Bluetooth Security with Link Key Generation

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As mentioned in Section 3.1, there are two methods in which link key generation is performed for Bluetooth. Security Modes 2 and 3 use one method, while Security Mode 4 uses another. Both methods are described below.

For Bluetooth v2.0 + EDR (and earlier), operating in Security Mode 2 or 3, two associated devices simultaneously derive link keys during the initialization phase when users enter an identical PIN into one or both devices, depending on the configuration and device type. The PIN entry, device association, and key derivation are depicted conceptually in Figure 3-2. Note that if the PIN is less than 16 bytes, the BD_ADDR is used to supplement the PIN value used to generate the initialization key. The Ex boxes
represent encryption algorithms that are used during the Bluetooth device association and key derivation processes. More details on the Bluetooth authentication and encryption procedures are outlined in Sections 3.4 and 3.5, respectively.

After initialization is complete, devices automatically and transparently authenticate and initiate the encryption procedure to secure the wireless link, if encryption is enabled. The PIN code used in Bluetooth devices can vary between one and 16 bytes. The typical four-digit PIN may be sufficient for low-risk situations; a longer PIN should be used for devices that require a higher level of security.

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