568 lines
19 KiB
Markdown
568 lines
19 KiB
Markdown
# Cross-signing devices with device signing keys
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## Background
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If a user has multiple devices, each device will have a different key for
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end-to-end encryption. Other users who want to communicate securely with this
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user must then verify each key on each of their own 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 three key pairs:
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- a *master* cross-signing key pair that is used to identify themselves and to
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sign their other cross-signing keys,
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- a *self-signing* key pair that is used to sign their own devices, and
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- a *user-signing* key pair that is used to sign other users' master keys.
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When one user (e.g. Alice) verifies another user's (Bob's) identity, Alice will
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sign Bob's master 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 public part of
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Bob's master key.) Alice's device will trust Bob's device if:
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- Alice's device is using a master key that has signed her user-signing key,
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- Alice's user-signing key has signed Bob's master key,
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- Bob's master 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 master key could allow an attacker to impersonate that user to other
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users, or other users to that user. Thus clients must ensure that the private
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part of the master key is treated securely. If clients do not have a secure
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means of storing the master key (such as a secret storage system provided by
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the operating system), then clients must not store the private part. If a user
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changes their master key, clients of users that they communicate with must
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notify their users about the change.
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A user's user-signing and self-signing keys are intended to be easily
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replaceable if they are compromised by re-issuing a new key signed by the
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user's master key and possibly by re-verifying devices or users. However,
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doing so relies on the user being able to notice when their keys have been
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compromised, and it involves extra work for the user, and so although clients
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do not have to treat the private parts as sensitively as the master key,
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clients should still make efforts to store the private part securely, or not
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store it at all. Clients will need to balance the security of the keys with
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the usability of signing users and devices when performing key verification.
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The private halves of a user's cross-signing keys may be stored encrypted on the
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server so that they may be retrieved by new devices, or shared between devices
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using [MSC1946](https://github.com/matrix-org/matrix-doc/pull/1946). When
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handled in this way, the keys must be base64-encoded, and use the names
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`m.cross_signing.master`, `m.cross_signing.self_signing`, and
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`m.cross_signing.user_signing` for the master, self-signing, and user-signing
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keys, respectively.
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### Signature distribution
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Currently, users will only be allowed to see
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* signatures made by their own master, self-signing or user-signing keys,
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* signatures made by their own devices about their own master key,
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* signatures made by other users' self-signing keys about their own respective
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devices,
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* signatures made by other users' master keys about their respective
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self-signing key, or
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* signatures made by other users' devices about their respective master keys
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(these signatures are used for [migrating from device
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verifications](#migrating-from-device-verifications)).
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This is done in order to preserve the privacy of social connections. Future
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proposals may define mechanisms for distributing signatures to other users in
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order to allow for other web-of-trust use cases.
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### Migrating from device verifications
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Users who have verified individual devices may wish to migrate these
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verifications to use cross-signing instead. In order to aid with this,
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signatures of a user's master key, made by their own devices, may be uploaded
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to the server. If another user's client sees that that a given user's master key has a valid
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signature from a device that was previously verified, then the client may
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choose to trust and sign the master key. The client should take precautions to
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ensure that a stolen device cannot be used to cause it to trust a malicious
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master key. For example, a client could prompt the user before signing the
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master key, or it could only do this migration on the first master key that it
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sees from a user.
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### API description
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#### Uploading signing keys
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Public keys for the cross-signing keys are uploaded to the servers using
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`/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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"master_key": {
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"user_id": "@alice:example.com",
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"usage": ["master"],
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"keys": {
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"ed25519:base64+master+public+key": "base64+self+master+key",
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}
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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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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+master+public+key": "base64+signature"
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}
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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+master+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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Cross-signing keys are JSON objects with the following 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 contain `"master"` for
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master keys, `"self_signing"` for self-signing keys, and `"user_signing"`
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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: {string: string}}): signatures of the key. A
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self-signing or user-signing key must be signed by the master key. A master
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key may be signed by a device.
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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 `M_FORBIDDEN` 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 `M_FORBIDDEN` error.
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If a self-signing or user-signing key is uploaded, it must be signed by the
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master key that is included in the request, or the current master key if no
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master key is included. If the signature from the master key is incorrect, the
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server should respond with an error code of `M_INVALID_SIGNATURE`.
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After uploading cross-signing keys, they will be included under the
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`/keys/query` endpoint under the `master_keys`, `self_signing_keys` and
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`user_signing_keys` properties. The `user_signing_keys` property will only be
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included when a 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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"master_keys": {
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"@alice:example.com": {
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"user_id": "@alice:example.com",
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"usage": ["master"],
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"keys": {
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"ed25519:base64+master+public+key": "base64+master+public+key"
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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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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+master+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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```
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Similarly, the federation endpoints `POST /user/keys/query` and `GET
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/user/devices/{userId}` will include the master and self-signing keys. (It
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will not include the user-signing key because it is not intended to be visible
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to other users.)
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`POST /user/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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"master_keys": {
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"@alice:example.com": {
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"user_id": "@alice:example.com",
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"usage": ["master"],
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"keys": {
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"ed25519:base64+master+public+key": "base64+master+public+key"
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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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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+master+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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```
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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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"master_key": {
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"user_id": "@alice:example.com",
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"usage": ["master"],
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"keys": {
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"ed25519:base64+master+public+key": "base64+master+public+key"
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}
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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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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+master+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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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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* `master_key` (object): The master key, as above.
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* `self_signing_key` (object): 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 device keys can be uploaded using `/keys/signatures/upload`.
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For example, Alice signs one of her devices (HIJKLMN) (using her self-signing
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key), her own master key (using her HIJKLMN device), Bob's master key (using
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her user-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+self+signing+public+key": "base64+signature+of+HIJKLMN"
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}
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}
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},
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"base64+master+public+key": {
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"user_id": "@alice:example.com",
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"usage": ["master"],
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"keys": {
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"ed25519:base64+master+public+key": "base64+master+public+key"
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},
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"signatures": {
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"@alice:example.com": {
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"ed25519:HIJKLMN": "base64+signature+of+master+key"
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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+master+public+key": "bobs+base64+master+public+key"
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},
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"usage": ["master"],
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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+master+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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response:
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``` json
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{
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"failures": {}
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}
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```
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The response contains a `failures` property, which is a map of user ID to
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device ID to failure reason, if any of the uploaded keys failed. The
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homeserver should verify that the signatures on the uploaded keys are valid.
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If a signature is not valid, the homeserver should set the corresponding entry
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in `failures` to a JSON object with the `errcode` property set to
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`M_INVALID_SIGNATURE`.
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After Alice uploads a signature for her own devices or master key, her
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signature will be included in the results of the `/keys/query` request when
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*anyone* requests her keys. However, signatures made for other users' keys,
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made by her user-signing key, will not be included.
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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+self+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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"master_keys": {
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"@alice:example.com": {
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"user_id": "@alice:example.com",
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"usage": ["master"],
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"keys": {
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"ed25519:base64+master+public+key": "base64+master+public+key"
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},
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"signatures": {
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"@alice:example.com": {
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"ed25519:HIJKLMN": "base64+signature+of+master+key"
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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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"signatures": {
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"@alice:example.com": {
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"ed25519:base64+master+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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```
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Similarly, the federation endpoints `POST /user/keys/query` and `GET
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/user/devices/{userId}` will include the new signatures for her own devices or
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master key, but not signatures made by her user-signing key.
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In addition, when Alice uploads signatures for her own device, Alice's server
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will send an `m.device_list_update` EDU to servers that have users who share
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encrypted rooms with Alice, updating her device to include her new signature.
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And when a signature of a master key is uploaded, Alice's server will send an
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`m.signing_key_update` EDU, updating her master key to include her new
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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, but will not be included when anyone else requests 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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"master_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+master+public+key": "bobs+base64+master+public+key"
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},
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"usage": ["master"],
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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+master+key"
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}
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}
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||
}
|
||
}
|
||
}
|
||
```
|
||
|
||
## Comparison with MSC1680
|
||
|
||
MSC1680 suffers from the fact that the attestation graph may be arbitrarily
|
||
complex and may become ambiguous how the graph should be interpreted. In
|
||
particular, it is not obvious exactly how revocations should be interpreted --
|
||
should they be interpreted as only revoking the signature created previously by
|
||
the device making the revocation, or should it be interpreted as a statement
|
||
that the device should not be trusted at all? As well, a revocation may split
|
||
the attestation graph, causing devices that were previously trusted to possibly
|
||
become untrusted. Logging out a device may also split the attestation graph.
|
||
Moreover, it may not be clear to a user what device verifications would be
|
||
needed to reattach the parts of the graph.
|
||
|
||
One way to solve this is by registering a "virtual device", which is used to
|
||
sign other devices. This solution would be similar to this proposal. However,
|
||
real devices would still form an integral part of the attestation graph. For
|
||
example, if Alice's Osborne 2 verifies Bob's Dynabook, the attestation graph might
|
||
look like:
|
||
|
||
|
||
|
||
If Bob replaces his Dynabook without re-verifying with Alice, this will split
|
||
the graph and Alice will not be able to verify Bob's other devices. In
|
||
contrast, in this proposal, Alice and Bob sign each other's master keys
|
||
with their user-signing keys, and the attestation graph would look like:
|
||
|
||
|
||
|
||
In this case, Bob's Dynabook can be replaced without breaking the graph.
|
||
|
||
With normal cross-signing, it is not clear how to recover from a stolen device.
|
||
For example, if Mallory steals one of Alice's devices and revokes Alice's other
|
||
devices, it is unclear how Alice can rebuild the attestation graph with her
|
||
devices, as there may be stale attestations and revocations lingering around.
|
||
(This also relates to the question of whether a revocation should only revoke
|
||
the signature created previously by the device making the attestation, or
|
||
whether it should be a statement that the device should not be trusted at all.)
|
||
In contrast, with this proposal, if a device is stolen, then only the
|
||
keys for which the device had access to the private keys must be re-issued,
|
||
along with any associated signatures. When the new keys are distributed, the
|
||
old keys and their signatures will no longer be part of the attestation graph.
|
||
|
||
## Security considerations
|
||
|
||
This proposal relies on servers to communicate when cross-signing keys are
|
||
deleted and replaced. An attacker who is able to both steal a user's device
|
||
and control their homeserver could prevent that device from being marked as
|
||
untrusted.
|
||
|
||
An attacker may be able to upload a large number of signatures in a DoS attack
|
||
against clients or servers, similar to the [attack against the SKS keyserver
|
||
network](https://gist.github.com/rjhansen/67ab921ffb4084c865b3618d6955275f).
|
||
Since clients are only sent a subset of signatures, and the attestation graph
|
||
is limited, a DoS attack is less likely to be successful in this case.
|
||
|
||
## Conclusion
|
||
|
||
This proposal presents an alternative cross-signing mechanism to MSC1680,
|
||
allowing users to trust another user's devices without needing to verify each
|
||
one individually.
|