diff --git a/changelogs/client_server/newsfragments/2188.clarification b/changelogs/client_server/newsfragments/2188.clarification new file mode 100644 index 000000000..d575444e1 --- /dev/null +++ b/changelogs/client_server/newsfragments/2188.clarification @@ -0,0 +1 @@ +Clarify terminology for keys in cross-signing module. diff --git a/content/client-server-api/modules/end_to_end_encryption.md b/content/client-server-api/modules/end_to_end_encryption.md index c06178d70..5db69897a 100644 --- a/content/client-server-api/modules/end_to_end_encryption.md +++ b/content/client-server-api/modules/end_to_end_encryption.md @@ -93,7 +93,7 @@ Example: ``` `ed25519` and `curve25519` keys are used for [device keys](#device-keys). -Additionally, `ed25519` keys are used for [cross-signing keys](#cross-signing). +Additionally, `ed25519` keys are used for [cross-signing](#cross-signing). `signed_curve25519` keys are used for [one-time and fallback keys](#one-time-and-fallback-keys). @@ -675,7 +675,7 @@ The process between Alice and Bob verifying each other would be: 15. Assuming they match, Alice and Bob's devices each calculate Message Authentication Codes (MACs) for: * Each of the keys that they wish the other user to verify (usually their - device ed25519 key and their master cross-signing key). + device ed25519 key and their master key, see below). * The complete list of key IDs that they wish the other user to verify. The MAC calculation is defined [below](#mac-calculation). @@ -931,16 +931,16 @@ and can be translated online: Rather than requiring Alice to verify each of Bob's devices with each of her own devices and vice versa, the cross-signing feature allows users to sign their device keys such that Alice and Bob only need to verify -once. With cross-signing, each user has a set of cross-signing keys that +once. With cross-signing, each user has a set of ed25519 key pairs that are used to sign their own device keys and other users' keys, and can be used to trust device keys that were not verified directly. -Each user has three ed25519 key pairs for cross-signing: +Each user has three ed25519 key pairs used for cross-signing: -- a master key (MSK) that serves as the user's identity in - cross-signing and signs their other cross-signing keys; +- a master key (MK) that serves as the user's identity in + cross-signing and signs their user-signing and self-signing keys; - a user-signing key (USK) -- only visible to the user that it belongs - to --that signs other users' master keys; and + to -- that signs other users' master keys; and - a self-signing key (SSK) that signs the user's own device keys. The master key may also be used to sign other items such as the backup @@ -950,13 +950,15 @@ previously verified Bob's device and Bob's device has signed his master key, then Alice's device can trust Bob's master key, and she can sign it with her user-signing key. -Users upload their cross-signing keys to the server using [POST +Users upload the public part of their master, user-signing and self-signing +key to the server using [POST /\_matrix/client/v3/keys/device\_signing/upload](/client-server-api/#post_matrixclientv3keysdevice_signingupload). When Alice uploads -new cross-signing keys, her user ID will appear in the `changed` +new keys, her user ID will appear in the `changed` property of the `device_lists` field of the `/sync` of response of all users who share an encrypted room with her. When Bob sees Alice's user ID in his `/sync`, he will call [POST /\_matrix/client/v3/keys/query](/client-server-api/#post_matrixclientv3keysquery) -to retrieve Alice's device and cross-signing keys. +to retrieve Alice's device keys, as well as their master, user-signing and +self-signing key. If Alice has a device and wishes to send an encrypted message to Bob, she can trust Bob's device if: @@ -971,13 +973,13 @@ The following diagram illustrates how keys are signed: ``` +------------------+ .................. +----------------+ - | +--------------+ | .................. : | +------------+ | - | | v v v : : v v v | | - | | +-----------+ : : +-----------+ | | - | | | Alice MSK | : : | Bob MSK | | | - | | +-----------+ : : +-----------+ | | - | | | : : : : | | | - | | +--+ :... : : ...: +--+ | | + | +--------------+ | ................... : | +------------+ | + | | v v v : : v v v | | + | | +----------+ : : +----------+ | | + | | | Alice MK | : : | Bob MK | | | + | | +----------+ : : +----------+ | | + | | | : : : : | | | + | | +--+ :.... : : ...: +---+ | | | | v v : : v v | | | | +-----------+ ............. : : ............. +-----------+ | | | | | Alice SSK | : Alice USK : : : : Bob USK : | Bob SSK | | | @@ -1004,11 +1006,11 @@ signatures that she cannot see: +------------------+ +----------------+ +----------------+ | +--------------+ | | | | +------------+ | | | v v | v v v | | - | | +-----------+ | +-----------+ | | - | | | Alice MSK | | | Bob MSK | | | - | | +-----------+ | +-----------+ | | - | | | | | | | | - | | +--+ +--+ | +--+ | | + | | +----------+ | +----------+ | | + | | | Alice MK | | | Bob MK | | | + | | +----------+ | +----------+ | | + | | | | | | | | + | | +--+ +---+ | +---+ | | | | v v | v | | | | +-----------+ +-----------+ | +-----------+ | | | | | Alice SSK | | Alice USK | | | Bob SSK | | | @@ -1024,16 +1026,16 @@ signatures that she cannot see: ``` [Verification methods](#device-verification) can be used to verify a -user's master key by using the master public key, encoded using unpadded +user's master key by treating the master public key, encoded using unpadded base64, as the device ID, and treating it as a normal device. For example, if Alice and Bob verify each other using SAS, Alice's `m.key.verification.mac` message to Bob may include `"ed25519:alices+master+public+key": "alices+master+public+key"` in the `mac` property. Servers therefore must ensure that device IDs will not -collide with cross-signing public keys. +collide with public keys used for cross-signing. -The cross-signing private keys can be stored on the server or shared with other -devices using the [Secrets](#secrets) module. When doing so, the master, +Using the [Secrets](#secrets) module the private keys used for cross-signing can +be stored on the server or shared with other devices. When doing so, the master, user-signing, and self-signing keys are identified using the names `m.cross_signing.master`, `m.cross_signing.user_signing`, and `m.cross_signing.self_signing`, respectively, and the keys are base64-encoded @@ -1052,14 +1054,14 @@ If a user's client sees that any other user has changed their master key, that client must notify the user about the change before allowing communication between the users to continue. -Since device key IDs (`ed25519:DEVICE_ID`) and cross-signing key IDs -(`ed25519:PUBLIC_KEY`) occupy the same namespace, clients must ensure that they -use the correct keys when verifying. +Since device key IDs (`ed25519:DEVICE_ID`) as well as master, user-signing and +self-signing key IDs (`ed25519:PUBLIC_KEY`) occupy the same namespace, clients +must ensure that they use the correct keys when verifying. -While servers MUST not allow devices to have the same IDs as cross-signing -keys, a malicious server could construct such a situation, so clients must not -rely on the server being well-behaved and should take the following precautions -against this. +While servers MUST not allow devices to have the same IDs as keys used for +cross-signing, a malicious server could construct such a situation, so clients +must not rely on the server being well-behaved and should take the following +precautions against this: 1. Clients MUST refer to keys by their public keys during the verification process, rather than only by the key ID. @@ -1067,7 +1069,8 @@ against this. verification process, and ensure that they do not change in the course of verification. 3. Clients SHOULD also display a warning and MUST refuse to verify a user when - they detect that the user has a device with the same ID as a cross-signing key. + they detect that the user has a device with the same ID as a key used for + cross-signing. A user's user-signing and self-signing keys are intended to be easily replaceable if they are compromised by re-issuing a new key signed by @@ -1104,7 +1107,7 @@ user-signing keys. Verifying by QR codes provides a quick way to verify when one of the parties has a device capable of scanning a QR code. The QR code encodes both parties' -master signing keys as well as a random shared secret that is used to allow +master keys as well as a random shared secret that is used to allow bi-directional verification from a single scan. To advertise the ability to show a QR code, clients use the names @@ -1202,15 +1205,14 @@ The binary segment MUST be of the following form: bytes of the ID as a UTF-8 string - the ID encoded as a UTF-8 string - the first key, as 32 bytes. The key to use depends on the mode field: - - if `0x00` or `0x01`, then the current user's own master cross-signing public key + - if `0x00` or `0x01`, then the current user's own master public key - if `0x02`, then the current device's Ed25519 signing key - the second key, as 32 bytes. The key to use depends on the mode field: - if `0x00`, then what the device thinks the other user's master - cross-signing public key is + public key is - if `0x01`, then what the device thinks the other device's Ed25519 signing public key is - - if `0x02`, then what the device thinks the user's master cross-signing public - key is + - if `0x02`, then what the device thinks the user's master public key is - a random shared secret, as a sequence of bytes. It is suggested to use a secret that is about 8 bytes long. Note: as we do not share the length of the secret, and it is not a fixed size, clients will just use the remainder of @@ -1221,14 +1223,14 @@ For example, if Alice displays a QR code encoding the following binary data: ``` "MATRIX" |ver|mode| len | event ID 4D 41 54 52 49 58 02 00 00 2D 21 41 42 43 44 ... -| user's cross-signing key | other user's cross-signing key | shared secret - 00 01 02 03 04 05 06 07 ... 10 11 12 13 14 15 16 17 ... 20 21 22 23 24 25 26 27 +| the first key | the second key | shared secret + 00 01 02 03 04 05 06 07 ... 10 11 12 13 14 15 16 17 ... 20 21 22 23 24 25 26 27 ``` -this indicates that Alice is verifying another user (say Bob), in response to -the request from event "$ABCD...", her cross-signing key is +Mode `0x00` indicates that Alice is verifying another user (say Bob), in +response to the request from event "$ABCD...", her master key is `0001020304050607...` (which is "AAECAwQFBg..." in base64), she thinks that -Bob's cross-signing key is `1011121314151617...` (which is "EBESExQVFh..." in +Bob's master key is `1011121314151617...` (which is "EBESExQVFh..." in base64), and the shared secret is `2021222324252627` (which is "ICEiIyQlJic" in base64). @@ -1300,8 +1302,8 @@ one of its variants. Clients must only store keys in backups after they have ensured that the `auth_data` is trusted. This can be done either by: -- checking that it is signed by the user's [master cross-signing - key](#cross-signing) or by a verified device belonging to the same user, or +- checking that it is signed by the user's [master key](#cross-signing) + or by a verified device belonging to the same user, or - deriving the public key from a private key that it obtained from a trusted source. Trusted sources for the private key include the user entering the key, retrieving the key stored in [secret storage](#secret-storage), or @@ -1786,13 +1788,14 @@ a way to identify the server's support for fallback keys. | Parameter | Type | Description | |------------|-----------|------------------------------------------------------------------------------------------------------------------------------------------------------------------| -| changed | [string] | List of users who have updated their device identity or cross-signing keys, or who now share an encrypted room with the client since the previous sync response. | +| changed | [string] | List of users who have updated their device identity or their master, self-signing or user-signing keys, or who now share an encrypted room with the client since the previous sync response. | | left | [string] | List of users with whom we do not share any encrypted rooms anymore since the previous sync response. | {{% boxes/note %}} For optimal performance, Alice should be added to `changed` in Bob's -sync only when she updates her devices or cross-signing keys, or when -Alice and Bob now share a room but didn't share any room previously. +sync only when she updates her devices or master, self-signing or +user-signing keys, or when Alice and Bob now share a room but didn't +share any room previously. However, for the sake of simpler logic, a server may add Alice to `changed` when Alice and Bob share a new room, even if they previously already shared a room. diff --git a/data/api/client-server/cross_signing.yaml b/data/api/client-server/cross_signing.yaml index 60fa9e5bc..6c197667c 100644 --- a/data/api/client-server/cross_signing.yaml +++ b/data/api/client-server/cross_signing.yaml @@ -21,16 +21,16 @@ paths: x-addedInMatrixVersion: "1.1" x-changedInMatrixVersion: "1.11": UIA is not always required for this endpoint. - summary: Upload cross-signing keys. + summary: Upload keys used for cross-signing. description: |- - Publishes cross-signing keys for the user. + Publishes keys used for cross-signing for the user. This API endpoint uses the [User-Interactive Authentication API](/client-server-api/#user-interactive-authentication-api). User-Interactive Authentication MUST be performed, except in these cases: - - there is no existing cross-signing master key uploaded to the homeserver, OR - - there is an existing cross-signing master key and it exactly matches the - cross-signing master key provided in the request body. If there are any additional + - there is no existing master key uploaded to the homeserver, OR + - there is an existing master key and it exactly matches the + master key provided in the request body. If there are any additional keys provided in the request (self-signing key, user-signing key) they MUST also match the existing keys stored on the server. In other words, the request contains no new keys. diff --git a/data/api/client-server/definitions/cross_signing_key.yaml b/data/api/client-server/definitions/cross_signing_key.yaml index d937daab5..a2928573c 100644 --- a/data/api/client-server/definitions/cross_signing_key.yaml +++ b/data/api/client-server/definitions/cross_signing_key.yaml @@ -13,7 +13,7 @@ # limitations under the License. type: object title: CrossSigningKey -description: Cross signing key +description: Key used for cross signing properties: user_id: type: string diff --git a/data/api/client-server/keys.yaml b/data/api/client-server/keys.yaml index de4501b34..fcc47d6de 100644 --- a/data/api/client-server/keys.yaml +++ b/data/api/client-server/keys.yaml @@ -219,7 +219,7 @@ paths: x-addedInMatrixVersion: "1.1" type: object description: |- - Information on the master cross-signing keys of the queried users. + Information on the master keys of the queried users. A map from user ID, to master key information. For each key, the information returned will be the same as uploaded via `/keys/device_signing/upload`, along with the signatures diff --git a/data/api/server-server/definitions/event-schemas/m.signing_key_update.yaml b/data/api/server-server/definitions/event-schemas/m.signing_key_update.yaml index 0748bc35c..1d44da269 100644 --- a/data/api/server-server/definitions/event-schemas/m.signing_key_update.yaml +++ b/data/api/server-server/definitions/event-schemas/m.signing_key_update.yaml @@ -17,7 +17,7 @@ type: object title: m.signing_key_update description: |- An EDU that lets servers push details to each other when one of their users - updates their cross-signing keys. + updates their keys used for cross-signing. allOf: - $ref: ../edu.yaml - type: object @@ -34,7 +34,7 @@ allOf: properties: user_id: type: string - description: The user ID whose cross-signing keys have changed. + description: The user ID whose keys have changed. example: "@alice:example.com" master_key: allOf: diff --git a/data/api/server-server/user_devices.yaml b/data/api/server-server/user_devices.yaml index 8a5669c4f..cfde5fec4 100644 --- a/data/api/server-server/user_devices.yaml +++ b/data/api/server-server/user_devices.yaml @@ -79,7 +79,7 @@ paths: - keys master_key: type: object - description: The user\'s master cross-signing key. + description: The user\'s master key. allOf: - $ref: ../client-server/definitions/cross_signing_key.yaml - example: diff --git a/data/api/server-server/user_keys.yaml b/data/api/server-server/user_keys.yaml index 059dcae41..21fdfb9fb 100644 --- a/data/api/server-server/user_keys.yaml +++ b/data/api/server-server/user_keys.yaml @@ -194,7 +194,7 @@ paths: x-addedInMatrixVersion: "1.1" type: object description: |- - Information on the master cross-signing keys of the queried users. + Information on the master keys of the queried users. A map from user ID, to master key information. For each key, the information returned will be the same as uploaded via `/keys/device_signing/upload`, along with the signatures