matrix-spec/proposals/1219-storing-megolm-keys-serverside.md

Storing megolm keys serverside

Background

A user who uses end-to-end encyrption will usually have many inbound session keys. Users who log into new devices and want to read old messages will need a convenient way to transfer the session keys from one device to another. While users can currently export their keys from one device and import them to another, this is involves several steps and may be cumbersome for many users. Users can also share keys from one device to another, but this has several limitations, such as the fact that key shares only share one key at a time, and require another logged-in device to be active.

To help resolve this, we optionally let clients store an encrypted copy of their megolm inbound session keys on the homeserver. Clients can keep the backup up to date, so that users will always have the keys needed to decrypt their conversations. The backup could be used not just for new logins, but also to try to fix UISIs that occur after a device has logged in (as an alternative to key sharing), or to support clients with limited local storage for keys (clients can store old keys to the backup, and remove their local copy, retrieving the key from the backup when needed).

To recover keys from the backup, a user will need to enter a recovery key to decrypt the backup. The backup will be encrypted using public key cryptography, so that any of a user's devices can back up keys without needing the user to enter the recovery key until they need to read from the backup.

See also:

• https://github.com/matrix-org/matrix-doc/issues/1219
• https://github.com/vector-im/riot-web/issues/3661
• https://github.com/vector-im/riot-web/issues/5675
• https://docs.google.com/document/d/1MOoIA9qEKIhUQ3UmKZG-loqA8e0BzgWKKlKRUGMynVc/edit# (old version of proposal)

Proposal

This proposal creates new APIs to allow clients to back up room decryption keys on the server. Decryption keys are encrypted (using public key crypto) before being sent to the server along with some unencrypted metadata to allow the server to manage the backups, overwriting backups with "better" versions of the keys. The user is given a private recovery key to save for recovering the keys from the backup.

Clients can create new versions of backups. Aside from the initial backup creation, a client might start a new version of a backup when, for example, a user loses a device, and wants to ensure that that device does not get any new decryption keys.

Once one client has created a backup version, other clients can fetch the public key for the backup from the server and add keys to the backup, if they trust that the backup was not created by a malicious device.

Possible UX for interactive clients

On receipt of encryption keys (1st time):

1. client checks if there is an existing backup: GET /room_keys/version
   1. if not, ask if the user wants to back up keys
      1. if yes:
         1. generate new curve25519 key pair
         2. create new backup version: POST /room_keys/version
         3. display private key to user to save (see below for the format)
      2. if no, exit and remember decision (user can change their mind later)
      3. while prompting, continue to poll GET /room_keys/versions, as another device may have created a backup. If so, go to 1.2.
   2. if yes, get public key, prompt user to verify a device that signed the key¹, or enter recovery key (which can derive the backup key).
      1. User can also decide to create a new backup, in which case, go to 1.1.
2. send key to backup: PUT /room_keys/keys/${roomId}/${sessionId}?version=$v
3. continue backing up keys as we receive them (may receive a M_WRONG_ROOM_KEYS_VERSION error if a new backup version has been created: see below)

On M_WRONG_ROOM_KEYS_VERSION error when trying to PUT keys:

1. get the current version
2. notify the user that there is a new backup version, and display relevant information
3. confirm with user that they want to use the backup (user may want use the backup, to stop backing up keys, or to create a new backup)
4. verify the device that signed the backup key¹, or enter recovery key

¹: cross-signing (when that is completed) can be used to verify the device that signed the key.

On receipt of undecryptable message:

1. ask user if they want to restore backup (ask whether to get individual key, room keys, or all keys). (This can be done in the same place as asking if the user wants to request keys from other devices.)
2. if yes, prompt for private key, and get keys: GET /room_keys/keys

Users can also set up, disable, or rotate backups, or restore from backup via user settings.

Recovery key

The recovery key is can either be saved by the user directly, or stored encrypted on the server (as proposed in MSC1687). If the key is saved directly by the user, then it the code is constructed as follows:

1. The 256-bit curve25519 private key is prepended by the bytes 0x8B and 0x01
2. All the bytes in the string are above are XORed together to form a parity byte. This parity byte is appended to the byte string.
3. The byte string is encoded using base58, using the same mapping as is used for Bitcoin addresses.

This 58-character string is presented to the user to save. Implementations may add whitespace to the recovery key.

When reading in a recovery key, clients must disregard whitespace. Clients must base58-decode the code, ensure that the first two bytes of the decoded string are 0x8B and 0x01, ensure that XOR-ing all the bytes together results in 0, and ensure that the total length of the decoded string is 35 bytes. Clients must then remove the first two bytes and the last byte, and use the resulting string as the private key to decrypt backups.

API

Backup versions

POST /room_keys/version

Create a new backup version.

Body parameters:

• algorithm (string): Required. The algorithm used for storing backups. Currently, only m.megolm_backup.v1.curve25519-aes-sha2 is defined.
• auth_data (object): Required. algorithm-dependent data. For m.megolm_backup.v1.curve25519-aes-sha2, this is a signedjson object with the following keys:
  • public_key (string): the curve25519 public key used to encrypt the backups
  • signatures (object): signatures of the public key

Example:

{
  "algorithm": "m.megolm_backup.v1.curve25519-aes-sha2",
  "auth_data": {
    "public_key": "abcdefg",
    "signatures": {
      "something": {
        "ed25519:something": "hijklmnop"
      }
    }
  }
}

On success, returns a JSON object with keys:

• version (string): the backup version

GET /room_keys/version/{version}

Get information about the given version, or the current version if {version} is omitted.

On success, returns a JSON object with keys:

• algorithm (string): Required. Same as in the body parameters for POST /room_keys/version.
• auth_data (object): Required. Same as in the body parameters for POST /room_keys/version.
• version (string): Required. The backup version.

Error codes:

• M_NOT_FOUND: No backup version has been created.

Storing keys

PUT /room_keys/keys/${roomId}/${sessionId}?version=$v

Store the key for the given session in the given room, using the given backup version.

If the server already has a backup in the backup version for the given session and room, then it will keep the "better" one. To determine which one is "better", key backups are compared first by the is_verified flag (true is better than false), then by the first_message_index (a lower number is better), and finally by forwarded_count (a lower number is better).

Body parameters:

• first_message_index (integer): Required. The index of the first message in the session that the key can decrypt.
• forwarded_count (integer): Required. The number of times this key has been forwarded.
• is_verified (boolean): Whether the device backing up the key has verified the device that the key is from.
• session_data (object): Algorithm-dependent data. For m.megolm_backup.v1.curve25519-aes-sha2, this is an object with the following keys (TODO: this stuff should be moved):
  • ciphertext (string): the encrypted version of the session key, as an unpadded base64 string. See below for how the session key is encoded.
  • ephemeral (string): the public ephemeral curve25519 key that was used to encrypt the session key, as an unpadded base64 string.
  • mac (string): the message authentication code for the ciphertext. FIXME: more details

On success, returns the empty JSON object.

Error codes:

• M_WRONG_ROOM_KEYS_VERSION: the version specified does not match the current backup version

PUT /room_keys/keys/${roomId}?version=$v

Store several keys for the given room, using the given backup version.

Behaves the same way as if the keys were added individually using PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Body parameters:

• sessions (object): an object where the keys are the session IDs, and the values are objects of the same form as the body in PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Returns the same as PUT /room_keys/keys/${roomId}/${sessionId}?version=$v

PUT /room_keys/keys?version=$v

Store several keys, using the given backup version.

Behaves the same way as if the keys were added individually using PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Body parameters:

• rooms (object): an object where the keys are the room IDs, and the values are objects of the same form as the body in PUT /room_keys/keys/${roomId}/?version=$v.

Returns the same as PUT /room_keys/keys/${roomId}/${sessionId}?version=$v

Retrieving keys

GET /room_keys/keys/${roomId}/${sessionId}?version=$v

Retrieve the key for the given session in the given room from the backup.

On success, returns a JSON object in the same form as the request body of PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

GET /room_keys/keys/${roomId}?version=$v

Retrieve the all the keys for the given room from the backup.

On success, returns a JSON object in the same form as the request body of PUT /room_keys/keys/${roomId}?version=$v.

GET /room_keys/keys?version=$v

Retrieve all the keys from the backup.

On success, returns a JSON object in the same form as the request body of PUT /room_keys/keys?version=$v.

Deleting keys

DELETE /room_keys/keys/${roomId}/${sessionId}?version=$v

DELETE /room_keys/keys/${roomId}?version=$v

DELETE /room_keys/keys/?version=$v

Deletes keys from the backup.

On success, returns the empty JSON object.

m.megolm_backup.v1.curve25519-aes-sha2 Key format

Each session key is encoded as a JSON object with the properties:

• algorithm (string): m.megolm.v1.aes-sha2
• sender_key (string): base64-encoded device curve25519 key
• sender_claimed_keys (object): object containing the identity keys for the sending device
• forwardingCurve25519KeyChain (array): zero or more curve25519 keys for devices who forwarded the session key
• session_key (string): base64-encoded (unpadded) session key

The JSON object is then encrypted by generating an ephemeral curve25519 key, performing an ECDH with the ephemeral key and the backup's public key to generate an AES key, and encrypting the stringified object using AES.

Security Considerations

An attacker who gains access to a user's account can delete or corrupt their key backup. This proposal does not attempt to protect against that.

An attacker who gains access to a user's account can create a new backup version using a key that they control. For this reason, clients SHOULD confirm with users before sending keys to a new backup version or verify that it was created by a trusted device by checking the signature.

Other Issues

Since many clients will receive encryption keys at around the same time, they will all want to back up their copies of the keys at around the same time, which may increase load on the server if this happens in a big room. (TODO: how much of an issue is this?) For this reason, clients should offset their backup requests randomly.

Conclusion

This proposal allows users to securely and conveniently back up and restore their decryption keys so that users logging into a new device can decrypt old messages.