Implementing End-to-End Encryption in Matrix clients

This guide is intended for authors of Matrix clients who wish to add support for end-to-end encryption. It is highly recommended that readers be familiar with the Matrix protocol and the use of access tokens before proceeding.

The libolm library

End-to-end encryption in Matrix is based on the Olm and Megolm cryptographic ratchets. The recommended starting point for any client authors is with the libolm library, which contains implementations of all of the cryptographic primitives required. The library itself is written in C/C++, but is architected in a way which makes it easy to write wrappers for higher-level languages.

Devices

We have a particular meaning for “device”. As a user, I might have several devices (a desktop client, some web browsers, an Android device, an iPhone, etc). When I first use a client, it should register itself as a new device. If I log out and log in again as a different user, the client must register as a new device. Critically, the client must create a new set of keys (see below) for each “device”.

The longevity of devices will depend on the client. In the web client, we create a new device every single time you log in. In a mobile client, it might be acceptable to reuse the device if a login session expires, provided the user is the same. Never share keys between different users.

Devices are identified by their device_id (which is unique within the scope of a given user). By default, the /login and /register endpoints will auto-generate a device_id and return it in the response; a client is also free to generate its own device_id or, as above, reuse a device, in which case the client should pass the device_id in the request body.

The lifetime of devices and access_tokens are closely related. In the simple case where a new device is created each time you log in, there is a one-to-one mapping between a device_id and an access_token. If a client reuses a device_id when logging in, there will be several access_tokens associated with a given device_id - but still, we would expect only one of these to be active at once (though we do not currently enforce that in Synapse).

Keys used in End-to-End encryption

There are a number of keys involved in encrypted communication: a summary of them follows.

Ed25519 fingerprint key pair

Ed25519 is a public-key cryptographic system for signing messages. In Matrix, each device has an Ed25519 key pair which serves to identify that device. The private part of the key pair should never leave the device, but the public part is published to the Matrix network.

Curve25519 identity key pair

Curve25519 is a public-key cryptographic system which can be used to establish a shared secret. In Matrix, each device has a long-lived Curve25519 identity key which is used to establish Olm sessions with that device. Again, the private key should never leave the device, but the public part is signed with the Ed25519 fingerprint key and published to the network.

Theoretically we should rotate the Curve25519 identity key from time to time, but we haven't implemented this yet.

Curve25519 one-time keys

As well as the identity key, each device creates a number of Curve25519 key pairs which are also used to establish Olm sessions, but can only be used once. Once again, the private part remains on the device.

At startup, Alice creates a number of one-time key pairs, and publishes them to her homeserver. If Bob wants to establish an Olm session with Alice, he needs to claim one of Alice’s one-time keys, and creates a new one of his own. Those two keys, along with Alice’s and Bob’s identity keys, are used in establishing an Olm session between Alice and Bob.

Megolm encryption keys

The Megolm key is used to encrypt group messages (in fact it is used to derive an AES-256 key, and an HMAC-SHA-256 key). It is initialised with random data. Each time a message is sent, a hash calculation is done on the Megolm key to derive the key for the next message. It is therefore possible to share the current state of the Megolm key with a user, allowing them to decrypt future messages but not past messages.

Ed25519 Megolm signing key pair

When a sender creates a Megolm session, he also creates another Ed25519 signing key pair. This is used to sign messages sent via that Megolm session, to authenticate the sender. Once again, the private part of the key remains on the device. The public part is shared with other devices in the room alongside the encryption key.

Creating and registering device keys

This process only happens once, when a device first starts.

It must create the Ed25519 fingerprint key pair and the Curve25519 identity key pair. This is done by calling olm_create_account in libolm. The (base64-encoded) keys are retrieved by calling olm_account_identity_keys. The account should be stored for future use.

It should then publish these keys to the homeserver. To do this, it should construct a JSON object as follows:

{
  "algorithms": ["m.olm.v1.curve25519-aes-sha2", "m.megolm.v1.aes-sha2"],
  "device_id": "<device_id>",
  "keys": {
    "curve25519:<device_id>": "<curve25519_key>",
    "ed25519:<device_id>": "<ed25519_key>"
  },
  "user_id: <user_id>"
}

The object should be formatted as Canonical JSON, then signed with olm_account_sign; the signature should be added to the JSON as signatures.<user_id>.ed25519:<device_id>.

The signed JSON is then uploaded via POST /_matrix/client/unstable/keys/upload.

Creating and registering one-time keys

At first start, and at regular intervals thereafter[1], the client should check how many one-time keys the homeserver has stored for it, and, if necessary, generate and upload some more.

[1]Every 10 minutes is suggested.

The number of one-time keys currently stored is returned by POST /_matrix/client/unstable/keys/upload. (Post an empty JSON object {} if you don’t want to upload the device keys.)

The maximum number of active keys supported by libolm is returned by olm_account_max_number_of_one_time_keys. The client should try to maintain about half this number on the homeserver.

To generate new one-time keys:

  • Call olm_account_generate_one_time_keys to generate new keys.

  • Call olm_account_one_time_keys to retrieve the unpublished keys. This returns a JSON-formatted object with the single property curve25519, which is itself an object mapping key id to base64-encoded Curve25519 key. For example:

    {
      "curve25519": {
        "AAAAAA": "wo76WcYtb0Vk/pBOdmduiGJ0wIEjW4IBMbbQn7aSnTo",
        "AAAAAB": "LRvjo46L1X2vx69sS9QNFD29HWulxrmW11Up5AfAjgU"
      }
    }
    
  • Each key should be signed with the account key. To do this:

    • Construct a JSON object as follows:

      {
        "key": "<curve25519_key>"
      }
      
    • Call olm_account_sign to calculate the signature.

    • Add the signature should be added to the JSON as signatures.<user_id>.ed25519:<device_id>.

    • The complete key object should now look like:

      {
        "key": "wo76WcYtb0Vk/pBOdmduiGJ0wIEjW4IBMbbQn7aSnTo",
        "signatures": {
          "@alice:example.com": {
            "ed25519:JLAFKJWSCS": "dSO80A01XiigH3uBiDVx/EjzaoycHcjq9lfQX0uWsqxl2giMIiSPR8a4d291W1ihKJL/a+myXS367WT6NAIcBA"
          }
        }
      }
      
  • Aggregate all the signed one-time keys into a single JSON object as follows:

    {
      "one_time_keys": {
        "signed_curve25519:<key_id>": {
          "key": "<curve25519_key>",
          "signatures": {
            "<user_id>": {
              "ed25519:<device_id>": "<signature>"
            }
          }
        },
        "signed_curve25519:<key_id>": {
          ...
        },
        ...
      }
    }
    
  • Upload the object via POST /_matrix/client/unstable/keys/upload.

  • Call olm_account_mark_keys_as_published to tell the olm library not to return the same keys from a future call to olm_account_one_time_keys.

Configuring a room to use encryption

To enable encryption in a room, a client should send a state event of type m.room.encryption, and content { "algorithm": "m.megolm.v1.aes-sha2" }.

Handling an m.room.encryption state event

When a client receives an m.room.encryption event as above, it should set a flag to indicate that messages sent in the room should be encrypted.

This flag should not be cleared if a later m.room.encryption event changes the configuration. This is to avoid a situation where a MITM can simply ask participants to disable encryption. In short: once encryption is enabled in a room, it can never be disabled.

The event should contain an algorithm property which defines which encryption algorithm should be used for encryption. Currently only m.megolm.v1-aes-sha2 is permitted here.

The event may also include other settings for how messages sent in the room should be encrypted (for example, rotation_period_ms to define how often the session should be replaced).

Handling an m.room.encrypted event

Encrypted events have a type of m.room.encrypted. They have a content property algorithm which gives the encryption algorithm in use, as well as other properties specific to the algorithm.

The encrypted payload is a JSON object with the properties type (giving the decrypted event type), and content (giving the decrypted content). Depending on the algorithm in use, the payload may contain additional keys.

There are currently two defined algorithms:

m.olm.v1.curve25519-aes-sha2

Encrypted events using this algorithm should have a sender_key and a ciphertext property.

The sender_key property of the event content gives the Curve25519 identity key of the sender. Clients should maintain a list of known Olm sessions for each device they speak to; it is recommended to index them by Curve25519 identity key.

Olm messages are encrypted separately for each recipient device. ciphertext is an object mapping from the Curve25519 identity key for the recipient device. The receiving client should, of course, look for its own identity key in this object. (If it isn't listed, the message wasn't sent for it, and the client can't decrypt it; it should show an error instead, or similar).

This should result in an object with the properties type and body. Messages of type '0' are 'prekey' messages which are used to establish a new Olm session between two devices; type '1' are normal messages which are used once a message has been received on the session.

When a message (of either type) is received, a client should first attempt to decrypt it with each of the known sessions for that sender. There are two steps to this:

  • If (and only if) type==0, the client should call olm_matches_inbound_session with the session and body. This returns a flag indicating whether the message was encrypted using that session.
  • The client calls olm_decrypt, with the session, type, and body. If this is successful, it returns the plaintext of the event.

If the client was unable to decrypt the message using any known sessions (or if there are no known sessions yet), and the message had type 0, and olm_matches_inbound_session wasn't true for any existing sessions, then the client can try establishing a new session. This is done as follows:

  • Call olm_create_inbound_session_from using the olm account, and the sender_key and body of the message.
  • If the session was established successfully:
    • call olm_remove_one_time_keys to ensure that the same one-time-key cannot be reused.
    • Call olm_decrypt with the new session
    • Store the session for future use

At the end of this, the client will hopefully have successfully decrypted the payload.

As well as the type and content properties, the payload should contain a number of other properties. Each of these should be checked as follows [2].

sender
The user ID of the sender. The client should check that this matches the sender in the event.
recipient
The user ID of the recipient. The client should check that this matches the local user ID.
keys
an object with a property ed25519, The client should check that the value of this property matches the sender's fingerprint key when marking the event as verified.

recipient_keys

an object with a property ed25519. The client should check that the value of this property matches its own fingerprint key.
[2]These tests prevent an attacker publishing someone else's curve25519 keys as their own and subsequently claiming to have sent messages which they didn't.

m.megolm.v1.aes-sha2

Encrypted events using this algorithm should have sender_key, session_id and ciphertext content properties. If the room_id, sender_key and session_id correspond to a known Megolm session (see below), the ciphertext can be decrypted by passing the ciphertext into olm_group_decrypt.

In order to avoid replay attacks a client should remember the megolm message_index returned by olm_group_decrypt of each event they decrypt for each session. If the client decrypts an event with the same message_index as one that it has already received using that session then it should treat the message as invalid.

The client should check that the sender's fingerprint key matches the keys.ed25519 property of the event which established the Megolm session when marking the event as verified.

Handling an m.room_key event

These events contain key data to allow decryption of other messages. They are sent to specific devices, so they appear in the to_device section of the response to GET /_matrix/client/r0/sync. They will also be encrypted, so will need decrypting as above before they can be seen. (These events are generated by other clients - see starting a megolm session).

The event content will contain an 'algorithm' property, indicating the encryption algorithm the key data is to be used for. Currently, this will always be m.megolm.v1.aes-sha2.

Room key events for Megolm will also have room_id, session_id, and session_key keys. They are used to establish a Megolm session. The room_id identifies which room the session will be used in. The room_id, together with the sender_key of the room_key event before it was decrypted, and the session_id, uniquely identify a Megolm session. If they do not represent a known session, the client should start a new inbound Megolm session by calling olm_init_inbound_group_session with the session_key.

The client should remember the value of the keys property of the payload of the encrypted m.room_key event and store it with the inbound session. This is used as above when marking the event as verified.

Downloading the device list for users in the room

Before an encrypted message can be sent, it is necessary to retrieve the list of devices for each user in the room. This can be done proactively, or deferred until the first message is sent. The information is also required to allow users to verify or block devices.

The client should build a JSON query object as follows:

{
  "<user_id>": {},
  ...
}

Each member in the room should be included in the query. This is then sent via POST /_matrix/client/unstable/keys/query.

The result includes, for each listed user id, a map from device ID to an object containing information on the device, as follows:

{
  "algorithms": [...],
  "device_id": "<device_id>",
  "keys": {
    "curve25519:<device_id>": "<curve25519_key>",
    "ed25519:<device_id>": "<ed25519_key>"
  },
  "signatures": {
    "<userId>": {
      "ed25519:<device_id>": "<signature>"
    },
  },
  "unsigned": {
    "device_display_name": "<display name>"
  },
  "user_id: <user_id>"
}

The client should first check the signature on this object. To do this, it should remove the signatures and unsigned properties, format the remainder as Canonical JSON, and pass the result into olm_ed25519_verify, using the Ed25519 key for the key parameter, and the corresponding signature for the signature parameter. If the signature check fails, no further processing should be done on the device.

The client must also check that the user_id and device_id fields in the object match those in the top-level map [3].

The client should check if the user_id/device_id correspond to a device it had seen previously. If it did, the client must check that the Ed25519 key hasn't changed. Again, if it has changed, no further processing should be done on the device.

Otherwise the client stores the information about this device.

[3]This prevents a malicious or compromised homeserver replacing the keys for the device with those of another.

Sending an encrypted event

When sending a message in a room configured to use encryption, a client first checks to see if it has an active outbound Megolm session. If not, it first creates one as per below. If an outbound session exists, it should check if it is time to rotate it, and create a new one if so.

The client then builds an encryption payload as follows:

{
  "type": "<event type>",
  "content": "<event content>",
  "room_id": "<id of destination room>"
}

and calls olm_group_encrypt to encrypt the payload. This is then packaged into event content as follows:

{
  "algorithm": "m.megolm.v1.aes-sha2",
  "sender_key": "<our curve25519 device key>",
  "ciphertext": "<encrypted payload>",
  "session_id": "<outbound group session id>",
  "device_id": "<our device ID>"
}

Finally, the encrypted event is sent to the room with POST /_matrix/client/r0/rooms/<room_id>/send/m.room.encrypted/<txn_id>.

Starting a Megolm session

When a message is first sent in an encrypted room, the client should start a new outbound Megolm session. This should not be done proactively, to avoid proliferation of unnecessary Megolm sessions.

To create the session, the client should call olm_init_outbound_group_session, and store the details of the outbound session for future use.

The client should then call olm_outbound_group_session_id to get the unique ID of the new session, and olm_outbound_group_session_key to retrieve the current ratchet key and index. It should store these details as an inbound session, just as it would when receiving them via an m.room_key event.

The client must then share the keys for this session with each device in the room. It must therefore download the device list if it hasn't already done so, and for each device in the room which has not been blocked, the client should:

  • Build a content object as follows:

    {
      "algorithm": "m.megolm.v1.aes-sha2",
      "room_id": "<id of destination room>",
      "session_id": "<session id>",
      "session_key": "<session_key>"
    }
    
  • Encrypt the content as an m.room_key event using Olm, as below.

Once all of the key-sharing event contents have been assembled, the events should be sent to the corresponding devices via PUT /_matrix/client/unstable/sendToDevice/m.room.encrypted/<txnId>.

Rotating Megolm sessions

Megolm sessions may not be reused indefinitely.

The number of messages which can be sent before a session should be rotated is given by the rotation_period_msgs property of the m.room.encryption event, or 100 if that property isn't present.

Similarly, the maximum age of a megolm session is given, in milliseconds, by the rotation_period_ms property of the m.room.encryption event. 604800000 (a week) is the recommended default here.

Once either the message limit or time limit have been reached, the client should start a new session before sending any more messages.

Encrypting an event with Olm

Olm is not used for encrypting room events, as it requires a separate copy of the ciphertext for each device, and because the receiving device can only decrypt received messages once. However, it is used for encrypting key-sharing events for Megolm.

When encrypting an event using Olm, the client should:

  • Build an encryption payload as follows:

    {
      "type": "<event type>",
      "content": "<event content>",
      "sender": "<our user ID>",
      "sender_device": "<our device ID>",
      "keys": {
        "ed25519": "<our ed25519 fingerprint key>"
      },
      "recipient": "<recipient user ID>",
      "recipient_keys": {
        "ed25519": "<recipient's ed25519 fingerprint key>"
      },
    }
    
  • Check if it has an existing Olm session; if it does not, start a new one. If it has several (as may happen due to races when establishing sessions), it should use the one with the first session_id when sorted by their ASCII codepoints (ie, 'A' would be before 'Z', which would be before 'a').

  • Encrypt the payload by calling olm_encrypt.

  • Package the payload into event content as follows:

    {
      "algorithm": "m.olm.v1.curve25519-aes-sha2",
      "sender_key": "<our curve25519 identity key>",
      "ciphertext": "<encrypted payload>"
    }
    

Starting an Olm session

To start a new Olm session with another device, a client must first claim one of the other device's one-time keys. To do this, it should create a query object as follows:

{
  "<user id>": {
    "<device_id>": "signed_curve25519",
    ...
  },
  ...
}

and send this via POST /_matrix/client/unstable/keys/claim. Claims for multiple devices should be aggregated into a single request.

This will return a result as follows:

{
  "one_time_keys": {
    "<user id>": {
      "<device_id>": {
        "signed_curve25519:<key_id>": {
          "key": "<curve25519_key>",
          "signatures": {
            "<user_id>": {
              "ed25519:<device_id>": "<signature>"
            }
          }
        },
      },
      ...
    },
    ...
  }
}

The client should first check the signatures on the signed key objects. As with checking the signatures on the device keys, it should remove the signatures and (if present) unsigned properties, format the remainder as Canonical JSON, and pass the result into olm_ed25519_verify, using the Ed25519 device key for the key parameter.

Provided the key object passes verification, the client should then pass the key, along with the Curve25519 Identity key for the remote device, into olm_create_outbound_session.

Handling membership changes

The client should monitor rooms which are configured to use encryption for membership changes.

When a member leaves a room, the client should invalidate any active outbound Megolm session, to ensure that a new session is used next time the user sends a message.

When a new member joins a room, the client should first download the device list for the new member, if it doesn't already have it.

After giving the user an opportunity to block any suspicious devices, the client should share the keys for the outbound Megolm session with all the new member's devices. This is done in the same way as creating a new session, except that there is no need to start a new Megolm session: due to the design of the Megolm ratchet, the new user will only be able to decrypt messages starting from the current state. The recommended method is to maintain a list of members who are waiting for the session keys, and share them when the user next sends a message.

Sending New Device announcements

When a user logs in on a new device, it is necessary to make sure that other devices in any rooms with encryption enabled are aware of the new device. This is done as follows.

Once the initial call to the /sync API completes, the client should iterate through each room where encryption is enabled. For each user (including the client's own user), it should build a content object as follows:

{
  "device_id": "<our device ID>",
  "rooms": ["<shared room id 1>", "<room id 2>", ... ]
}

Once all of these have been constructed, they should be sent to all of the relevant user's devices (using the wildcard * in place of the device_id) via PUT /_matrix/client/unstable/sendToDevice/m.new_device/<txnId>.

Handling an m.new_device event

As with m.room_key events, these will appear in the to_device section of the /sync response.

The client should download the device list of the sender, to get the details of the new device.

The event content will contain a rooms property, as well as the device_id of the new device. For each room in the list, the client should check if encryption is enabled, and if the sender of the event is a member of that room. If so, the client should share the keys for the outbound Megolm session with the new device, in the same way as handling a new user in the room.

Blocking / Verifying devices

It should be possible for a user to mark each device belonging to another user as 'Blocked' or 'Verified'.

When a user chooses to block a device, this means that no further encrypted messages should be shared with that device. In short, it should be excluded when sharing room keys when starting a new Megolm session. Any active outbound Megolm sessions whose keys have been shared with the device should also be invalidated so that no further messages are sent over them.

Verifying a device involves ensuring that the device belongs to the claimed user. Currently this must be done by showing the user the Ed25519 fingerprint key for the device, and prompting the user to verify out-of-band that it matches the key shown on the other user's device.

Marking events as 'verified'

Once a device has been verified, it is possible to verify that events have been sent from a particular device. See the section on Handling an m.room.encrypted event for notes on how to do this for each algorithm. Events sent from a verified device can be decorated in the UI to show that they have been sent from a verified device.