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@ -396,8 +396,8 @@ recommended that clients provide mechanisms by which the user can see:
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Ed25519 signing key for each device, again encoded using unpadded Base64.
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Alice can then meet Bob in person, or contact him via some other trusted
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medium, and ask him to read out the Ed25519 key shown on his device. She
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compares this with the value shown for his device on her client.
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medium, and use `SAS Verification`_ or ask him to read out the Ed25519 key
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shown on his device, comparing it to the one shown on Alice's device.
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Device verification may reach one of several conclusions. For example:
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@ -423,6 +423,327 @@ Device verification may reach one of several conclusions. For example:
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decrypted by such a device. For the Olm protocol, this is documented at
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https://matrix.org/git/olm/about/docs/signing.rst.
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Key verification framework
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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Verifying keys manually by reading out the Ed25519 key is not very user friendly,
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and can lead to errors. In order to help mitigate errors, and to make the process
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eaiser for users, some verification methods are supported by the specification.
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The methods all use a common framework for negotiating the key verification.
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To use this framework, Alice's client would send ``m.key.verification.request``
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events to Bob's devices. All of the ``to_device`` messages sent to Bob MUST have
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the same ``transaction_id`` to indicate they are part of the same request. This
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allows Bob to reject the request on one device, and have it apply to all of his
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devices. Similarly, it allows Bob to process the verification on one device without
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having to involve all of his devices.
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When Bob's device receives a ``m.key.verification.request``, it should prompt Bob
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to verify keys with Alice using one of the supported methods in the request. If
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Bob's device does not understand any of the methods, it should not cancel the request
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as one of his other devices may support the request. Instead, Bob's device should
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tell Bob that an unsupported method was used for starting key verification. The
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prompt for Bob to accept/reject Alice's request (or the unsupported method prompt)
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should be automatically dismissed 10 minutes after the ``timestamp`` field or 2
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minutes after Bob's client receives the message, whichever comes first, if Bob
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does not interact with the prompt. The prompt should additionally be hidden if
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an appropriate ``m.key.verification.cancel`` message is received.
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If Bob rejects the request, Bob's client must send a ``m.key.verification.cancel``
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message to Alice's device. Upon receipt, Alice's device should tell her that Bob
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does not want to verify her device and send ``m.key.verification.cancel`` messages
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to all of Bob's devices to notify them that the request was rejected.
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If Bob accepts the request, Bob's device starts the key verification process by
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sending a ``m.key.verification.start`` message to Alice's device. Upon receipt
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of this message, Alice's device should send a ``m.key.verification.cancel`` message
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to all of Bob's other devices to indicate the process has been started. The start
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message must use the same ``transaction_id`` from the original key verification
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request if it is in response to the request. The start message can be sent indepdently
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of any request.
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Individual verification methods may add additional steps, events, and properties to
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the verification messages. Event types for methods defined in this specification must
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be under the ``m.key.verification`` namespace and any other event types must be namespaced
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according to the Java package naming convention.
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Any of Alice's or Bob's devices can cancel the key verification request or process
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at any time with a ``m.key.verification.cancel`` message to all applicable devices.
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This framework yields the following handshake, assuming both Alice and Bob each have
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2 devices, Bob's first device accepts the key verification request, and Alice's second
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device initiates the request. Note how Alice's first device is not involved in the
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request or verification process.
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::
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+---------------+ +---------------+ +-------------+ +-------------+
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| AliceDevice1 | | AliceDevice2 | | BobDevice1 | | BobDevice2 |
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+---------------+ +---------------+ +-------------+ +-------------+
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| | | |
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| | m.key.verification.request | |
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| |---------------------------------->| |
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| | | |
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| | m.key.verification.request | |
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| |-------------------------------------------------->|
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| | | |
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| | m.key.verification.start | |
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| |<----------------------------------| |
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| | | |
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| | m.key.verification.cancel | |
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| |-------------------------------------------------->|
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| | | |
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After the handshake, the verification process begins.
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{{m_key_verification_request_event}}
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{{m_key_verification_start_event}}
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{{m_key_verification_cancel_event}}
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.. _`SAS Verification`:
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Short Authentication String (SAS) verification
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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SAS verification is a user-friendly key verification process built off the common
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framework outlined above. SAS verification is intended to be a highly interactive
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process for users, and as such exposes verfiication methods which are easier for
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users to use.
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The verification process is heavily inspired by Phil Zimmerman's ZRTP key agreement
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handshake. A key part of key agreement in ZRTP is the hash commitment: the party that
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begins the Diffie-Hellman key sharing sends a hash of their part of the Diffie-Hellman
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exchange, and does not send their part of the Diffie-Hellman exchange until they have
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received the other party's part. Thus an attacker essentially only has one attempt to
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attack the Diffie-Hellman exchange, and hence we can verify fewer bits while still
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achieving a high degree of security: if we verify n bits, then an attacker has a 1 in
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2\ :sup:`n` chance of success. For example, if we verify 40 bits, then an attacker has
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a 1 in 1,099,511,627,776 chance (or less than 1 in 1012 chance) of success. A failed
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attack would result in a mismatched Short Authentication String, alerting users to the
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attack.
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The verification process takes place over `to-device`_ messages in two phases:
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1. Key agreement phase (based on `ZRTP key agreement <https://tools.ietf.org/html/rfc6189#section-4.4.1>`_).
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#. Key verification phase (based on HMAC).
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The process between Alice and Bob verifying each other would be:
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.. |AlicePublicKey| replace:: :math:`K_{A}^{public}`
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.. |AlicePrivateKey| replace:: :math:`K_{A}^{private}`
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.. |AliceCurve25519| replace:: :math:`K_{A}^{private}K_{A}^{public}`
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.. |BobPublicKey| replace:: :math:`K_{B}^{public}`
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.. |BobPrivateKey| replace:: :math:`K_{B}^{private}`
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.. |BobCurve25519| replace:: :math:`K_{B}^{private}K_{B}^{public}`
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.. |AliceBobCurve25519| replace:: :math:`K_{A}^{private}K_{B}^{public}`
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.. |BobAliceCurve25519| replace:: :math:`K_{B}^{private}K_{A}^{public}`
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.. |AliceBobECDH| replace:: :math:`ECDH(K_{A}^{private},K_{B}^{public})`
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1. Alice and Bob establish a secure connection, likely meeting in-person. "Secure"
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here means that either party cannot be impersonated, not explicit secrecy.
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#. Alice and Bob communicate which devices they'd like to verify with each other.
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#. Alice selects Bob's device from the device list and begins verification.
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#. Alice's client ensures it has a copy of Bob's device key.
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#. Alice's device sends Bob's device a ``m.key.verification.start`` message.
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#. Bob's device receives the message and selects a key agreement protocol, hash
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algorithm, message authentication code, and SAS method supported by Alice's
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device.
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#. Bob's device ensures it has a copy of Alice's device key.
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#. Bob's device creates an ephemeral Curve25519 key pair (|BobCurve25519|), and
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calculates the hash (using the chosen algorithm) of the public key |BobPublicKey|.
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#. Bob's device replies to Alice's device with a ``m.key.verification.accept`` message.
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#. Alice's device receives Bob's message and stores the commitment hash for later use.
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#. Alice's device creates an ephemeral Curve25519 key pair (|AliceCurve25519|) and
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replies to Bob's device with a ``m.key.verification.key``, sending only the public
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key |AlicePublicKey|.
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#. Bob's device receives Alice's message and replies with its own ``m.key.verification.key``
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message containing its public key |BobPublicKey|.
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#. Alice's device receives Bob's message and verifies the commitment hash from earlier
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matches the hash of the key Bob's device just sent and the content of Alice's
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``m.key.verification.start`` message.
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#. Both Alice and Bob's devices perform an Elliptic-curve Diffie-Hellman (|AliceBobECDH|),
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using the result as the shared secret.
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#. Both Alice and Bob's devices display a SAS to their users, which is derived
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from the shared key using one of the methods in this section. If multiple SAS
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methods are available, clients should allow the users to select a method.
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#. Alice and Bob compare the strings shown by their devices, and tell their devices if
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they match or not.
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#. Assuming they match, Alice and Bob's devices calculate the HMAC of their own device
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and a comma-separated sorted list of of the key IDs that they wish the other user
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to verify. HMAC is defined in RFC 2104, and SHA-256 as the hash function. The key for
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the HMAC is different for each item and is calculated by generating 32 bytes (256 bits)
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using `the key verification HKDF <#SAS-HKDF>`_.
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#. Alice's device sends Bob's device a ``m.key.verification.mac`` message containing the
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MAC of Alice's device keys and the MAC of her key IDs to be verified. Bob's device does
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the same for Bob's device keys and key IDs concurrently with Alice.
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#. When the other device receives the ``m.key.verification.mac`` message, the device
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calculates the HMAC of its copies of the other device's keys given in the message,
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as well as the HMAC of the comma-seperated, sorted, list of key IDs in the message.
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The device compares these with the HMAC values given in the message, and if everything
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matches then the device keys are verified.
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The wire protocol looks like the following between Alice and Bob's devices::
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+-------------+ +-----------+
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| AliceDevice | | BobDevice |
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+-------------+ +-----------+
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| |
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| m.key.verification.start |
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|-------------------------------->|
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| |
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| m.key.verification.accept |
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|<--------------------------------|
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| |
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| m.key.verification.key |
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|-------------------------------->|
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| |
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| m.key.verification.key |
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|<--------------------------------|
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| |
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| m.key.verification.mac |
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|-------------------------------->|
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| |
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| m.key.verification.mac |
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|<--------------------------------|
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| |
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Error and exception handling
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<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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At any point the interactive verfication can go wrong. The following describes what
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to do when an error happens:
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* Alice or Bob can cancel the verification at any time. A ``m.key.verification.cancel``
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message must be sent to signify the cancelation.
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* The verification can time out. Clients should time out a verification that does not
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complete within 5 minutes. Additionally, clients should expire a ``transaction_id``
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which goes unused for 5 minutes after having last sent/received it. The client should
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inform the user that the verification timed out, and send an appropriate ``m.key.verification.cancel``
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message to the other device.
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* When the same device attempts to intiate multiple verification attempts, cancel all
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attempts with that device.
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* When a device receives an unknown ``transaction_id``, it should send an appropriate
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``m.key.verfication.cancel`` message to the other device indicating as such. This
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does not apply for inbound ``m.key.verification.start`` or ``m.key.verification.cancel``
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messages.
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* If the two devices do not share a common key share, hash, HMAC, or SAS method then
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the device should notify the other device with an appropriate ``m.key.verification.cancel``
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message.
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* If the user claims the Short Authentication Strings do not match, the device should
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send an appropriate ``m.key.verification.cancel`` message to the other device.
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* If the device receives a message out of sequence or that it was not expecting, it should
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notify the other device with an appropriate ``m.key.verification.cancel`` message.
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Verification messages specific to SAS
|
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<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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Building off the common framework, the following events are involved in SAS verification.
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The ``m.key.verification.cancel`` event is unchanged, however the following error codes
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are used in addition to those already specified:
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* ``m.unknown_method``: The devices are unable to agree on the key agreement, hash, MAC,
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or SAS method.
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* ``m.mismatched_commitment``: The hash commitment did not match.
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* ``m.mismatched_sas``: The SAS did not match.
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{{m_key_verification_start_m_sas_v1_event}}
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{{m_key_verification_accept_event}}
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{{m_key_verification_key_event}}
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{{m_key_verification_mac_event}}
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|
.. _`SAS-HKDF`:
|
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|
|
HKDF calculation
|
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|
|
<<<<<<<<<<<<<<<<
|
|
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|
|
|
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|
|
In all of the SAS methods, HKDF is as defined in RFC 5869 and uses the previously
|
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|
|
agreed upon hash function for the hash function. The shared secret is supplied
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|
|
as the input keying material. No salt is used, and the input parameter is the
|
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|
|
concatenation of:
|
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|
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|
|
* The string ``MATRIX_KEY_VERIFICATION_SAS``.
|
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|
|
* The Matrix ID of the user who sent the ``m.key.verification.start`` message.
|
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|
|
* The Device ID of the device which sent the ``m.key.verification.start`` message.
|
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|
|
* The Matrix ID of the user who sent the ``m.key.verification.accept`` message.
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|
* The Device ID of the device which sent the ``m.key.verification.accept`` message.
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|
|
* The ``transaction_id`` being used.
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|
HKDF is used over the plain shared secret as it results in a harder attack
|
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|
|
as well as more uniform data to work with.
|
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|
|
For verification of each party's device keys, HKDF is as defined in RFC 5869 and
|
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|
|
uses SHA-256 as the hash function. The shared secret is supplied as the input keying
|
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|
|
material. No salt is used, and in the input parameter is the concatenation of:
|
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|
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|
|
* The string ``MATRIX_KEY_VERIFICATION_MAC``.
|
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|
|
* The Matrix ID of the user whose key is being MAC-ed.
|
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|
|
* The Device ID of the device sending the MAC.
|
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|
|
* The Matrix ID of the other user.
|
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|
|
* The Device ID of the device receiving the MAC.
|
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|
|
* The ``transaction_id`` being used.
|
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|
|
* The Key ID of the key being MAC-ed, or the string ``KEY_IDS`` if the item
|
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|
|
being MAC-ed is the list of key IDs.
|
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SAS method: ``decimal``
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<<<<<<<<<<<<<<<<<<<<<<<
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Generate 5 bytes using `HKDF <#SAS-HKDF>`_ then take sequences of 13 bits to
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convert to decimal numbers (resulting in 3 numbers between 0 and 8191 inclusive
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each). Add 1000 to each calculated number.
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The bitwise operations to get the numbers given the 5 bytes
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:math:`B_{0}, B_{1}, B_{2}, B_{3}, B_{4}` would be:
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* First: :math:`(B_{0} \ll 5 | B_{1} \gg 3) + 1000`
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* Second: :math:`(B_{1} \& 0x7 | B_{2} \ll 2 | B_{3} \gg 6) + 1000`
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* Third: :math:`((B_{3} \& 0x3F) \ll 7 | B_{4} \gg 1) + 1000`
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The digits are displayed to the user either with an appropriate separator,
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such as dashes, or with the numbers on individual lines.
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SAS method: ``emoji``
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<<<<<<<<<<<<<<<<<<<<<
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Generate 6 bytes using `HKDF <#SAS-HKDF>`_ then split the first 42 bits into
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7 groups of 6 bits, similar to how one would base64 encode something. Convert
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each group of 6 bits to a number and use the following table to get the corresponding
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emoji:
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{{sas_emoji_table}}
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.. Note::
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This table is available as JSON at
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https://github.com/matrix-org/matrix-doc/blob/master/data-definitions/sas-emoji.json
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.. admonition:: Rationale
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The emoji above were chosen to:
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* Be recognisable without colour.
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* Be recognisable at a small size.
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* Be recognisable by most cultures.
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* Be distinguishable from each other.
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* Easily described by a few words.
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* Avoid symbols with negative connotations.
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* Be likely similar across multiple platforms.
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Clients SHOULD show the emoji with the descriptions from the table, or appropriate
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translation of those descriptions. Client authors SHOULD collaborate to create a
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common set of translations for all languages.
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.. section name changed, so make sure that old links keep working
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.. _key-sharing:
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Travis Ralston
5 years ago