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425 lines
14 KiB
Markdown
425 lines
14 KiB
Markdown
# Cross-signing devices with device signing keys
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## Background
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A user with multiple devices will have a different key for end-to-end
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encryption for each device. Other users who want to communicate securely with
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this user must then verify each key on each of their devices. If Alice has *n*
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devices, and Bob has *m* devices, then for Alice to be able to communicate with
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Bob on any of their devices, this involves *n×m* key verifications.
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One way to address this is for each user to use a device signing key to sign
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all of their devices. Thus another user who wishes to verify their identity
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only needs to verify the device signing key and can use the signatures created
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by the device signing key to verify their devices.
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[MSC1680](https://github.com/matrix-org/matrix-doc/issues/1680) presents a
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different solution to the problem. A comparison between this proposal and
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MSC1680 is presented below.
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## Proposal
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Each user has a self-signing key pair that is used to sign their own devices,
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and a user-signing key pair that is used to sign other users' signing keys. A
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user's user-signing key is also signed by their own self-signing key. When one
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user (e.g. Alice) verifies another user's (Bob's) identity, Alice will sign
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Bob's self-signing key with her user-signing key. (This will mean that
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verification methods will need to be modified to pass along the self-signing
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identity key.) Alice's device will trust Bob's device if:
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- Alice's device is using a self-signing key that has signed her user-signing key,
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- Alice's user-signing key has signed Bob's self-signing key, and
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- Bob's self-signing key has signed Bob's device key.
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### Key security
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A user's private half of their user-signing key pair may be kept unlocked on a
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device, but their self-signing key should not; the private half of the
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self-signing key pair should only be stored encrypted, requiring a passphrase
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to access. By keeping the user-signing key unlocked, Alice can verify Bob's
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identity and distribute signatures to all her devices without needing to enter
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a passphrase to decrypt the key.
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If a user's device is compromised, they can issue a new user-signing key,
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signed by their self-signing key, rendering the old user-signing key useless.
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If they are certain that the old user-signing key has not yet been used by an
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attacker, then they may also reissue signatures made by the old user-signing
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key by using the new user-signing key. Otherwise, they will need to re-verify
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the other users.
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If a user's self-signing key is compromised, then the user will need to issue
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both a new self-signing key and a new device-signing key. The user may sign
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their new self-signing key with their old self-signing key, allowing other
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users who have verified the old self-signing key to automatically trust the new
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self-signing key if they wish to. Otherwise, the users will need to re-verify
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each other.
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The private halves of the user-signing key pair and self-signing key pair may
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be stored encrypted on the server (possibly along with the megolm key backup)
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so that they may be retrieved by new devices. FIXME: explain how to do this
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### Signature distribution
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Currently, users will only be allowed to see signatures made by their own
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self-signing or user-signing keys, or signatures made by other users'
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self-signing keys about their own devices. This is done in order to preserve
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the privacy of social connections. Future proposals may define mechanisms for
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distributing signatures to other users in order to allow for other web-of-trust
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use cases.
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### API description
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#### Uploading signing keys
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Public keys for the self-signing and user-signing keys are uploaded to the
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servers using `/keys/device_signing/upload`. This endpoint requires [UI
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Auth](https://matrix.org/docs/spec/client_server/r0.4.0.html#user-interactive-authentication-api).
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`POST /keys/device_signing/upload`
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``` json
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{
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"self_signing_key": {
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"user_id": "@alice:example.com",
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"usage": ["self_signing"],
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"keys": {
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"ed25519:base64+self+signing+public+key": "base64+self+signing+public+key",
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}
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},
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"user_signing_key": {
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"user_id": "@alice:example.com",
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"keys": {
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"ed25519:base64+device+signing+public+key": "base64+device+signing+public+key",
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},
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"usage": ["user_signing"],
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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+self+signing+public+key": "base64+signature"
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}
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}
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}
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}
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```
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Self-signing and user-signing keys are JSON objects with the following
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properties:
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* `user_id` (string): The user who owns the key
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* `usage` ([string]): Allowed uses for the key. Must be `["self_signing"]` for
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self-signing keys, and `["user_signing"]` for user-signing keys.
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* `keys` ({string: string}): an object that must have one entry, whose name is
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"`ed25519:`" followed by the unpadded base64 encoding of the public key, and
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whose value is the unpadded base64 encoding of the public key.
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* `signatures` ({string: {stringg: string}}): signatures of the key. A
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user-signing key must be signed by the self-signing key.
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In order to ensure that there will be no collisions in the `signatures`
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property, the server must respond with an error (FIXME: what error?) if any of
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the uploaded public keys match an existing device ID for the user. Similarly,
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if a user attempts to log in specifying a device ID matching one of the signing
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keys, the server must respond with an error (FIXME: what error?).
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If a user-signing key is uploaded, it must be signed by the current
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self-signing key (or the self-signing key that is included in the request)
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If a previous self-signing key exists, then the new self-signing key must have
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a `replaces` property whose value is the previous public self-signing key.
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Otherwise the server must respond with an error (FIXME: what error?). The new
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self-signing key may also be signed with the old self-signing key.
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After uploading self-signing and user-signing keys, they will be included under
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the `/keys/query` endpoint under the `self_signing_key` and `user_signing_key`
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properties, respectively. The `user_signing_key` will only be included when a
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user requests their own keys.
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`POST /keys/query`
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``` json
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{
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"device_keys": {
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"@alice:example.com": []
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},
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"token": "string"
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}
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```
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response:
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``` json
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{
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"failures": {},
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"device_keys": {
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"@alice:example.com": {
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// ...
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}
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},
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"self_signing_keys": {
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"@alice:example.com": {
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"user_id": "@alice:example.com",
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"usage": ["self_signing"],
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"keys": {
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"ed25519:base64+self+signing+public+key": "base64+self+signing+public+key"
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}
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}
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}
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}
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```
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Similarly, the federation endpoints `GET /user/keys/query` and
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`POST /user/devices/{userId}` will include the self-signing key.
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`POST /keys/query`
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``` json
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{
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"device_keys": {
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"@alice:example.com": []
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}
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}
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```
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response:
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``` json
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{
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"device_keys": {
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"@alice:example.com": {
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// ...
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}
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},
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"self_signing_keys": {
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"@alice:example.com": {
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"user_id": "@alice:example.com",
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"usage": ["self_signing"],
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"keys": {
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"ed25519:base64+self+signing+public+key": "base64+self+signing+public+key"
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}
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}
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}
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}
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```
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`GET /user/devices/%40alice%3Aexample.com`
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response:
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``` json
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{
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"user_id": "@alice:example.com",
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"stream_id": 5,
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"devices": [
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// ...
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],
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"self_signing_keys": {
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"user_id": "@alice:example.com",
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"usage": ["self_signing"],
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"keys": {
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"ed25519:base64+self+signing+public+key": "base64+self+signing+public+key"
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}
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}
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}
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```
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In addition, Alice's homeserver will send a `m.signing_key_update` EDU to
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servers that have users who share encrypted rooms with Alice. The `content` of
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that EDU has the following properties:
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* `user_id` (string): Required. The user ID who owns the signing key
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* `self_signing_key` (object): Required. The self-signing key, as above.
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After uploading self-signing and user-signing keys, the user will show up in
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the `changed` property of the `device_lists` field of the sync result of any
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others users who share an encrypted room with that user.
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#### Uploading signatures
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Signatures of keys can be uploaded using `/keys/signatures/upload`.
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For example, Alice signs one of her devices (HIJKLMN), and Bob's self-signing key.
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`POST /keys/signatures/upload`
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``` json
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{
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"@alice:example.com": {
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"HIJKLMN": {
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"user_id": "@alice:example.com",
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"device_id": "HIJKLMN",
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"algorithms": [
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"m.olm.curve25519-aes-sha256",
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"m.megolm.v1.aes-sha"
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],
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"keys": {
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"curve25519:HIJKLMN": "base64+curve25519+key",
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"ed25519:HIJKLMN": "base64+ed25519+key"
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},
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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+user+signing+public+key": "base64+signature+of+HIJKLMN"
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}
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}
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}
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},
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"@bob:example.com": {
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"bobs+base64+self+signing+public+key": {
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"user_id": "@bob:example.com",
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"keys": {
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"ed25519:bobs+base64+self+signing+public+key": "bobs+base64+self+signing+public+key"
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},
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"usage": ["self_signing"],
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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+user+signing+public+key": "base64+signature+of+bobs+self+signing+key"
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}
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}
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}
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}
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}
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```
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After Alice uploads a signature for her own devices, her signature will be
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included in the results of the `/keys/query` request when *anyone* requests her
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keys:
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`POST /keys/query`
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``` json
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{
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"device_keys": {
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"@alice:example.com": []
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},
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"token": "string"
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}
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```
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response:
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``` json
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{
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"failures": {},
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"device_keys": {
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"@alice:example.com": {
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"HIJKLMN": {
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"user_id": "@alice:example.com",
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"device_id": "HIJKLMN",
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"algorithms": [
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"m.olm.v1.curve25519-aes-sha256",
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"m.megolm.v1.aes-sha"
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],
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"keys": {
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"curve25519:HIJKLMN": "base64+curve25519+key",
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"ed25519:HIJKLMN": "base64+ed25519+key"
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},
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"signatures": {
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"@alice:example.com": {
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"ed25519:HIJKLMN": "base64+self+signature",
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"ed25519:base64+user+signing+public+key": "base64+signature+of+HIJKLMN"
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}
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},
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"unsigned": {
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"device_display_name": "Alice's Osborne 2"
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}
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}
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}
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},
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"self_signing_keys": {
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"@alice:example.com": {
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"user_id": "@alice:example.com",
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"usage": ["self_signing"],
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"keys": {
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"ed25519:base64+self+signing+public+key": "base64+self+signing+public+key",
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}
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}
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}
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}
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```
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Similarly, the federation endpoints `GET /user/keys/query` and
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`POST /user/devices/{userId}` will include the new signature.
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In addition, Alice's server will send an `m.device_list_update` EDU to servers
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that have users who share encrypted rooms with Alice, updating her device to
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include her new signature.
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After Alice uploads a signature for Bob's user-signing key, her signature will
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be included in the results of the `/keys/query` request when Alice requests
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Bob's key:
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`GET /keys/query`
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``` json
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{
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"failures": {},
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"device_keys": {
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"@bob:example.com": {
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// ...
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}
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},
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"self_signing_keys": {
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"@bob:example.com": {
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"user_id": "@bob:example.com",
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"keys": {
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"ed25519:bobs+base64+self+signing+public+key": "bobs+base64+self+signing+public+key"
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},
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"usage": ["self_signing"],
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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+user+signing+public+key": "base64+signature+of+bobs+self+signing+key"
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}
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}
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}
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}
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}
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```
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## Comparison with MSC1680
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MSC1680 suffers from the fact that the attestation graph may be arbitrarily
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complex and may become ambiguous how the graph should be interpreted. In
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particular, it is not obvious exactly how revocations should be interpreted --
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should they be interpreted as only revoking the signature created previously by
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the device making the revocation, or should it be interpreted as a statement
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that the device should not be trusted at all? As well, a revocation may split
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the attestation graph, causing devices that were previously trusted to possibly
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become untrusted. Logging out a device may also split the attestation graph.
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Moreover, it may not be clear to a user what device verifications would be
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needed to reattach the parts of the graph.
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One way to solve this is by registering a "virtual device", which is used to
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sign other devices. This solution would be similar to this proposal. However,
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real devices would still form an integral part of the attestation graph. For
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example, if Alice's Osborne 2 verifies Bob's Dynabook, the attestation graph might
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look like:
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If Bob replaces his Dynabook without re-verifying with Alice, this will split
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the graph and Alice will not be able to verify Bob's other devices. In
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contrast, in this proposal, Alice and Bob sign each other's self-signing key
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with their user-signing keys, and the attestation graph would look like:
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In this case, Bob's Dynabook can be replaced without breaking the graph.
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With normal cross-signing, it is not clear how to recover from a stolen device.
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For example, if Mallory steals one of Alice's devices and revokes Alice's other
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devices, it is unclear how Alice can rebuild the attestation graph with her
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devices, as there may be stale attestations and revocations lingering around.
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(This also relates to the question of whether a revocation should only revoke
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the signature created previously by the device making the attestation, or
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whether it should be a statement that the device should not be trusted at all.)
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In contrast, with this proposal, if a device is stolen, then only the
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user-signing key must be re-issued.
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## Security considerations
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This proposal relies on servers to communicate when self-signing or
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user-signing keys are deleted and replaced. An attacker who is able to both
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steal a user's device and control their homeserver could prevent that device
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from being marked as untrusted.
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## Conclusion
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This proposal presents an alternative cross-signing mechanism to MSC1680.
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