matrix-spec/content/client-server-api/modules/end_to_end_encryption.md

End-to-End Encryption

Matrix optionally supports end-to-end encryption, allowing rooms to be created whose conversation contents are not decryptable or interceptable on any of the participating homeservers.

Key Distribution

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

Overview

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

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

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

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

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

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

Key algorithms

Different key algorithms are used for different purposes. Each key algorithm is identified by a name and is represented in a specific way.

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

"SogYyrkTldLz0BXP+GYWs0qaYacUI0RleEqNT8J3riQ"

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

"JGLn/yafz74HB2AbPLYJWIVGnKAtqECOBf11yyXac2Y"

The name signed_curve25519 also corresponds to the Curve25519 ECDH algorithm, but the key is signed so that it can be authenticated. A key using this algorithm is represented by an object with the following properties:

KeyObject

Parameter	Type	Description
key	string	Required. The unpadded Base64-encoded 32-byte Curve25519 public key.
signatures	Signatures	Required. Signatures of the key object. The signature is calculated using the process described at Signing JSON.
fallback	boolean	Indicates whether this is a fallback key. Defaults to false.

Example:

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

ed25519 and curve25519 keys are used for device keys. Additionally, ed25519 keys are used for cross-signing keys.

signed_curve25519 keys are used for one-time and fallback keys.

Device keys

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

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

For Olm version 1, each device also requires a single Curve25519 identity key.

One-time and fallback keys

In addition to the device keys, which are long-lived, some encryption algorithms require that devices may also have a number of one-time keys, which are only used once and discarded after use. For Olm version 1, devices use signed_curve25519 one-time keys, signed by the device's Ed25519 key.

Devices will generate one-time keys and upload them to the server; these will later be claimed by other users. Servers must ensure that each one-time key is only claimed once: a homeserver should discard the one time key once it has been given to another user.

{{% added-in v="1.2" %}} Fallback keys are similar to one-time keys, but are not consumed once used. If a fallback key has been uploaded, it will be returned by the server when the device has run out of one-time keys and a user tries to claim a key. Fallback keys should be replaced with new fallback keys as soon as possible after they have been used.

{{% boxes/warning %}} Fallback keys are used to prevent one-time key exhaustion when devices are offline/unable to upload additional keys, though sessions started using fallback keys could be vulnerable to replay attacks. {{% /boxes/warning %}}

Devices will be informed, via /sync, about the number of one-time keys remaining that can be claimed, as well as whether the fallback keys have been used. The device can thus ensure that, while it is online, there is a sufficient supply of one-time keys available, and that the fallback keys get replaced if they have been used.

Uploading keys

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

One-time and fallback keys are also uploaded to the homeserver using the /keys/upload API. New one-time and fallback keys are uploaded as needed. Fallback keys for key algorithms whose format is a signed JSON object should contain a property named fallback with a value of true.

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

{{% boxes/warning %}} Clients should not store the private half of fallback keys indefinitely to avoid situations where attackers can decrypt past messages sent using that fallback key.

Instead, clients should keep the private keys for at most 2 fallback keys: the current, unused, fallback key and the key immediately preceding it. Once the client is reasonably certain it has received all messages that used the old fallback key, such as after an hour since the first message, it should remove that fallback key. {{% /boxes/warning %}}

Tracking the device list for a user

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

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

It is therefore expected that each client will maintain a list of devices for a number of users (in practice, typically each user with whom we share an encrypted room). Furthermore, it is likely that this list will need to be persisted between invocations of the client application (to preserve device verification data and to alert Alice if Bob suddenly gets a new device).

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

1. It first sets a flag to record that it is now tracking Bob's device list, and a separate flag to indicate that its list of Bob's devices is outdated. Both flags should be in storage which persists over client restarts.
2. It then makes a request to /keys/query, passing Bob's user ID in the device_keys parameter. When the request completes, it stores the resulting list of devices in persistent storage, and clears the 'outdated' flag.
3. During its normal processing of responses to /sync, Alice's client inspects the changed property of the device_lists field. If it is tracking the device lists of any of the listed users, then it marks the device lists for those users outdated, and initiates another request to /keys/query for them.
4. Periodically, Alice's client stores the next_batch field of the result from /sync in persistent storage. If Alice later restarts her client, it can obtain a list of the users who have updated their device list while it was offline by calling /keys/changes, passing the recorded next_batch field as the from parameter. If the client is tracking the device list of any of the users listed in the response, it marks them as outdated. It combines this list with those already flagged as outdated, and initiates a /keys/query request for all of them.

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

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

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

Clients MUST guard against these situations. For example, a client could ensure that only one request to /keys/query is in flight at a time for each user, by queuing additional requests until the first completes. Alternatively, the client could make a new request immediately, but ensure that the first request's results are ignored (possibly by cancelling the request). {{% /boxes/warning %}}

{{% boxes/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. {{% /boxes/note %}}

Sending encrypted attachments

When encryption is enabled in a room, files should be uploaded encrypted on the homeserver.

In order to achieve this, a client should generate a single-use 256-bit AES key, and encrypt the file using AES-CTR. The counter should be 64-bit long, starting at 0 and prefixed by a random 64-bit Initialization Vector (IV), which together form a 128-bit unique counter block.

{{% boxes/warning %}} An IV must never be used multiple times with the same key. This implies that if there are multiple files to encrypt in the same message, typically an image and its thumbnail, the files must not share both the same key and IV. {{% /boxes/warning %}}

Then, the encrypted file can be uploaded to the homeserver. The key and the IV must be included in the room event along with the resulting mxc:// in order to allow recipients to decrypt the file. As the event containing those will be Megolm encrypted, the server will never have access to the decrypted file.

A hash of the ciphertext must also be included, in order to prevent the homeserver from changing the file content.

A client should send the data as an encrypted m.room.message event, using either m.file as the msgtype, or the appropriate msgtype for the file type. The key is sent using the JSON Web Key format, with a W3C extension.

Extensions to m.room.message msgtypes

This module adds file and thumbnail_file properties, of type EncryptedFile, to m.room.message msgtypes that reference files, such as m.file and m.image, replacing the url and thumbnail_url properties.

EncryptedFile

Parameter	Type	Description
url	string	Required. The URL to the file.
key	JWK	Required. A JSON Web Key object.
iv	string	Required. The 128-bit unique counter block used by AES-CTR, encoded as unpadded base64.
hashes	{string: string}	Required. A map from an algorithm name to a hash of the ciphertext, encoded as unpadded base64. Clients should support the SHA-256 hash, which uses the key sha256.
v	string	Required. Version of the encrypted attachment's protocol. Must be v2.

JWK

Parameter	Type	Description
kty	string	Required. Key type. Must be oct.
key_ops	[string]	Required. Key operations. Must at least contain encrypt and decrypt.
alg	string	Required. Algorithm. Must be A256CTR.
k	string	Required. The key, encoded as urlsafe unpadded base64.
ext	boolean	Required. Extractable. Must be true. This is a W3C extension.

Example:

{
  "content": {
    "body": "something-important.jpg",
    "file": {
      "url": "mxc://example.org/FHyPlCeYUSFFxlgbQYZmoEoe",
      "v": "v2",
      "key": {
        "alg": "A256CTR",
        "ext": true,
        "k": "aWF6-32KGYaC3A_FEUCk1Bt0JA37zP0wrStgmdCaW-0",
        "key_ops": ["encrypt","decrypt"],
        "kty": "oct"
      },
      "iv": "w+sE15fzSc0AAAAAAAAAAA",
      "hashes": {
        "sha256": "fdSLu/YkRx3Wyh3KQabP3rd6+SFiKg5lsJZQHtkSAYA"
      }
    },
    "info": {
      "mimetype": "image/jpeg",
      "h": 1536,
      "size": 422018,
      "thumbnail_file": {
        "hashes": {
          "sha256": "/NogKqW5bz/m8xHgFiH5haFGjCNVmUIPLzfvOhHdrxY"
        },
        "iv": "U+k7PfwLr6UAAAAAAAAAAA",
        "key": {
          "alg": "A256CTR",
          "ext": true,
          "k": "RMyd6zhlbifsACM1DXkCbioZ2u0SywGljTH8JmGcylg",
          "key_ops": ["encrypt", "decrypt"],
          "kty": "oct"
        },
        "url": "mxc://example.org/pmVJxyxGlmxHposwVSlOaEOv",
        "v": "v2"
      },
      "thumbnail_info": {
        "h": 768,
        "mimetype": "image/jpeg",
        "size": 211009,
        "w": 432
      },
      "w": 864
    },
    "msgtype": "m.image"
  },
  "event_id": "$143273582443PhrSn:example.org",
  "origin_server_ts": 1432735824653,
  "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
  "sender": "@example:example.org",
  "type": "m.room.message",
  "unsigned": {
      "age": 1234
  }
}

Device verification

Before Alice sends Bob encrypted data, or trusts data received from him, she may want to verify that she is actually communicating with him, rather than a man-in-the-middle. This verification process requires an out-of-band channel: there is no way to do it within Matrix without trusting the administrators of the homeservers.

In Matrix, verification works by Alice meeting Bob in person, or contacting him via some other trusted medium, and using one of the verification methods defined below to interactively verify Bob's devices. Alice and Bob may also read aloud their unpadded base64 encoded Ed25519 public key, as returned by /keys/query.

Device verification may reach one of several conclusions. For example:

• Alice may "accept" the device. This means that she is satisfied that the device belongs to Bob. She can then encrypt sensitive material for that device, and knows that messages received were sent from that device.
• Alice may "reject" the device. She will do this if she knows or suspects that Bob does not control that device (or equivalently, does not trust Bob). She will not send sensitive material to that device, and cannot trust messages apparently received from it.
• Alice may choose to skip the device verification process. She is not able to verify that the device actually belongs to Bob, but has no reason to suspect otherwise. The encryption protocol continues to protect against passive eavesdroppers.

{{% boxes/note %}} Once the signing key has been verified, it is then up to the encryption protocol to verify that a given message was sent from a device holding that Ed25519 private key, or to encrypt a message so that it may only be decrypted by such a device. For the Olm protocol, this is documented at https://matrix.org/docs/olm_signing.html. {{% /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 room between the two parties, or by using to-device 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 a key verification request event. If the verification is being requested in a room, this will be an event with type m.room.message and msgtype: m.key.verification.request; if the verification is being requested using to-device messaging, this will be an event with type m.key.verification.request. 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           |               |
            |                 |---------------------------------->|               |
            |                 |                                   |               |

In contrast with the case of using to-devices messages, when using in-room messages, Alice only sends one request event (an event with type m.room.message with msgtype: m.key.verification.request, rather than an event with type m.key.verification.request), to the room. In addition, Alice does not send an m.key.verification.cancel event to tell Bob's other devices that the request as already been accepted; instead, when Bob's other devices see his m.key.verification.ready event, they will know that the request has already been accepted, and that they should ignore the request.

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.

When verifying using to-device messages, an m.key.verification.start message can also be sent independently of any request, specifying the verification method to use. This behaviour is deprecated, and new clients should not begin verifications in this way. However, clients should handle such verifications started by other clients.

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.room.message$m.key.verification.request" title="m.room.message with msgtype: m.key.verification.request" %}}

{{% 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 2ⁿ chance of success. For example, if we verify 40 bits, then an attacker has a 1 in 1,099,511,627,776 chance (or less than 1 in 10¹² chance) of success. A failed attack would result in a mismatched Short Authentication String, alerting users to the attack.

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 in two phases:

1. Key agreement phase (based on ZRTP key agreement).
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 begin a key verification using the key verification framework as described above.

3. Alice's device sends Bob's device an m.key.verification.start message. Alice's device ensures it has a copy of Bob's device key.

4. 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.

5. Bob's device ensures it has a copy of Alice's device key.

6. Bob's device creates an ephemeral Curve25519 key pair (K_B^private, K_B^public), and calculates the hash (using the chosen algorithm) of the public key K_B^public.

7. Bob's device replies to Alice's device with an m.key.verification.accept message.

8. Alice's device receives Bob's message and stores the commitment hash for later use.

9. Alice's device creates an ephemeral Curve25519 key pair (K_A^private, K_A^public) and replies to Bob's device with an m.key.verification.key, sending only the public key K_A^public.

10. Bob's device receives Alice's message and replies with its own m.key.verification.key message containing its public key K_B^public.

11. 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.

12. Both Alice's and Bob's devices perform an Elliptic-curve Diffie-Hellman using their private ephemeral key, and the other device's ephemeral public key (ECDH(K_A^private, K_B^public) for Alice's device and ECDH(K_B^private, K_A^public) for Bob's device), using the result as the shared secret.

13. 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.

14. Alice and Bob compare the strings shown by their devices, and tell their devices if they match or not.

15. Assuming they match, Alice and Bob's devices each calculate Message Authentication Codes (MACs) for:

    • Each of the keys that they wish the other user to verify (usually their device ed25519 key and their master cross-signing key).
    • The complete list of key IDs that they wish the other user to verify.

    The MAC calculation is defined below.

16. 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.

17. When the other device receives the m.key.verification.mac message, the device calculates the MACs of its copies of the other device's keys given in the message, as well as the MAC of the comma-separated, sorted, list of key IDs in the message. The device compares these with the MAC values given in the message, and if everything matches then the device keys are verified.

18. Alice and Bob's devices send m.key.verification.done messages to complete the verification.

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" title="m.key.verification.start with method: m.sas.v1" %}}

{{% event event="m.key.verification.accept" %}}

{{% event event="m.key.verification.key" %}}

{{% event event="m.key.verification.mac" %}}

MAC calculation

During the verification process, Message Authentication Codes (MACs) are calculated for keys and lists of key IDs.

The method used to calculate these MACs depends upon the value of the message_authentication_code property in the m.key.verification.accept message. All current implementations should use the hkdf-hmac-sha256.v2 method which is defined as follows:

1. An HMAC key is generated using HKDF, as defined in RFC 5869, using 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.

2. A MAC is then generated using HMAC as defined in RFC 2104 with the key generated above and using SHA-256 as the hash function.

   If a key is being MACed, the MAC is performed on the public key as encoded according to the key algorithm. For example, for ed25519 keys, it is the unpadded base64-encoded key.

   If the key list is being MACed, the list is sorted lexicographically and comma-separated with no extra whitespace added, with each key written in the form {algorithm}:{keyId}. For example, the key list could look like: ed25519:Cross+Signing+Key,ed25519:DEVICEID. In this way, the recipient can reconstruct the list from the names in the mac property of the m.key.verification.mac message and ensure that no keys were added or removed.

3. The MAC values are base64-encoded and sent in a m.key.verification.mac message.

{{% boxes/note %}} The MAC method hkdf-hmac-sha256 used an incorrect base64 encoding, due to a bug in the original implementation in libolm. To remedy this, hkdf-hmac-sha256.v2 was introduced, which calculates the MAC in the same way, but uses a correct base64 encoding. hkdf-hmac-sha256 is deprecated and will be removed in a future version of the spec. Use of hkdf-hmac-sha256 should be avoided whenever possible: if both parties support hkdf-hmac-sha256.v2, then hkdf-hmac-sha256 MUST not be used. {{% /boxes/note %}}

SAS HKDF calculation

In all of the SAS methods, HKDF is as defined in RFC 5869 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, encoded as unpadded base64, 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, encoded as unpadded base64, 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 %}}

SAS method: decimal

Generate 5 bytes using HKDF then take sequences of 13 bits to convert to decimal numbers (resulting in 3 numbers between 0 and 8191 inclusive each). Add 1000 to each calculated number.

The bitwise operations to get the numbers given the 5 bytes B₀, B₁, B₂, B₃, B₄ would be:

• First: (B₀ ≪ 5|B₁ ≫ 3) + 1000
• Second: ((B₁&0x7) ≪ 10|B₂ ≪ 2|B₃ ≫ 6) + 1000
• Third: ((B₃&0x3F) ≪ 7|B₄ ≫ 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 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-spec/blob/main/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/v3/keys/device_signing/upload. 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/v3/keys/query 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 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.

The cross-signing private keys can be stored on the server or shared with other devices using the Secrets module. When doing so, the master, user-signing, and self-signing keys are identified using the names m.cross_signing.master, m.cross_signing.user_signing, and m.cross_signing.self_signing, respectively, and the keys are base64-encoded before being encrypted.

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.

Since device key IDs (ed25519:DEVICE_ID) and cross-signing key IDs (ed25519:PUBLIC_KEY) occupy the same namespace, clients must ensure that they use the correct keys when verifying.

While servers MUST not allow devices to have the same IDs as cross-signing keys, a malicious server could construct such a situation, so clients must not rely on the server being well-behaved and should take the following precautions against this.

1. Clients MUST refer to keys by their public keys during the verification process, rather than only by the key ID.
2. Clients MUST fix the keys that are being verified at the beginning of the verification process, and ensure that they do not change in the course of verification.
3. Clients SHOULD also display a warning and MUST refuse to verify a user when they detect that the user has a device with the same ID as a cross-signing key.

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" %}}

QR codes

{{% added-in v="1.1" %}}

Verifying by QR codes provides a quick way to verify when one of the parties has a device capable of scanning a QR code. The QR code encodes both parties' master signing keys as well as a random shared secret that is used to allow bi-directional verification from a single scan.

To advertise the ability to show a QR code, clients use the names m.qr_code.show.v1 and m.reciprocate.v1 in the methods fields of the m.key.verification.request and m.key.verification.ready events. To advertise the ability to scan a QR code, clients use the names m.qr_code.scan.v1 and m.reciprocate.v1 in the methods fields of the m.key.verification.request and m.key.verification.ready events. Clients that support both showing and scanning QR codes would advertise m.qr_code.show.v1, m.qr_code.scan.v1, and m.reciprocate.v1 as methods.

The process between Alice and Bob verifying each other would be:

1. Alice and Bob meet in person, and want to verify each other's keys.

2. Alice and Bob begin a key verification using the key verification framework as described above.

3. Alice's client displays a QR code that Bob is able to scan if Bob's client indicated the ability to scan, an option to scan Bob's QR code if her client is able to scan. Bob's client prompts displays a QR code that Alice can scan if Alice's client indicated the ability to scan, and an option to scan Alice's QR code if his client is able to scan. The format for the QR code is described below. Other options, like starting SAS Emoji verification, can be presented alongside the QR code if the devices have appropriate support.

4. Alice scans Bob's QR code.

5. Alice's device ensures that the keys encoded in the QR code match the expected values for the keys. If not, Alice's device displays an error message indicating that the code is incorrect, and sends a m.key.verification.cancel message to Bob's device.

   Otherwise, at this point:

   • Alice's device has now verified Bob's key, and
   • Alice's device knows that Bob has the correct key for her.

   Thus for Bob to verify Alice's key, Alice needs to tell Bob that he has the right key.

6. Alice's device displays a message saying that the verification was successful because the QR code's keys will have matched the keys expected for Bob. Bob's device hasn't had a chance to verify Alice's keys yet so wouldn't show the same message. Bob will know that he has the right key for Alice because Alice's device will have shown this message, as otherwise the verification would be cancelled.

7. Alice's device sends an m.key.verification.start message with method set to m.reciprocate.v1 to Bob (see below). The message includes the shared secret from the QR code. This signals to Bob's device that Alice has scanned Bob's QR code.

   This message is merely a signal for Bob's device to proceed to the next step, and is not used for verification purposes.

8. Upon receipt of the m.key.verification.start message, Bob's device ensures that the shared secret matches.

   If the shared secret does not match, it should display an error message indicating that an attack was attempted. (This does not affect Alice's verification of Bob's keys.)

   If the shared secret does match, it asks Bob to confirm that Alice has scanned the QR code.

9. Bob sees Alice's device confirm that the key matches, and presses the button on his device to indicate that Alice's key is verified.

   Bob's verification of Alice's key hinges on Alice telling Bob the result of her scan. Since the QR code includes what Bob thinks Alice's key is, Alice's device can check whether Bob has the right key for her. Alice has no motivation to lie about the result, as getting Bob to trust an incorrect key would only affect communications between herself and Bob. Thus Alice telling Bob that the code was scanned successfully is sufficient for Bob to trust Alice's key, under the assumption that this communication is done over a trusted medium (such as in-person).

10. Both devices send an m.key.verification.done message.

QR code format

The QR codes to be displayed and scanned MUST be compatible with ISO/IEC 18004:2015 and contain a single segment that uses the byte mode encoding.

The error correction level can be chosen by the device displaying the QR code.

The binary segment MUST be of the following form:

• the string MATRIX encoded as one ASCII byte per character (i.e. 0x4D, 0x41, 0x54, 0x52, 0x49, 0x58)
• one byte indicating the QR code version (must be 0x02)
• one byte indicating the QR code verification mode. Should be one of the following values:
  • 0x00 verifying another user with cross-signing
  • 0x01 self-verifying in which the current device does trust the master key
  • 0x02 self-verifying in which the current device does not yet trust the master key
• the event ID or transaction_id of the associated verification request event, encoded as:
  • two bytes in network byte order (big-endian) indicating the length in bytes of the ID as a UTF-8 string
  • the ID encoded as a UTF-8 string
• the first key, as 32 bytes. The key to use depends on the mode field:
  • if 0x00 or 0x01, then the current user's own master cross-signing public key
  • if 0x02, then the current device's Ed25519 signing key
• the second key, as 32 bytes. The key to use depends on the mode field:
  • if 0x00, then what the device thinks the other user's master cross-signing public key is
  • if 0x01, then what the device thinks the other device's Ed25519 signing public key is
  • if 0x02, then what the device thinks the user's master cross-signing public key is
• a random shared secret, as a sequence of bytes. It is suggested to use a secret that is about 8 bytes long. Note: as we do not share the length of the secret, and it is not a fixed size, clients will just use the remainder of binary segment as the shared secret.

For example, if Alice displays a QR code encoding the following binary data:

      "MATRIX"    |ver|mode| len   | event ID
 4D 41 54 52 49 58  02  00   00 2D   21 41 42 43 44 ...
| user's cross-signing key    | other user's cross-signing key | shared secret
  00 01 02 03 04 05 06 07 ...   10 11 12 13 14 15 16 17 ...      20 21 22 23 24 25 26 27

this indicates that Alice is verifying another user (say Bob), in response to the request from event "$ABCD...", her cross-signing key is 0001020304050607... (which is "AAECAwQFBg..." in base64), she thinks that Bob's cross-signing key is 1011121314151617... (which is "EBESExQVFh..." in base64), and the shared secret is 2021222324252627 (which is "ICEiIyQlJic" in base64).

Verification messages specific to QR codes

{{% event event="m.key.verification.start$m.reciprocate.v1" title="m.key.verification.start with method: m.reciprocate.v1" %}}

Sharing keys between devices

If Bob has an encrypted conversation with Alice on his computer, and then logs in through his phone for the first time, he may want to have access to the previously exchanged messages. To address this issue, several methods are provided to allow users to transfer keys from one device to another.

Key requests

When a device is missing keys to decrypt messages, it can request the keys by sending m.room_key_request to-device messages to other devices with action set to request.

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

If a device does not wish to share keys with that device, it can indicate this by sending an m.room_key.withheld to-device message, as described in Reporting that decryption keys are withheld.

{{% boxes/note %}} Key sharing can be a big attack vector, thus it must be done very carefully. Clients should only send keys requested by the verified devices of the same user, and should only request and accept forwarded keys from 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, 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, the client requires a decryption key to decrypt the session keys.

To create a backup, a client will call POST /_matrix/client/v3/room_keys/version 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/v3/room_keys/version. Keys are encrypted according to the backup's auth_data and added to the backup by calling PUT /_matrix/client/v3/room_keys/keys or one of its variants, and can be retrieved by calling GET /_matrix/client/v3/room_keys/keys 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/v3/room_keys/version/{version}, or individual keys can be deleted using DELETE /_matrix/client/v3/room_keys/keys or one of its variants.

Clients must only store keys in backups after they have ensured that the auth_data is trusted. This can be done either by:

• checking that it is signed by the user's master cross-signing key or by a verified device belonging to the same user, or
• deriving the public key from a private key that it obtained from a trusted source. Trusted sources for the private key include the user entering the key, retrieving the key stored in secret storage, or obtaining the key via secret sharing from a verified device belonging to the same user.

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, if is_verified and first_message_index are equal, then it will keep the key with a lower forwarded_count.

Decryption key

Normally, the decryption key (i.e. the secret part of the encryption key) is stored on the server or shared with other devices using the Secrets module. When doing so, it is identified using the name m.megolm_backup.v1, and the key is base64-encoded before being encrypted.

If the backup decryption key is given directly to the user, the key should be presented as a string using the common cryptographic key representation.

{{% boxes/note %}} The backup decryption key was previously referred to as a "recovery key". However, this conflicted with common practice in client user interfaces, which often use the term "recovery key" to refer to the secret storage key. The term "recovery key" is no longer used in this specification. {{% /boxes/note %}}

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:

{{% definition path="api/client-server/definitions/key_backup_auth_data" %}}

The session_data field in the backups is constructed as follows:

1. Encode the session key to be backed up as a JSON object using the BackedUpSessionData format defined below.

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 an empty string 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.

{{% boxes/warning %}} Step 5 was intended to pass the raw encrypted data, but due to a bug in libolm, all implementations have since passed an empty string instead.

Future versions of the spec will fix this problem. See MSC4048 for a potential new key backup algorithm version that would fix this issue. {{% /boxes/warning %}}

{{% definition path="api/client-server/definitions/key_backup_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.

2. Generate a 512-bit key from the user-entered passphrase by computing PBKDF2(HMAC-SHA-512, passphrase, S, N, 512), where S is a 128-bit cryptographically-random salt and N is the number of rounds. N should be at least 100,000. The keys K and K' are set to the first and last 256 bits of this generated key, respectively. K is used as an AES-256 key, and K' is used as an HMAC-SHA-256 key.

3. Serialize the JSON object as a UTF-8 string, and encrypt it using AES-CTR-256 with the key K generated above, and with a 128-bit cryptographically-random initialization vector, IV, that has bit 63 set to zero. (Setting bit 63 to zero in IV is needed to work around differences in implementations of AES-CTR.)

4. Concatenate the following data:

Size (bytes)	Description
1	Export format version, which must be 0x01.
16	The salt S.
16	The initialization vector IV.
4	The number of rounds N, as a big-endian unsigned 32-bit integer.
variable	The encrypted JSON object.
32	The HMAC-SHA-256 of all the above string concatenated together, using K' as the key.

5. Base64-encode the string above. Newlines may be added to avoid overly long lines.

6. Prepend the resulting string with -----BEGIN MEGOLM SESSION DATA-----, with a trailing newline, and append -----END MEGOLM SESSION DATA-----, with a leading and trailing newline.

Key export format

The exported sessions are formatted as a JSON array of ExportedSessionData objects described as follows:

{{% definition path="api/client-server/definitions/megolm_export_session_data" %}}

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:

{
  "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:

{
  "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 property in the cleartext m.room.encrypted event body, and the keys.ed25519 property in the decrypted plaintext, match the keys returned by /keys/query for the given user. Clients must also verify the signature of the keys from the /keys/query response. 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 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.

{{% boxes/note %}} Clients should send key requests for unknown sessions to all devices for the user which used the session rather than just the device_id or sender_key denoted on the event.

This is due to a deprecation of the fields. See m.megolm.v1.aes-sha2 for more information. {{% /boxes/note %}}

m.megolm.v1.aes-sha2

{{% changed-in v="1.3" %}}

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:

{
  "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:

{
  "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.

Similar to Olm events, clients should confirm that the user who sent the message corresponds to the user the message was expected to come from. For room events, this means ensuring the event's sender, room_id, and the recorded session_id match a trusted session (eg: the session_id is already known and validated to the client).

{{% boxes/note %}} As of v1.3, the sender_key and device_id keys are deprecated. They SHOULD continue to be sent, however they MUST NOT be used to verify the message's source.

Clients MUST NOT store or lookup sessions using the sender_key or device_id.

In a future version of the specification the keys can be removed completely, including for sending new messages. {{% /boxes/note %}}

{{% boxes/rationale %}} Removing the fields (eventually) improves privacy and security by masking the device which sent the encrypted message as well as reducing the client's dependence on untrusted data: a malicious server (or similar attacker) could change these values, and other devices/users can simply lie about them too.

We can remove the fields, particularly the sender_key, because the session_id is already globally unique, therefore making storage and lookup possible without the need for added context from the sender_key or device_id.

Removing the dependence on the fields gives a privacy gain while also increasing the security of messages transmitted over Matrix. {{% /boxes/rationale %}}

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.

When a client receives a Megolm session, the client MUST ensure that the session was received via an Olm channel, in order to ensure the authenticity of the messages.

When a client is updating a Megolm session in its store, the client MUST ensure:

• that the updated session data comes from a trusted source, such as via a m.forwarded_room_key event from a verified device belonging to the same user, or from a m.room_key event.
• that the new session key has a lower message index than the existing session key.

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 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 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 device_one_time_keys_count property. Note the spelling difference with the one_time_key_counts property in the /keys/upload response.

{{% added-in v="1.2" %}} Finally, a device_unused_fallback_key_types property is added to list the key algorithms where the device has a fallback key that has not been used in a /keys/claim response. When a previously uploaded fallback key's algorithm is missing from this list, the device should upload a replacement key alongside any necessary one-time keys to avoid the fallback key's further usage. This property is required for inclusion, though previous versions of the specification did not have it. In addition to /versions, this can be a way to identify the server's support for fallback keys.

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. If an algorithm is unlisted, the count for that algorithm is assumed to be zero. If this entire parameter is missing, the count for all algorithms is assumed to be zero.
device_unused_fallback_key_types	[string]	Required. The unused fallback key algorithms.

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:

{
  "next_batch": "s72595_4483_1934",
  "rooms": {"leave": {}, "join": {}, "invite": {}},
  "device_lists": {
    "changed": [
       "@alice:example.com",
    ],
    "left": [
       "@bob:example.com",
    ],
  },
  "device_one_time_keys_count": {
    "signed_curve25519": 20
  },
  "device_unused_fallback_key_types": ["signed_curve25519"]
}

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, the device from which the key is being requested may want to tell the requester that it is purposely not sharing the key.

If Alice withholds a megolm session from Bob for some messages in a room, and then later on decides to allow Bob to decrypt later messages, she can send Bob the megolm session, ratcheted up to the point at which she allows Bob to decrypt the messages. If Bob logs into a new device and uses key sharing to obtain the decryption keys, the new device will be sent the megolm sessions that have been ratcheted up. Bob's old device can include the reason that the session was initially not shared by including a withheld property in the m.forwarded_room_key message that is an object with the code and reason properties from the m.room_key.withheld message.

{{% event event="m.room_key.withheld" %}}