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Device Pairing

To add a device to an existing BlazeNet network, it will need to be paired to the network. This pairing process can take place over both an in-band mechanism (over the air) or out of band (via additional interfaces) but the general scheme of operation is the same regardless.

Overview

Pairing is the process of registering a device’s authentication key (which is the public part of a public/private keypair unique and internal to the device) with the network control element. This allows the network control element to issue authentication challenges (and more importantly, verify the responses) to the device when it attempts to join the network.

Additionally, during the pairing process, the end device will receive a list of all public keys used by coordinators in the network. This allows the device to verify the identity of the network when joining. (This list can be updated over the air whenever the device associates to the network, though these changed key lists must be cross-signed with the key of a coordinator the end device already trusts.)

Depending on the pairing mechanism, additional data (such as the network identity, channel, regulatory region, etc.) may be provided to the device as well.

Out-of-band Pairing

In this section is a description of how devices are paired to the network using out-of-band communication mechanisms.

NFC

The end device exposes an NFC “dynamic tag,” which can be read and written by a separate user device (such as a mobile phone,) or by specifically outfitted coordinator devices. This tag contains a fixed region, which holds information about the device type, as well as its public key.

During the pairing process, the end device and user device exchange messages:

  1. Read out information: User device queries the read-only area of the NFC tag, which holds the device’s public key, and stores it for later reference.
  2. Issue challenge to end device: The user device issues a challenge to the end device, which it will sign and return.
  3. Validate challenge: User device verifies the response to the challenge. This ensures the device works correctly and is in control of the private key corresponding to its public key.
  4. Key exchange: Run ECDH key exchange to derive a shared secret key that’s used to encrypt the network configuration data that will be uploaded next. 
    1. Once key exchange has completed, the user device will register the device’s public key (along with the additional metadata in the fixed area of the tag) with the network control element for authentication.
    2. XXX: Is this step really necessary?
  5. Upload network configuration: Using the previously established encryption key, encrypt the network configuration information. This consists of the network name (UUID,) channel, regulatory information, as well as the public keys of all coordinator nodes.

Once this is complete, the end device will attempt to associate to the network. During this process, the user device may periodically poll the end device as to its association status to determine when it’s completed the process. The pairing process is considered complete when the end device either successfully associates to the network, or the association fails (due to invalid credentials, inability to find a coordinator, timeout, or cancelled by user device.) If the pairing process fails at any stage prior to successful association, the network information is not saved and the end device will continue to use its previous network settings, if any.

In-band Pairing

This section describes the process of pairing a new device to a network in-band. It exists as a fall-back for other pairing mechanisms, but should be used as a last resort as it is less secure, less convenient, and much slower than other supported out-of-band mechanisms. Due to its nature, the end device will have to blindly trust the network it is associating to, and must already know the regulatory region the network is operating in.

The basic premise of in-band pairing is that every device will have an unique “pairing code” associated with it. It’s usually derived from its serial number, and should be printed on the device itself. The device will use this pairing code to authenticate to the network during association, instead of its device key.

First Stage

A network will need to be made “joinable,” which sets a flag in its beacon. This also activates an additional authentication mechanism during association, allowing the device will attempt to authenticate with its pairing code. Coordinator nodes should have a timeout that disables the joinable mode (and purges all permitted pairing codes) automatically.

During this time, the end device will scan all radio channels in the currently active bandplan to find active networks broadcasting beacon frames. It will attempt to authenticate with the pairing code against all networks that are currently joinable. (Note that this does not allow the radio to be updated with a new regulatory region, as with out-of-band pairing.)

Second Stage

If the device can associate successfully with its pairing code, it should perform the second stage of the in-band pairing process immediately after. It should not attempt to negotiate group encryption keys, power saving modes, or attempt to pass any other traffic, as it will be dropped by the coordinator for all end devices authenticated with pairing codes.

  1. Request in-band pairing: Notify the coordinator that the device wishes to perform in-band pairing. This request includes the device’s public key, as well as some basic metadata about the device.
  2. Handle pairing challenge: The coordinator issues a challenge (which is just a blob of random bytes) that the device will concatenate with the pairing code, then signs using its private key before returning the resulting signature to the coordinator.
  3. Complete pairing: If the challenge response was correctly verified, the coordinator provides the public keys of all coordinator nodes and other information that would have otherwise been shared out of band.

Once the second stage of pairing has completed, the end device is expected to re-associate to the network using its device key. The coordinator will invalidate the device’s association at the conclusion of the pairing process, or if any of the steps during the second stage fail.

 

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