Bluetooth Low Energy Fundamentals

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Pairing process

The most common practice of protecting wireless communication is to encrypt the connection, which converts the data being sent into a form that can only be read by those with the permission to do so.

To encrypt the link, both peers need to have the same keys. The process of generating, distributing, and authenticating these keys for encryption is referred to as the pairing process.

The pairing process requires the devices to repeat the process every time they want to encrypt the link again. In addition to “pairing”, the term “bonding” is used when the peers store the encryption key so they can re-encrypt the link in future connections with the same peer. During bonding, they can also exchange and store identity keys so they can recognize each other for future connections through the random resolvable private address.

Definition

Pairing: The process of generating, distributing, and authenticating keys for encryption purposes.
Bonding: The process of pairing followed by distribution of keys used to encrypt the link in future reconnections.

Bluetooth LE defines 3 phases in the encryption process.

Phase 1: Initiate pairing

To initiate the pairing and, in some cases, the bonding process, the central needs to send a Pairing Request and the peripheral responds with a Pairing Response. In this phase, the two devices exchange their pairing features, that will be used to determine what pairing method they will use in phase 2 and what keys are distributed in phase 3.

Most importantly, the peers exchange their I/O (input/output) capabilities, selected from one of the following

  • DisplayOnly: The peer only has a display
  • DisplayYesNo: The peer has a display and the option to select “yes” or “no”
  • KeyboardOnly: The peer has keyboard only
  • NoInputNoOutput: The peer has no input and no output capabilities
  • KeyboardDisplay: The peer has keyboard and display capabilities

In addition, they exchange what security features they support, whether or not bonding is requested, and more.

Note

Only the central can send a Pairing Request. The peripheral, however, can send a Security Request which can trigger a Pairing Request from the central, but it’s not a common practice.

Phase 2: Perform pairing

In phase 2, the keys used to encrypt the connection are generated. The pairing method used here depends on the information exchanged in phase 1.

In LE Legacy pairing, the peers exchange a Temporary Key (TK) used to generate a Short Term Key (STK) that is then used to encrypt the link. However, since the STK can easily be cracked, Bluetooth v4.2 introduced something called Bluetooth LE Secure Connections. In LE Secure Connections, the devices generate and exchange a more secure type of key, and use it to generate a single Long Term Key (LTK) used to encrypt the connection. See Legacy pairing vs LE Secure Connections later in this lesson for more information on the difference between these two security methods.

Legacy pairing defines three different methods to exchange the TK, called pairing methods. LE Secure Connections supports these three pairing methods but also a fourth (numeric comparison) that is not supported in Legacy pairing. The security of the pairing process depends on which pairing method is used in this phase.

Pairing methods

  • Just Works: Both peers generate the STK based on information exchanged in plain text, and the user is just asked to accept the connection. This method is unauthenticated.
  • Passkey Entry: 6-digit number is displayed on one device, and needs to be typed in on the other device. The I/O capabilities of the devices determind which one displays the number and which one inputs it.
  • Out of Band (OOB): The encryption keys are exchanged by some other means than Bluetooth LE, for example by using NFC.
  • Numeric Comparison (LE Secure Connections only): Both devices display a 6-digit number and the user selects “yes” or “no” to confirm the display.

Which pairing method to use is decided based on the OOB flag, the Man-In-The-Middle (MITM) flag, and the I/O capabilities of the peers, exchanged during phase 1.

The OOB and MITM flags first determine whether to use the OOB pairing method directly or determine the pairing method based on the I/O capabilities.

Rules for using OOB and MITM flags

Notice that in LE Secure Connections, only one of the peers needs to have the OOB flag set, for this pairing method to be used.

Depending on the OOB and MITM flags, the I/O capabilities of the peers might be used to determine the pairing method. In this case, the following table is used.

Mapping of I/O capabilities to key generation method

The key generated at this phase will be used to encrypt the link after phase 2. If you are only doing pairing, not bonding, then only these 2 phases will be performed and the peers will skip phase 3.

Phase 3: Key distribution

In this phase, the Long Term Key (LTK) is used to distribute the rest of the keys. In legacy pairing, the LTK is also generated in this phase (in LE Secure Connections, the LTK is generated in phase 2). Other keys are also generated and exchanged in this phase, to identify the peers the next time they re-connect and to be able to re-encrypt the link using the same LTK.

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