There are three main architectural layers in the Bluetooth® core specification: controllers, hosts, and applications. The host layer has a security manager module that defines methods and protocols for pairing and key distribution, a corresponding security toolkit, and a security manager protocol that defines the form of the pairing instruction framework, framework, and timeout restrictions. The security manager performs the identity and encryption functions in the radio communication by means of key distribution.
Pairing to establish a key, then the key can be used to encrypt the link. A transport specific key assignment is then performed to share the key. The key can be used to encrypt future reconnections, verify signature data, or perform random address resolution. In general, there are three stages of pairing:
Phase 1: Matching feature exchange
The second stage (low power traditional pairing): generate Short Term Key (STK)
(Low Power Secure Link): Generate Long Term Key (LTK)
Phase III: Transferring a specific key distribution
Most readers may not necessarily understand low-power traditional pairing and low-power secure connections. Low Energy (LE) is one of the main features of Bluetooth 4.0 and above in the Bluetooth specification. The Bluetooth 4.2 specification adds secure connectivity to low-power physical transport, upgrades pairing, and uses Federal Information Processing (FIPS) licensed algorithms (AES-CMAC and P-256 elliptic curves) for Bluetooth low-power physical transmission. . To distinguish between a secure connection and a low-power pairing as defined in the Bluetooth 4.0 and 4.1 specifications, we turn the latter into a low-power legacy pairing. Figure 1 is a pairing flow diagram for both traditional pairing and secure connections.
Figure 1: Pairing flow chart
In this article, we mainly discuss the first phase, the pairing feature exchange. Pairing is the exchange of security features, including input/output (IO) functions, requirements for intermediaries protection, and more. The exchange of pairing information between the two devices is achieved through a pairing request (PairingRequest) and a pairing response (PairingResponse) packet. For the contents of these two kinds of information, please refer to Table 1: Pairing Request/Response.
Table 1 Pairing Request/Response
"code"
IO function
IO is the input/output, and the IO functions are combined to generate the value of this field.
For the input function, it may be "no input", "yes/no" or "keyboard", the details are as follows:
For the output function, it may be “no output†or “digital outputâ€, as follows:
After combining the input and output functions, the following matrix is ​​defined, which defines the IO functions that Bluetooth devices should have.
1 There is no pairing algorithm that can use the "yes/no" input and "no output", so its IO function result is "NoInputNoOutput".
From the above matrix, you can get an overview of the corresponding IO functions, and select the appropriate values ​​from the table below to place the pairing request/response packets.
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