matrix-spec/proposals/1756-cross-signing.md

Cross-signing devices with device signing keys

Background

If a user has multiple devices, each device will have a different key for end-to-end encryption. Other users who want to communicate securely with this user must then verify each key on each of their own devices. If Alice has n devices, and Bob has m devices, then for Alice to be able to communicate with Bob on any of their devices, this involves n×m key verifications.

One way to address this is for each user to use a device signing key to sign all of their devices. Thus another user who wishes to verify their identity only needs to verify the device signing key and can use the signatures created by the device signing key to verify their devices.

MSC1680 presents a different solution to the problem. A comparison between this proposal and MSC1680 is presented below.

Proposal

Each user has three key pairs:

• a master cross-signing key pair that is used to identify themselves and to sign their other cross-signing keys,
• a self-signing key pair that is used to sign their own devices, and
• a user-signing key pair that is used to sign other users' master keys.

When one user (e.g. Alice) verifies another user's (Bob's) identity, Alice will sign Bob's master key with her user-signing key. (This will mean that verification methods will need to be modified to pass along the public part of Bob's master key.) Alice's device will trust Bob's device if:

• Alice's device is using a master key that has signed her user-signing key,
• Alice's user-signing key has signed Bob's master key,
• Bob's master key has signed Bob's self-signing key, and
• Bob's self-signing key has signed Bob's device key.

Key security

A user's master key could allow an attacker to impersonate that user to other users, or other users to that user. Thus clients must ensure that the private part of the master key is treated securely. If clients do not have a secure means of storing the master key (such as a secret storage system provided by the operating system), then clients must not store the private part. If a user changes their master key, clients of users that they communicate with must notify their users about the change.

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 the user's master key and possibly by re-verifying devices or users. However, doing so relies on the user being able to notice when their keys have been compromised, and it involves extra work for the user, and so although clients do not have to treat the private parts as sensitively as the master key, clients should still make efforts to store the private part securely, or not store it at all. Clients will need to balance the security of the keys with the usability of signing users and devices when performing key verification.

The private halves of a user's cross-signing keys may be stored encrypted on the server so that they may be retrieved by new devices, or shared between devices using MSC1946. When handled in this way, the keys must be base64-encoded, and use the names m.cross_signing.master, m.cross_signing.self_signing, and m.cross_signing.user_signing for the master, self-signing, and user-signing keys, respectively.

Signature distribution

Currently, users will only be allowed to see

• signatures made by their own master, self-signing or user-signing keys,
• signatures made by their own devices about their own master key,
• signatures made by other users' self-signing keys about their own respective devices,
• signatures made by other users' master keys about their respective self-signing key, or
• signatures made by other users' devices about their respective master keys (these signatures are used for migrating from device verifications).

This is done in order to preserve the privacy of social connections. Future proposals may define mechanisms for distributing signatures to other users in order to allow for other web-of-trust use cases.

Migrating from device verifications

Users who have verified individual devices may wish to migrate these verifications to use cross-signing instead. In order to aid with this, signatures of a user's master key, made by their own devices, may be uploaded to the server. If another user's client sees that that a given user's master key has a valid signature from a device that was previously verified, then the client may choose to trust and sign the master key. The client should take precautions to ensure that a stolen device cannot be used to cause it to trust a malicious master key. For example, a client could prompt the user before signing the master key, or it could only do this migration on the first master key that it sees from a user.

API description

Uploading signing keys

Public keys for the cross-signing keys are uploaded to the servers using /keys/device_signing/upload. This endpoint requires UI Auth.

POST /keys/device_signing/upload

{
  "master_key": {
    "user_id": "@alice:example.com",
    "usage": ["master"],
    "keys": {
      "ed25519:base64+master+public+key": "base64+self+master+key",
    }
  },
  "self_signing_key": {
    "user_id": "@alice:example.com",
    "usage": ["self_signing"],
    "keys": {
      "ed25519:base64+self+signing+public+key": "base64+self+signing+public+key",
    },
    "signatures": {
      "@alice:example.com": {
        "ed25519:base64+master+public+key": "base64+signature"
      }
    }
  },
  "user_signing_key": {
    "user_id": "@alice:example.com",
    "keys": {
      "ed25519:base64+device+signing+public+key": "base64+device+signing+public+key",
    },
    "usage": ["user_signing"],
    "signatures": {
      "@alice:example.com": {
        "ed25519:base64+master+public+key": "base64+signature"
      }
    }
  }
}

Cross-signing keys are JSON objects with the following properties:

• user_id (string): The user who owns the key
• usage ([string]): Allowed uses for the key. Must contain "master" for master keys, "self_signing" for self-signing keys, and "user_signing" for user-signing keys.
• keys ({string: string}): an object that must have one entry, whose name is "ed25519:" followed by the unpadded base64 encoding of the public key, and whose value is the unpadded base64 encoding of the public key.
• signatures ({string: {string: string}}): signatures of the key. A self-signing or user-signing key must be signed by the master key. A master key may be signed by a device.

In order to ensure that there will be no collisions in the signatures property, the server must respond with an M_FORBIDDEN error if any of the uploaded public keys match an existing device ID for the user. Similarly, if a user attempts to log in specifying a device ID matching one of the signing keys, the server must respond with an M_FORBIDDEN error.

If a self-signing or user-signing key is uploaded, it must be signed by the master key that is included in the request, or the current master key if no master key is included. If the signature from the master key is incorrect, the server should respond with an error code of M_INVALID_SIGNATURE.

After uploading cross-signing keys, they will be included under the /keys/query endpoint under the master_keys, self_signing_keys and user_signing_keys properties. The user_signing_keys property will only be included when a user requests their own keys.

POST /keys/query

{
   "device_keys": {
      "@alice:example.com": []
   },
   "token": "string"
}

response:

{
  "failures": {},
  "device_keys": {
    "@alice:example.com": {
      // ...
    }
  },
  "master_keys": {
    "@alice:example.com": {
      "user_id": "@alice:example.com",
      "usage": ["master"],
      "keys": {
        "ed25519:base64+master+public+key": "base64+master+public+key"
      }
    }
  },
  "self_signing_keys": {
    "@alice:example.com": {
      "user_id": "@alice:example.com",
      "usage": ["self_signing"],
      "keys": {
        "ed25519:base64+self+signing+public+key": "base64+self+signing+public+key"
      },
      "signatures": {
        "@alice:example.com": {
          "ed25519:base64+master+public+key": "base64+signature"
        }
      }
    }
  }
}

Similarly, the federation endpoints POST /user/keys/query and GET /user/devices/{userId} will include the master and self-signing keys. (It will not include the user-signing key because it is not intended to be visible to other users.)

POST /user/keys/query

{
   "device_keys": {
      "@alice:example.com": []
   }
}

response:

{
  "device_keys": {
    "@alice:example.com": {
      // ...
    }
  },
  "master_keys": {
    "@alice:example.com": {
      "user_id": "@alice:example.com",
      "usage": ["master"],
      "keys": {
        "ed25519:base64+master+public+key": "base64+master+public+key"
      }
    }
  },
  "self_signing_keys": {
    "@alice:example.com": {
      "user_id": "@alice:example.com",
      "usage": ["self_signing"],
      "keys": {
        "ed25519:base64+self+signing+public+key": "base64+self+signing+public+key"
      },
      "signatures": {
        "@alice:example.com": {
          "ed25519:base64+master+public+key": "base64+signature"
        }
      }
    }
  }
}

GET /user/devices/%40alice%3Aexample.com

response:

{
  "user_id": "@alice:example.com",
  "stream_id": 5,
  "devices": [
     // ...
  ],
  "master_key": {
    "user_id": "@alice:example.com",
    "usage": ["master"],
    "keys": {
      "ed25519:base64+master+public+key": "base64+master+public+key"
    }
  },
  "self_signing_key": {
    "user_id": "@alice:example.com",
    "usage": ["self_signing"],
    "keys": {
      "ed25519:base64+self+signing+public+key": "base64+self+signing+public+key"
    },
    "signatures": {
      "@alice:example.com": {
        "ed25519:base64+master+public+key": "base64+signature"
      }
    }
  }
}

In addition, Alice's homeserver will send a m.signing_key_update EDU to servers that have users who share encrypted rooms with Alice. The content of that EDU has the following properties:

• user_id (string): Required. The user ID who owns the signing key
• master_key (object): The master key, as above.
• self_signing_key (object): The self-signing key, as above.

After uploading self-signing and user-signing keys, the user will show up in the changed property of the device_lists field of the sync result of any others users who share an encrypted room with that user.

Uploading signatures

Signatures of device keys can be uploaded using /keys/signatures/upload.

For example, Alice signs one of her devices (HIJKLMN) (using her self-signing key), her own master key (using her HIJKLMN device), Bob's master key (using her user-signing key).

POST /keys/signatures/upload

{
  "@alice:example.com": {
    "HIJKLMN": {
      "user_id": "@alice:example.com",
      "device_id": "HIJKLMN",
      "algorithms": [
        "m.olm.curve25519-aes-sha256",
        "m.megolm.v1.aes-sha"
      ],
      "keys": {
        "curve25519:HIJKLMN": "base64+curve25519+key",
        "ed25519:HIJKLMN": "base64+ed25519+key"
      },
      "signatures": {
        "@alice:example.com": {
          "ed25519:base64+self+signing+public+key": "base64+signature+of+HIJKLMN"
        }
      }
    },
    "base64+master+public+key": {
      "user_id": "@alice:example.com",
      "usage": ["master"],
      "keys": {
        "ed25519:base64+master+public+key": "base64+master+public+key"
      },
      "signatures": {
        "@alice:example.com": {
          "ed25519:HIJKLMN": "base64+signature+of+master+key"
        }
      }
    }
  },
  "@bob:example.com": {
    "bobs+base64+self+signing+public+key": {
      "user_id": "@bob:example.com",
      "keys": {
        "ed25519:bobs+base64+master+public+key": "bobs+base64+master+public+key"
      },
      "usage": ["master"],
      "signatures": {
        "@alice:example.com": {
          "ed25519:base64+user+signing+public+key": "base64+signature+of+bobs+master+key"
        }
      }
    }
  }
}

response:

{
  "failures": {}
}

The response contains a failures property, which is a map of user ID to device ID to failure reason, if any of the uploaded keys failed. The homeserver should verify that the signatures on the uploaded keys are valid. If a signature is not valid, the homeserver should set the corresponding entry in failures to a JSON object with the errcode property set to M_INVALID_SIGNATURE.

After Alice uploads a signature for her own devices or master key, her signature will be included in the results of the /keys/query request when anyone requests her keys. However, signatures made for other users' keys, made by her user-signing key, will not be included.

POST /keys/query

{
   "device_keys": {
      "@alice:example.com": []
   },
   "token": "string"
}

response:

{
  "failures": {},
  "device_keys": {
    "@alice:example.com": {
      "HIJKLMN": {
        "user_id": "@alice:example.com",
        "device_id": "HIJKLMN",
        "algorithms": [
          "m.olm.v1.curve25519-aes-sha256",
          "m.megolm.v1.aes-sha"
        ],
        "keys": {
          "curve25519:HIJKLMN": "base64+curve25519+key",
          "ed25519:HIJKLMN": "base64+ed25519+key"
        },
        "signatures": {
          "@alice:example.com": {
            "ed25519:HIJKLMN": "base64+self+signature",
            "ed25519:base64+self+signing+public+key": "base64+signature+of+HIJKLMN"
          }
        },
        "unsigned": {
          "device_display_name": "Alice's Osborne 2"
        }
      }
    }
  },
  "master_keys": {
    "@alice:example.com": {
      "user_id": "@alice:example.com",
      "usage": ["master"],
      "keys": {
        "ed25519:base64+master+public+key": "base64+master+public+key"
      },
      "signatures": {
        "@alice:example.com": {
          "ed25519:HIJKLMN": "base64+signature+of+master+key"
        }
      }
    }
  },
  "self_signing_keys": {
    "@alice:example.com": {
      "user_id": "@alice:example.com",
      "usage": ["self_signing"],
      "keys": {
        "ed25519:base64+self+signing+public+key": "base64+self+signing+public+key"
      },
      "signatures": {
        "@alice:example.com": {
          "ed25519:base64+master+public+key": "base64+signature"
        }
      }
    }
  }
}

Similarly, the federation endpoints POST /user/keys/query and GET /user/devices/{userId} will include the new signatures for her own devices or master key, but not signatures made by her user-signing key.

In addition, when Alice uploads signatures for her own device, Alice's server will send an m.device_list_update EDU to servers that have users who share encrypted rooms with Alice, updating her device to include her new signature. And when a signature of a master key is uploaded, Alice's server will send an m.signing_key_update EDU, updating her master key to include her new signature.

After Alice uploads a signature for Bob's user-signing key, her signature will be included in the results of the /keys/query request when Alice requests Bob's key, but will not be included when anyone else requests Bob's key:

GET /keys/query

{
  "failures": {},
  "device_keys": {
    "@bob:example.com": {
      // ...
    }
  },
  "master_keys": {
    "@bob:example.com": {
      "user_id": "@bob:example.com",
      "keys": {
        "ed25519:bobs+base64+master+public+key": "bobs+base64+master+public+key"
      },
      "usage": ["master"],
      "signatures": {
        "@alice:example.com": {
          "ed25519:base64+user+signing+public+key": "base64+signature+of+bobs+master+key"
        }
      }
    }
  }
}

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. 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.