she can include his new devices for later encrypted
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messages. A naive solution to this would be to call [`/keys/query`](/client-server-api/#post_matrixclientr0keysquery)
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before sending each message -however, the number of users and devices
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may be large and this would be inefficient.
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It is therefore expected that each client will maintain a list of
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devices for a number of users (in practice, typically each user with
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whom we share an encrypted room). Furthermore, it is likely that this
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list will need to be persisted between invocations of the client
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application (to preserve device verification data and to alert Alice if
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Bob suddenly gets a new device).
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Alice's client can maintain a list of Bob's devices via the following
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process:
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1. It first sets a flag to record that it is now tracking Bob's device
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list, and a separate flag to indicate that its list of Bob's devices
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is outdated. Both flags should be in storage which persists over
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client restarts.
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2. It then makes a request to [`/keys/query`](/client-server-api/#post_matrixclientr0keysquery), passing Bob's user ID in
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the `device_keys` parameter. When the request completes, it stores
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the resulting list of devices in persistent storage, and clears the
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'outdated' flag.
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3. During its normal processing of responses to [`/sync`](/client-server-api/#get_matrixclientr0sync), Alice's client
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inspects the `changed` property of the [`device_lists`](/client-server-api/#extensions-to-sync-1) field. If it
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is tracking the device lists of any of the listed users, then it
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marks the device lists for those users outdated, and initiates
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another request to [`/keys/query`](/client-server-api/#post_matrixclientr0keysquery) for them.
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4. Periodically, Alice's client stores the `next_batch` field of the
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result from [`/sync`](/client-server-api/#get_matrixclientr0sync) in persistent storage. If Alice later restarts her
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client, it can obtain a list of the users who have updated their
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device list while it was offline by calling [`/keys/changes`](/client-server-api/#get_matrixclientr0keyschanges),
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passing the recorded `next_batch` field as the `from` parameter. If
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the client is tracking the device list of any of the users listed in
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the response, it marks them as outdated. It combines this list with
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those already flagged as outdated, and initiates a [`/keys/query`](/client-server-api/#post_matrixclientr0keysquery)
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request for all of them.
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{{% boxes/warning %}}
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Bob may update one of his devices while Alice has a request to
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`/keys/query` in flight. Alice's client may therefore see Bob's user ID
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in the `device_lists` field of the `/sync` response while the first
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request is in flight, and initiate a second request to `/keys/query`.
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This may lead to either of two related problems.
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The first problem is that, when the first request completes, the client
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will clear the 'outdated' flag for Bob's devices. If the second request
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fails, or the client is shut down before it completes, this could lead
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to Alice using an outdated list of Bob's devices.
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The second possibility is that, under certain conditions, the second
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request may complete *before* the first one. When the first request
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completes, the client could overwrite the later results from the second
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request with those from the first request.
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Clients MUST guard against these situations. For example, a client could
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ensure that only one request to `/keys/query` is in flight at a time for
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each user, by queuing additional requests until the first completes.
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Alternatively, the client could make a new request immediately, but
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ensure that the first request's results are ignored (possibly by
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cancelling the request).
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{{% /boxes/warning %}}
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{{% boxes/note %}}
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When Bob and Alice share a room, with Bob tracking Alice's devices, she
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may leave the room and then add a new device. Bob will not be notified
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of this change, as he doesn't share a room anymore with Alice. When they
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start sharing a room again, Bob has an out-of-date list of Alice's
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devices. In order to address this issue, Bob's homeserver will add
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Alice's user ID to the `changed` property of the `device_lists` field,
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thus Bob will update his list of Alice's devices as part of his normal
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processing. Note that Bob can also be notified when he stops sharing any
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room with Alice by inspecting the `left` property of the `device_lists`
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field, and as a result should remove her from its list of tracked users.
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{{% /boxes/note %}}
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##### Sending encrypted attachments
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When encryption is enabled in a room, files should be uploaded encrypted
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on the homeserver.
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In order to achieve this, a client should generate a single-use 256-bit
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AES key, and encrypt the file using AES-CTR. The counter should be
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64-bit long, starting at 0 and prefixed by a random 64-bit
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Initialization Vector (IV), which together form a 128-bit unique counter
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block.
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{{% boxes/warning %}}
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An IV must never be used multiple times with the same key. This implies
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that if there are multiple files to encrypt in the same message,
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typically an image and its thumbnail, the files must not share both the
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same key and IV.
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{{% /boxes/warning %}}
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Then, the encrypted file can be uploaded to the homeserver. The key and
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the IV must be included in the room event along with the resulting
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`mxc://` in order to allow recipients to decrypt the file. As the event
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containing those will be Megolm encrypted, the server will never have
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access to the decrypted file.
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A hash of the ciphertext must also be included, in order to prevent the
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homeserver from changing the file content.
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A client should send the data as an encrypted `m.room.message` event,
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using either `m.file` as the msgtype, or the appropriate msgtype for the
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file type. The key is sent using the [JSON Web
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Key](https://tools.ietf.org/html/rfc7517#appendix-A.3) format, with a
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[W3C extension](https://w3c.github.io/webcrypto/#iana-section-jwk).
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Extensions to `m.room.message` msgtypes
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<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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This module adds `file` and `thumbnail_file` properties, of type
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`EncryptedFile`, to `m.room.message` msgtypes that reference files, such
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as [m.file](#mfile) and [m.image](#mimage), replacing the `url` and `thumbnail_url`
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properties.
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`EncryptedFile`
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| Parameter | Type | Description |
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|-----------|------------------|------------------------------------------------------------------------------------------------|
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| url | string | **Required.** The URL to the file. |
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| key | JWK | **Required.** A [JSON Web Key](https://tools.ietf.org/html/rfc7517#appendix-A.3) object. |
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| iv | string | **Required.** The 128-bit unique counter block used by AES-CTR, encoded as unpadded base64. |
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| 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`. |
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| v | string | **Required.** Version of the encrypted attachments protocol. Must be `v2`. |
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`JWK`
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| Parameter | Type | Description |
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| --------- |----------|--------------------------------------------------------------------------------------------------------------------------|
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| kty | string | **Required.** Key type. Must be `oct`. |
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| key_ops | [string] | **Required.** Key operations. Must at least contain `encrypt` and `decrypt`. |
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| alg | string | **Required.** Algorithm. Must be `A256CTR`. |
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| k | string | **Required.** The key, encoded as urlsafe unpadded base64. |
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| ext | boolean | **Required.** Extractable. Must be `true`. This is a [W3C extension](https://w3c.github.io/webcrypto/#iana-section-jwk). |
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Example:
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```json
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{
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"content": {
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"body": "something-important.jpg",
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"file": {
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"url": "mxc://example.org/FHyPlCeYUSFFxlgbQYZmoEoe",
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"mimetype": "image/jpeg",
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"v": "v2",
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"key": {
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"alg": "A256CTR",
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"ext": true,
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"k": "aWF6-32KGYaC3A_FEUCk1Bt0JA37zP0wrStgmdCaW-0",
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"key_ops": ["encrypt","decrypt"],
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"kty": "oct"
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},
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"iv": "w+sE15fzSc0AAAAAAAAAAA",
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"hashes": {
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"sha256": "fdSLu/YkRx3Wyh3KQabP3rd6+SFiKg5lsJZQHtkSAYA"
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}
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},
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"info": {
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"mimetype": "image/jpeg",
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"h": 1536,
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"size": 422018,
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"thumbnail_file": {
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"hashes": {
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"sha256": "/NogKqW5bz/m8xHgFiH5haFGjCNVmUIPLzfvOhHdrxY"
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},
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"iv": "U+k7PfwLr6UAAAAAAAAAAA",
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"key": {
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"alg": "A256CTR",
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"ext": true,
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"k": "RMyd6zhlbifsACM1DXkCbioZ2u0SywGljTH8JmGcylg",
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"key_ops": ["encrypt", "decrypt"],
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"kty": "oct"
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},
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"mimetype": "image/jpeg",
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"url": "mxc://example.org/pmVJxyxGlmxHposwVSlOaEOv",
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"v": "v2"
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},
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"thumbnail_info": {
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"h": 768,
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"mimetype": "image/jpeg",
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"size": 211009,
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"w": 432
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},
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"w": 864
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},
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"msgtype": "m.image"
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},
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"event_id": "$143273582443PhrSn:example.org",
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"origin_server_ts": 1432735824653,
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"room_id": "!jEsUZKDJdhlrceRyVU:example.org",
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"sender": "@example:example.org",
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"type": "m.room.message",
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"unsigned": {
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"age": 1234
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}
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}
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```
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##### Claiming one-time keys
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A client wanting to set up a session with another device can claim a
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one-time key for that device. This is done by making a request to the
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[`/keys/claim`](/client-server-api/#post_matrixclientr0keysclaim) API.
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A homeserver should rate-limit the number of one-time keys that a given
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user or remote server can claim. A homeserver should discard the public
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part of a one time key once it has given that key to another user.
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#### Device verification
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Before Alice sends Bob encrypted data, or trusts data received from him,
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she may want to verify that she is actually communicating with him,
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rather than a man-in-the-middle. This verification process requires an
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out-of-band channel: there is no way to do it within Matrix without
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trusting the administrators of the homeservers.
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In Matrix, verification works by Alice meeting Bob in person, or
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contacting him via some other trusted medium, and using [SAS
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Verification](#SAS Verification) to interactively verify Bob's devices.
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Alice and Bob may also read aloud their unpadded base64 encoded Ed25519
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public key, as returned by `/keys/query`.
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Device verification may reach one of several conclusions. For example:
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- Alice may "accept" the device. This means that she is satisfied that
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the device belongs to Bob. She can then encrypt sensitive material
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for that device, and knows that messages received were sent from
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that device.
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- Alice may "reject" the device. She will do this if she knows or
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suspects that Bob does not control that device (or equivalently,
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does not trust Bob). She will not send sensitive material to that
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device, and cannot trust messages apparently received from it.
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- Alice may choose to skip the device verification process. She is not
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able to verify that the device actually belongs to Bob, but has no
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reason to suspect otherwise. The encryption protocol continues to
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protect against passive eavesdroppers.
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{{% boxes/note %}}
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Once the signing key has been verified, it is then up to the encryption
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protocol to verify that a given message was sent from a device holding
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|
|
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>.
|
|
|
{{% /boxes/note %}}
|
|
|
|
|
|
##### 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 and use messages exchanged by the user's devices
|
|
|
to assist in the verification. The methods all use a common framework
|
|
|
for negotiating the key verification.
|
|
|
|
|
|
Verification messages can be sent either in a room shared by the two parties,
|
|
|
which should be a [direct messaging](#direct-messaging) room between the two
|
|
|
parties, or by using [to-device](#send-to-device-messaging) messages sent
|
|
|
directly between the two devices involved. In both cases, the messages
|
|
|
exchanged are similar, with minor differences as detailed below. Verifying
|
|
|
between two different users should be performed using in-room messages, whereas
|
|
|
verifying two devices belonging to the same user should be performed using
|
|
|
to-device messages.
|
|
|
|
|
|
A key verification session is identified by an ID that is established by the
|
|
|
first message sent in that session. For verifications using in-room messages,
|
|
|
the ID is the event ID of the initial message, and for verifications using
|
|
|
to-device messages, the first message contains a `transaction_id` field that is
|
|
|
shared by the other messages of that session.
|
|
|
|
|
|
In general, verification operates as follows:
|
|
|
|
|
|
- Alice requests a key verification with Bob by sending an
|
|
|
`m.key.verification.request` event. This event indicates the verification
|
|
|
methods that Alice's client supports. (Note that "Alice" and "Bob" may in
|
|
|
fact be the same user, in the case where a user is verifying their own
|
|
|
devices.)
|
|
|
- Bob's client prompts Bob to accept the key verification. When Bob accepts
|
|
|
the verification, Bob's client sends an `m.key.verification.ready` event.
|
|
|
This event indicates the verification methods, corresponding to the
|
|
|
verification methods supported by Alice's client, that Bob's client supports.
|
|
|
- Alice's or Bob's devices allow their users to select one of the verification
|
|
|
methods supported by both devices to use for verification. When Alice or Bob
|
|
|
selects a verification method, their device begins the verification by
|
|
|
sending an `m.key.verification.start` event, indicating the selected
|
|
|
verification method. Note that if there is only one verification method in
|
|
|
common between the devices then the receiver's device (Bob) can auto-select
|
|
|
it.
|
|
|
- Alice and Bob complete the verification as defined by the selected
|
|
|
verification method. This could involve their clients exchanging messages,
|
|
|
Alice and Bob exchanging information out-of-band, and/or Alice and Bob
|
|
|
interacting with their devices.
|
|
|
- Alice's and Bob's clients send `m.key.verification.done` events to indicate
|
|
|
that the verification was successful.
|
|
|
|
|
|
Verifications can be cancelled by either device at any time by sending an
|
|
|
`m.key.verification.cancel` event with a `code` field that indicates the reason
|
|
|
it was cancelled.
|
|
|
|
|
|
When using to-device messages, Alice may not know which of Bob's devices to
|
|
|
verify, or may not want to choose a specific device. In this case, Alice will
|
|
|
send `m.key.verification.request` events to all of Bob's devices. All of these
|
|
|
events will use the same transaction ID. When Bob accepts or declines the
|
|
|
verification on one of his devices (sending either an
|
|
|
`m.key.verification.ready` or `m.key.verification.cancel` event), Alice will
|
|
|
send an `m.key.verification.cancel` event to Bob's other devices with a `code`
|
|
|
of `m.accepted` in the case where Bob accepted the verification, or `m.user` in
|
|
|
the case where Bob rejected the verification. This yields the following
|
|
|
handshake when using to-device messages, 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. Also note that, although in
|
|
|
this example, Bob's device sends the `m.key.verification.start`, Alice's device
|
|
|
could also send that message. As well, the order of the
|
|
|
`m.key.verification.done` messages could be reversed.
|
|
|
|
|
|
```
|
|
|
+---------------+ +---------------+ +-------------+ +-------------+
|
|
|
| AliceDevice1 | | AliceDevice2 | | BobDevice1 | | BobDevice2 |
|
|
|
+---------------+ +---------------+ +-------------+ +-------------+
|
|
|
| | | |
|
|
|
| | m.key.verification.request | |
|
|
|
| |---------------------------------->| |
|
|
|
| | | |
|
|
|
| | m.key.verification.request | |
|
|
|
| |-------------------------------------------------->|
|
|
|
| | | |
|
|
|
| | m.key.verification.ready | |
|
|
|
| |<----------------------------------| |
|
|
|
| | | |
|
|
|
| | m.key.verification.cancel | |
|
|
|
| |-------------------------------------------------->|
|
|
|
| | | |
|
|
|
| | m.key.verification.start | |
|
|
|
| |<----------------------------------| |
|
|
|
| | | |
|
|
|
.
|
|
|
. (verification messages)
|
|
|
.
|
|
|
| | | |
|
|
|
| | m.key.verification.done | |
|
|
|
| |<----------------------------------| |
|
|
|
| | | |
|
|
|
| | m.key.verification.done | |
|
|
|
| |---------------------------------->| |
|
|
|
| | | |
|
|
|
```
|
|
|
|
|
|
When using in-room messages and the room has encryption enabled, clients should
|
|
|
ensure that encryption does not hinder the verification. For example, if the
|
|
|
verification messages are encrypted, clients must ensure that all the
|
|
|
recipient's unverified devices receive the keys necessary to decrypt the
|
|
|
messages, even if they would normally not be given the keys to decrypt messages
|
|
|
in the room. Alternatively, verification messages may be sent unencrypted,
|
|
|
though this is not encouraged.
|
|
|
|
|
|
Upon receipt of Alice's `m.key.verification.request` message, 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 no supported method was found, and allow him to manually reject the
|
|
|
request.
|
|
|
|
|
|
The prompt for Bob to accept/reject Alice's request (or the unsupported method
|
|
|
prompt) should be automatically dismissed 10 minutes after the `timestamp` (in
|
|
|
the case of to-device messages) or `origin_ts` (in the case of in-room
|
|
|
messages) 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 an
|
|
|
`m.key.verification.cancel` event with `code` set to `m.user`. Upon receipt,
|
|
|
Alice's device should tell her that Bob does not want to verify her device and,
|
|
|
if the request was sent as a to-device message, send
|
|
|
`m.key.verification.cancel` messages to all of Bob's devices to notify them
|
|
|
that the request was rejected.
|
|
|
|
|
|
If Alice's and Bob's clients both send an `m.key.verification.start` message,
|
|
|
and both specify the same verification method, then the
|
|
|
`m.key.verification.start` message sent by the user whose ID is the
|
|
|
lexicographically largest user ID should be ignored, and the situation should
|
|
|
be treated the same as if only the user with the lexicographically smallest
|
|
|
user ID had sent the `m.key.verification.start` message. In the case where the
|
|
|
user IDs are the same (that is, when a user is verifying their own device),
|
|
|
then the device IDs should be compared instead. If the two
|
|
|
`m.key.verification.start` messages do not specify the same verification
|
|
|
method, then the verification should be cancelled with a `code` of
|
|
|
`m.unexpected_message`.
|
|
|
|
|
|
An `m.key.verification.start` message can also be sent independently of any
|
|
|
request, specifying the verification method to use.
|
|
|
|
|
|
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.
|
|
|
|
|
|
{{% event event="m.key.verification.request" %}}
|
|
|
|
|
|
{{% event event="m.key.verification.ready" %}}
|
|
|
|
|
|
{{% event event="m.key.verification.start" %}}
|
|
|
|
|
|
{{% event event="m.key.verification.done" %}}
|
|
|
|
|
|
{{% event event="m.key.verification.cancel" %}}
|
|
|
|
|
|
##### 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 verification
|
|
|
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</sup> 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 10<sup>12</sup> chance) of
|
|
|
success. A failed attack would result in a mismatched Short
|
|
|
Authentication String, alerting users to the attack.
|
|
|
|
|
|
To advertise support for this method, clients use the name `m.sas.v1` in the
|
|
|
`methods` fields of the `m.key.verification.request` and
|
|
|
`m.key.verification.ready` events.
|
|
|
|
|
|
The verification process takes place over [to-device](#send-to-device-messaging) messages in two
|
|
|
phases:
|
|
|
|
|
|
1. Key agreement phase (based on [ZRTP key
|
|
|
agreement](https://tools.ietf.org/html/rfc6189#section-4.4.1)).
|
|
|
2. Key verification phase (based on HMAC).
|
|
|
|
|
|
The process between Alice and Bob verifying each other would be:
|
|
|
|
|
|
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.
|
|
|
2. Alice and Bob communicate which devices they'd like to verify with
|
|
|
each other.
|
|
|
3. Alice selects Bob's device from the device list and begins
|
|
|
verification.
|
|
|
4. Alice's client ensures it has a copy of Bob's device key.
|
|
|
5. Alice's device sends Bob's device an `m.key.verification.start`
|
|
|
message.
|
|
|
6. 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.
|
|
|
7. Bob's device ensures it has a copy of Alice's device key.
|
|
|
8. Bob's device creates an ephemeral Curve25519 key pair
|
|
|
(*K<sub>B</sub><sup>private</sup>*, *K<sub>B</sub><sup>public</sup>*),
|
|
|
and calculates the hash (using the chosen algorithm) of the public
|
|
|
key *K<sub>B</sub><sup>public</sup>*.
|
|
|
9. Bob's device replies to Alice's device with an
|
|
|
`m.key.verification.accept` message.
|
|
|
10. Alice's device receives Bob's message and stores the commitment hash
|
|
|
for later use.
|
|
|
11. Alice's device creates an ephemeral Curve25519 key pair
|
|
|
(*K<sub>A</sub><sup>private</sup>*, *K<sub>A</sub><sup>public</sup>*)
|
|
|
and replies to Bob's device with an `m.key.verification.key`,
|
|
|
sending only the public key
|
|
|
*K<sub>A</sub><sup>public</sup>*.
|
|
|
12. Bob's device receives Alice's message and replies with its own
|
|
|
`m.key.verification.key` message containing its public key
|
|
|
*K<sub>B</sub><sup>public</sup>*.
|
|
|
13. 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.
|
|
|
14. Both Alice and Bob's devices perform an Elliptic-curve
|
|
|
Diffie-Hellman
|
|
|
(*ECDH(K<sub>A</sub><sup>private</sup>*, *K<sub>B</sub><sup>public</sup>*)),
|
|
|
using the result as the shared secret.
|
|
|
15. 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.
|
|
|
16. Alice and Bob compare the strings shown by their devices, and tell
|
|
|
their devices if they match or not.
|
|
|
17. Assuming they match, Alice and Bob's devices calculate the HMAC of
|
|
|
their own device keys and a comma-separated sorted list 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](#hkdf-calculation).
|
|
|
18. Alice's device sends Bob's device an `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.
|
|
|
19. 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-separated, 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 verification can go wrong. The following
|
|
|
describes what to do when an error happens:
|
|
|
|
|
|
- Alice or Bob can cancel the verification at any time. An
|
|
|
`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 initiate multiple verification
|
|
|
attempts, the recipient should cancel all attempts with that device.
|
|
|
- When a device receives an unknown `transaction_id`, it should send
|
|
|
an appropriate `m.key.verification.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.
|
|
|
|
|
|
{{% event event="m.key.verification.start$m.sas.v1" %}}
|
|
|
|
|
|
{{% event event="m.key.verification.accept" %}}
|
|
|
|
|
|
{{% event event="m.key.verification.key" %}}
|
|
|
|
|
|
{{% event event="m.key.verification.mac" %}}
|
|
|
|
|
|
###### 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. When the
|
|
|
`key_agreement_protocol` is `curve25519-hkdf-sha256`, 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, followed by `|`.
|
|
|
- The Device ID of the device which sent the
|
|
|
`m.key.verification.start` message, followed by `|`.
|
|
|
- The public key from the `m.key.verification.key` message sent by
|
|
|
the device which sent the `m.key.verification.start` message,
|
|
|
followed by `|`.
|
|
|
- The Matrix ID of the user who sent the `m.key.verification.accept`
|
|
|
message, followed by `|`.
|
|
|
- The Device ID of the device which sent the
|
|
|
`m.key.verification.accept` message, followed by `|`.
|
|
|
- The public key from the `m.key.verification.key` message sent by
|
|
|
the device which sent the `m.key.verification.accept` message,
|
|
|
followed by `|`.
|
|
|
- The `transaction_id` being used.
|
|
|
|
|
|
When the `key_agreement_protocol` is the deprecated method `curve25519`,
|
|
|
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.
|
|
|
|
|
|
New implementations are discouraged from implementing the `curve25519`
|
|
|
method.
|
|
|
|
|
|
{{% boxes/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.
|
|
|
{{% /boxes/rationale %}}
|
|
|
|
|
|
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](#hkdf-calculation) 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
|
|
|
*B<sub>0</sub>*, *B<sub>1</sub>*, *B<sub>2</sub>*, *B<sub>3</sub>*, *B<sub>4</sub>*
|
|
|
would be:
|
|
|
|
|
|
- First: (*B<sub>0</sub>* ≪ 5|*B<sub>1</sub>* ≫ 3) + 1000
|
|
|
- Second:
|
|
|
((*B<sub>1</sub>*&0x7) ≪ 10|*B<sub>2</sub>* ≪ 2|*B<sub>3</sub>* ≫ 6) + 1000
|
|
|
- Third: ((*B<sub>3</sub>*&0x3F) ≪ 7|*B<sub>4</sub>* ≫ 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](#hkdf-calculation) 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-emojis %}}
|
|
|
|
|
|
{{% boxes/note %}}
|
|
|
This table is available as JSON at
|
|
|
<https://github.com/matrix-org/matrix-doc/blob/master/data-definitions/sas-emoji.json>
|
|
|
{{% /boxes/note %}}
|
|
|
|
|
|
{{% boxes/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.
|
|
|
{{% /boxes/rationale %}}
|
|
|
|
|
|
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.
|
|
|
|
|
|
{{% boxes/note %}}
|
|
|
Known translations for the emoji are available from
|
|
|
<https://github.com/matrix-org/matrix-doc/blob/master/data-definitions/>
|
|
|
and can be translated online:
|
|
|
<https://translate.riot.im/projects/matrix-doc/sas-emoji-v1>
|
|
|
{{% /boxes/note %}}
|
|
|
|
|
|
##### Cross-signing
|
|
|
|
|
|
Rather than requiring Alice to verify each of Bob's devices with each of
|
|
|
her own devices and vice versa, the cross-signing feature allows users
|
|
|
to sign their device keys such that Alice and Bob only need to verify
|
|
|
once. With cross-signing, each user has a set of cross-signing keys that
|
|
|
are used to sign their own device keys and other users' keys, and can be
|
|
|
used to trust device keys that were not verified directly.
|
|
|
|
|
|
Each user has three ed25519 key pairs for cross-signing:
|
|
|
|
|
|
- a master key (MSK) that serves as the user's identity in
|
|
|
cross-signing and signs their other cross-signing keys;
|
|
|
- a user-signing key (USK) -- only visible to the user that it belongs
|
|
|
to --that signs other users' master keys; and
|
|
|
- a self-signing key (SSK) that signs the user's own device keys.
|
|
|
|
|
|
The master key may also be used to sign other items such as the backup
|
|
|
key. The master key may also be signed by the user's own device keys to
|
|
|
aid in migrating from device verifications: if Alice's device had
|
|
|
previously verified Bob's device and Bob's device has signed his master
|
|
|
key, then Alice's device can trust Bob's master key, and she can sign it
|
|
|
with her user-signing key.
|
|
|
|
|
|
Users upload their cross-signing keys to the server using [POST
|
|
|
/\_matrix/client/r0/keys/device\_signing/upload](/client-server-api/#post_matrixclientr0keysdevice_signingupload). When Alice uploads
|
|
|
new cross-signing keys, her user ID will appear in the `changed`
|
|
|
property of the `device_lists` field of the `/sync` of response of all
|
|
|
users who share an encrypted room with her. When Bob sees Alice's user
|
|
|
ID in his `/sync`, he will call [POST /\_matrix/client/r0/keys/query](/client-server-api/#post_matrixclientr0keysquery)
|
|
|
to retrieve Alice's device and cross-signing keys.
|
|
|
|
|
|
If Alice has a device and wishes to send an encrypted message to Bob,
|
|
|
she can trust Bob's device if:
|
|
|
|
|
|
- Alice's device is using a master key that has signed her
|
|
|
user-signing key,
|
|
|
- Alice's user-signing key has signed Bob's master key,
|
|
|
- Bob's master key has signed Bob's self-signing key, and
|
|
|
- Bob's self-signing key has signed Bob's device key.
|
|
|
|
|
|
The following diagram illustrates how keys are signed:
|
|
|
|
|
|
```
|
|
|
+------------------+ .................. +----------------+
|
|
|
| +--------------+ | .................. : | +------------+ |
|
|
|
| | v v v : : v v v | |
|
|
|
| | +-----------+ : : +-----------+ | |
|
|
|
| | | Alice MSK | : : | Bob MSK | | |
|
|
|
| | +-----------+ : : +-----------+ | |
|
|
|
| | | : : : : | | |
|
|
|
| | +--+ :... : : ...: +--+ | |
|
|
|
| | v v : : v v | |
|
|
|
| | +-----------+ ............. : : ............. +-----------+ | |
|
|
|
| | | Alice SSK | : Alice USK : : : : Bob USK : | Bob SSK | | |
|
|
|
| | +-----------+ :...........: : : :...........: +-----------+ | |
|
|
|
| | | ... | : : : : | ... | | |
|
|
|
| | V V :........: :........: V V | |
|
|
|
| | +---------+ -+ +---------+ -+ | |
|
|
|
| | | Devices | ...| | Devices | ...| | |
|
|
|
| | +---------+ -+ +---------+ -+ | |
|
|
|
| | | ... | | ... | | |
|
|
|
| +------+ | | +----+ |
|
|
|
+----------------+ +--------------+
|
|
|
```
|
|
|
|
|
|
In the diagram, boxes represent keys and lines represent signatures with
|
|
|
the arrows pointing from the signing key to the key being signed. Dotted
|
|
|
boxes and lines represent keys and signatures that are only visible to
|
|
|
the user who created them.
|
|
|
|
|
|
The following diagram illustrates Alice's view, hiding the keys and
|
|
|
signatures that she cannot see:
|
|
|
|
|
|
```
|
|
|
+------------------+ +----------------+ +----------------+
|
|
|
| +--------------+ | | | | +------------+ |
|
|
|
| | v v | v v v | |
|
|
|
| | +-----------+ | +-----------+ | |
|
|
|
| | | Alice MSK | | | Bob MSK | | |
|
|
|
| | +-----------+ | +-----------+ | |
|
|
|
| | | | | | | |
|
|
|
| | +--+ +--+ | +--+ | |
|
|
|
| | v v | v | |
|
|
|
| | +-----------+ +-----------+ | +-----------+ | |
|
|
|
| | | Alice SSK | | Alice USK | | | Bob SSK | | |
|
|
|
| | +-----------+ +-----------+ | +-----------+ | |
|
|
|
| | | ... | | | | ... | | |
|
|
|
| | V V +--------+ V V | |
|
|
|
| | +---------+ -+ +---------+ -+ | |
|
|
|
| | | Devices | ...| | Devices | ...| | |
|
|
|
| | +---------+ -+ +---------+ -+ | |
|
|
|
| | | ... | | ... | | |
|
|
|
| +------+ | | +----+ |
|
|
|
+----------------+ +--------------+
|
|
|
```
|
|
|
|
|
|
[Verification methods](#device-verification) can be used to verify a
|
|
|
user's master key by using the master public key, encoded using unpadded
|
|
|
base64, as the device ID, and treating it as a normal device. For
|
|
|
example, if Alice and Bob verify each other using SAS, Alice's
|
|
|
`m.key.verification.mac` message to Bob may include
|
|
|
`"ed25519:alices+master+public+key": "alices+master+public+key"` in the
|
|
|
`mac` property. Servers therefore must ensure that device IDs will not
|
|
|
collide with cross-signing public keys.
|
|
|
|
|
|
###### Key and signature security
|
|
|
|
|
|
A user's master key could allow an attacker to impersonate that user to
|
|
|
other users, or other users to that user. Thus clients must ensure that
|
|
|
the private part of the master key is treated securely. If clients do
|
|
|
not have a secure means of storing the master key (such as a secret
|
|
|
storage system provided by the operating system), then clients must not
|
|
|
store the private part.
|
|
|
|
|
|
If a user's client sees that any other user has changed their master
|
|
|
key, that client must notify the user about the change before allowing
|
|
|
communication between the users to continue.
|
|
|
|
|
|
A user's user-signing and self-signing keys are intended to be easily
|
|
|
replaceable if they are compromised by re-issuing a new key signed by
|
|
|
the user's master key and possibly by re-verifying devices or users.
|
|
|
However, doing so relies on the user being able to notice when their
|
|
|
keys have been compromised, and it involves extra work for the user, and
|
|
|
so although clients do not have to treat the private parts as
|
|
|
sensitively as the master key, clients should still make efforts to
|
|
|
store the private part securely, or not store it at all. Clients will
|
|
|
need to balance the security of the keys with the usability of signing
|
|
|
users and devices when performing key verification.
|
|
|
|
|
|
To avoid leaking of social graphs, servers will only allow users to see:
|
|
|
|
|
|
- signatures made by the user's own master, self-signing or
|
|
|
user-signing keys,
|
|
|
- signatures made by the user's own devices about their own master
|
|
|
key,
|
|
|
- signatures made by other users' self-signing keys about their
|
|
|
respective devices,
|
|
|
- signatures made by other users' master keys about their respective
|
|
|
self-signing key, or
|
|
|
- signatures made by other users' devices about their respective
|
|
|
master keys.
|
|
|
|
|
|
Users will not be able to see signatures made by other users'
|
|
|
user-signing keys.
|
|
|
|
|
|
{{% http-api spec="client-server" api="cross_signing" %}}
|
|
|
|
|
|
#### 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](#mroom_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](#mforwarded_room_key) to-device message. The first device should
|
|
|
then send an [m.room\_key\_request](#mroom_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`.
|
|
|
|
|
|
If a device does not wish to share keys with that device, it can
|
|
|
indicate this by sending an [m.room\_key.withheld](#mroom_key.withheld) to-device message,
|
|
|
as described in [Reporting that decryption keys are
|
|
|
withheld](#reporting-that-decryption-keys-are-withheld).
|
|
|
|
|
|
{{% boxes/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.
|
|
|
{{% /boxes/note %}}
|
|
|
|
|
|
##### Server-side key backups
|
|
|
|
|
|
Devices may upload encrypted copies of keys to the server. When a device
|
|
|
tries to read a message that it does not have keys for, it may request
|
|
|
the key from the server and decrypt it. Backups are per-user, and users
|
|
|
may replace backups with new backups.
|
|
|
|
|
|
In contrast with [Key requests](#key-requests), Server-side key backups
|
|
|
do not require another device to be online from which to request keys.
|
|
|
However, as the session keys are stored on the server encrypted, it
|
|
|
requires users to enter a decryption key to decrypt the session keys.
|
|
|
|
|
|
To create a backup, a client will call [POST
|
|
|
/\_matrix/client/r0/room\_keys/version](#post_matrixclientr0room_keysversion) and define how the keys are to
|
|
|
be encrypted through the backup's `auth_data`; other clients can
|
|
|
discover the backup by calling [GET
|
|
|
/\_matrix/client/r0/room\_keys/version](#get_matrixclientr0room_keysversion). Keys are encrypted according
|
|
|
to the backup's `auth_data` and added to the backup by calling [PUT
|
|
|
/\_matrix/client/r0/room\_keys/keys](#put_matrixclientr0room_keyskeys) or one of its variants, and can
|
|
|
be retrieved by calling [GET /\_matrix/client/r0/room\_keys/keys](#get_matrixclientr0room_keyskeys) or
|
|
|
one of its variants. Keys can only be written to the most recently
|
|
|
created version of the backup. Backups can also be deleted using [DELETE
|
|
|
/\_matrix/client/r0/room\_keys/version/{version}](#delete_matrixclientr0room_keysversionversion), or individual keys
|
|
|
can be deleted using [DELETE /\_matrix/client/r0/room\_keys/keys](#delete_matrixclientr0room_keyskeys) or
|
|
|
one of its variants.
|
|
|
|
|
|
Clients must only store keys in backups after they have ensured that the
|
|
|
`auth_data` is trusted, either by checking the signatures on it, or by
|
|
|
deriving the public key from a private key that it obtained from a
|
|
|
trusted source.
|
|
|
|
|
|
When a client uploads a key for a session that the server already has a
|
|
|
key for, the server will choose to either keep the existing key or
|
|
|
replace it with the new key based on the key metadata as follows:
|
|
|
|
|
|
- if the keys have different values for `is_verified`, then it will
|
|
|
keep the key that has `is_verified` set to `true`;
|
|
|
- if they have the same values for `is_verified`, then it will keep
|
|
|
the key with a lower `first_message_index`;
|
|
|
- and finally, is `is_verified` and `first_message_index` are equal,
|
|
|
then it will keep the key with a lower `forwarded_count`.
|
|
|
|
|
|
###### Recovery key
|
|
|
|
|
|
If the recovery key (the private half of the backup encryption key) is
|
|
|
presented to the user to save, it is presented as a string constructed
|
|
|
as follows:
|
|
|
|
|
|
1. The 256-bit curve25519 private key is prepended by the bytes `0x8B`
|
|
|
and `0x01`
|
|
|
2. All the bytes in the string above, including the two header bytes,
|
|
|
are XORed together to form a parity byte. This parity byte is
|
|
|
appended to the byte string.
|
|
|
3. The byte string is encoded using base58, using the same [mapping as
|
|
|
is used for Bitcoin
|
|
|
addresses](https://en.bitcoin.it/wiki/Base58Check_encoding#Base58_symbol_chart),
|
|
|
that is, using the alphabet
|
|
|
`123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz`.
|
|
|
4. A space should be added after every 4th character.
|
|
|
|
|
|
When reading in a recovery key, clients must disregard whitespace, and
|
|
|
perform the reverse of steps 1 through 3.
|
|
|
|
|
|
###### Backup algorithm: `m.megolm_backup.v1.curve25519-aes-sha2`
|
|
|
|
|
|
When a backup is created with the `algorithm` set to
|
|
|
`m.megolm_backup.v1.curve25519-aes-sha2`, the `auth_data` should have
|
|
|
the following format:
|
|
|
|
|
|
`AuthData`
|
|
|
|
|
|
| Parameter | Type | Description |
|
|
|
| -----------| -----------|--------------------------------------------------------------------------------------------------|
|
|
|
| public_key | string | **Required.** The curve25519 public key used to encrypt the backups, encoded in unpadded base64. |
|
|
|
| signatures | Signatures | Optional. Signatures of the ``auth_data``, as Signed JSON |
|
|
|
|
|
|
The `session_data` field in the backups is constructed as follows:
|
|
|
|
|
|
1. Encode the session key to be backed up as a JSON object with the
|
|
|
properties:
|
|
|
|
|
|
| Parameter | Type | Description |
|
|
|
| --------------------------------|-------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|
|
|
| algorithm | string | **Required.** The end-to-end message encryption algorithm that the key is for. 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](#mforwarded_room_key) events. |
|
|
|
| sender_key | string | **Required.** Unpadded base64-encoded device curve25519 key. |
|
|
|
| sender_claimed_keys | {string: string} | **Required.** A map from algorithm name (`ed25519`) to the identity key for the sending device. |
|
|
|
| session_key | string | **Required.** Unpadded base64-encoded session key in [session-sharing format](https://gitlab.matrix.org/matrix-org/olm/blob/master/docs/megolm.md#session-sharing-format). |
|
|
|
|
|
|
2. Generate an ephemeral curve25519 key, and perform an ECDH with the
|
|
|
ephemeral key and the backup's public key to generate a shared
|
|
|
secret. The public half of the ephemeral key, encoded using unpadded
|
|
|
base64, becomes the `ephemeral` property of the `session_data`.
|
|
|
|
|
|
3. Using the shared secret, generate 80 bytes by performing an HKDF
|
|
|
using SHA-256 as the hash, with a salt of 32 bytes of 0, and with
|
|
|
the empty string as the info. The first 32 bytes are used as the AES
|
|
|
key, the next 32 bytes are used as the MAC key, and the last 16
|
|
|
bytes are used as the AES initialization vector.
|
|
|
|
|
|
4. Stringify the JSON object, and encrypt it using AES-CBC-256 with
|
|
|
PKCS\#7 padding. This encrypted data, encoded using unpadded base64,
|
|
|
becomes the `ciphertext` property of the `session_data`.
|
|
|
|
|
|
5. Pass the raw encrypted data (prior to base64 encoding) through
|
|
|
HMAC-SHA-256 using the MAC key generated above. The first 8 bytes of
|
|
|
the resulting MAC are base64-encoded, and become the `mac` property
|
|
|
of the `session_data`.
|
|
|
|
|
|
{{% http-api spec="client-server" api="key_backup" %}}
|
|
|
|
|
|
##### 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](#key-export-format).
|
|
|
|
|
|
2. Generate a 512-bit key from the user-entered passphrase by computing
|
|
|
[PBKDF2](https://tools.ietf.org/html/rfc2898#section-5.2)(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`
|
|
|
|
|
|
| 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](#mforwarded_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: string} | Required. The Ed25519 key of the device which initiated the session originally. |
|
|
|
| session_id | string | Required. The ID of the session. |
|
|
|
| session_key | string | Required. The key for the session. |
|
|
|
|
|
|
This is similar to the format before encryption used for the session
|
|
|
keys in [Server-side key backups](#server-side-key-backups) but adds the
|
|
|
`room_id` and `session_id` fields.
|
|
|
|
|
|
Example:
|
|
|
|
|
|
```
|
|
|
[
|
|
|
{
|
|
|
"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](http://matrix.org/docs/spec/olm.html). 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:
|
|
|
|
|
|
```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:
|
|
|
|
|
|
```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`](/client-server-api/#post_matrixclientr0keysquery) 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.
|
|
|
|
|
|
{{% boxes/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.
|
|
|
{{% /boxes/note %}}
|
|
|
|
|
|
To establish a new session, the client sends an [m.dummy](#mdummy)
|
|
|
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](http://matrix.org/docs/spec/megolm.html). 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:
|
|
|
|
|
|
```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:
|
|
|
|
|
|
```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 an
|
|
|
`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. An
|
|
|
`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
|
|
|
|
|
|
{{% event event="m.room.encryption" %}}
|
|
|
|
|
|
{{% event event="m.room.encrypted" %}}
|
|
|
|
|
|
{{% event event="m.room_key" %}}
|
|
|
|
|
|
{{% event event="m.room_key_request" %}}
|
|
|
|
|
|
{{% event event="m.forwarded_room_key" %}}
|
|
|
|
|
|
{{% event event="m.dummy" %}}
|
|
|
|
|
|
##### Key management API
|
|
|
|
|
|
{{% http-api spec="client-server" api="keys" %}}
|
|
|
|
|
|
##### Extensions to /sync
|
|
|
|
|
|
This module adds an optional `device_lists` property to the [`/sync`](/client-server-api/#get_matrixclientr0sync) response,
|
|
|
as specified below. The server need only populate this property for an
|
|
|
incremental `/sync` (i.e., one where the `since` parameter was
|
|
|
specified). The client is expected to use [`/keys/query`](/client-server-api/#post_matrixclientr0keysquery) or
|
|
|
[`/keys/changes`](/client-server-api/#get_matrixclientr0keyschanges) for the equivalent functionality after an initial
|
|
|
sync, as documented in [Tracking the device list for a
|
|
|
user](#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`](/client-server-api/#post_matrixclientr0keysupload) response.
|
|
|
|
|
|
| Parameter | Type | Description |
|
|
|
|----------------------------|--------------------|------------------------------------------------------------------------------------------------------------------------|
|
|
|
| device_lists | DeviceLists | Optional. Information on e2e device updates. Note: only present on an incremental sync. |
|
|
|
| device_one_time_keys_count | {string: integer} | Optional. For each key algorithm, the number of 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 or cross-signing 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. |
|
|
|
|
|
|
{{% boxes/note %}}
|
|
|
For optimal performance, Alice should be added to `changed` in Bob's
|
|
|
sync only when she updates her devices or cross-signing keys, 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.
|
|
|
{{% /boxes/note %}}
|
|
|
|
|
|
Example response:
|
|
|
|
|
|
```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
|
|
|
}
|
|
|
}
|
|
|
```
|
|
|
|
|
|
#### Reporting that decryption keys are withheld
|
|
|
|
|
|
When sending an encrypted event to a room, a client can optionally
|
|
|
signal to other devices in that room that it is not sending them the
|
|
|
keys needed to decrypt the event. In this way, the receiving client can
|
|
|
indicate to the user why it cannot decrypt the event, rather than just
|
|
|
showing a generic error message.
|
|
|
|
|
|
In the same way, when one device requests keys from another using [Key
|
|
|
requests](#key-requests), the device from which the key is being
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requested may want to tell the requester that it is purposely not
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sharing the key.
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If Alice withholds a megolm session from Bob for some messages in a
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room, and then later on decides to allow Bob to decrypt later messages,
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she can send Bob the megolm session, ratcheted up to the point at which
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she allows Bob to decrypt the messages. If Bob logs into a new device
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and uses key sharing to obtain the decryption keys, the new device will
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be sent the megolm sessions that have been ratcheted up. Bob's old
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device can include the reason that the session was initially not shared
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by including a `withheld` property in the `m.forwarded_room_key` message
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that is an object with the `code` and `reason` properties from the
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`m.room_key.withheld` message.
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{{% event event="m.room_key.withheld" %}}
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