Bluetooth pairing - low power traditional pairing, master key entry

In the previous article "Bluetooth Pairing - Key Generation Method" , we discussed how to generate keys - if both the initiating device and the responding device satisfy certain IO functions, they can choose low-power traditional pairing (Legacy). Pairing) Passkey Entry method.

Bluetooth pairing - low power traditional pairing, master key entry

In this article, let's continue to learn more about how to use Passkey Entry for traditional pairing and how it works.

Figure 1: Entering a low-power traditional pairing with Passkey Entry

Figure 1: Entering a low-power traditional pairing with Passkey Entry

1. Temporary key (TK) and random number generation

When using traditional Bluetooth pairing with low power consumption, both devices generate a Temporary Key (TK) for pairing.

If the IO of any of the initiating device or the responding device has a display function, any TK value between "000000" and "999999" is generated and displayed. The other device should have an input function, such as a keyboard, so that the user can enter this temporary key value.

If the IO functions of the initiating device and the responding device do not have the display function, but all are "Keyboard Only", then the user needs to ensure that the temporary key between the initiating device and the responding device is the same, this is A special case of Passkey Entry.

The following figure shows the scenario when a device named “AuthenTIcaTIon” is paired with an iOS device. The temporary key is displayed on the input interface of the “AuthenTIcaTIon” device. After that, the iOS device will pop up a dialog box prompting the user to enter the temporary key value.

Figure 2: Master key entry for iOS devices

Figure 2: Master key entry for iOS devices

After entering the temporary key value, the initiating device and the responding device should generate a temporary value of 128 bits called the Mrand ("master random") and the response device called Srand ("slave random").

2.Mconfirm and Sconfirm

Mconfirm and Sconfirm are 128-bit acknowledgment values ​​that can be calculated by confirming the value generation function c1. For details on this function, please refer to Bluetooth Core Specification Version 4.2, Volume 4, Part H, Section 2.2.3.

The input parameters for the c1 function include:

Temporary key (TK)

Mrand for calculating Mconfirm, or Srand for calculating Sconfirm

Pairing request instruction

Pairing response command

Initiating device address type

Initiating device address

Response device address type

Response device address

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