Where is that headset now?
Who hasn't experienced this? You want to connect your smartphone to a Bluetooth device, start the search, and suddenly you can't see the forest for the trees, er... available clients. It would be nice to have an easy way to connect your smartphone to this one, specific device. And this is where NFC-initiated Bluetooth pairing comes into play, which I'd like to explain in more detail:
How can I conveniently establish a Bluetooth Low Energy connection?
Secure Simple Pairing with Bluetooth Low Energy (BLE) offers this possibility. This allows the necessary information to be transmitted via a second communication channel to establish a BLE connection. Using NFC for maximum convenience is also known as tap-to-pair.
The basic process can be seen in the following illustration.
The smartphone reads the data required to establish a BLE connection from a tag via NFC. This data must be in the NFC Data Exchange Format (NDEF). I'll explain the content in more detail below, but it at least includes the client's static BLE device address. Using this address, the smartphone can then send a connection request to the client and, if the client is configured accordingly, automatically establish the connection. This function is implemented in all current smartphones.
How do I build a Bluetooth Low Energy connection with NFC?
In the simplest case, the NDEF message on the NFC chip is structured as follows:
The record type specifies the content of the NDEF message. In addition to the data for establishing the BLE connection, it can also specify, for example, an app to be launched or a website to be accessed.
How these messages are to be stored on the NFC chip and, if necessary, configured can be found in detail in the data sheets or application notes of the chips used.
What options do I have to BLE connection to secure?
The connection setup described so far is called Just-Works and offers no security for the connection setup. However, data transmission once a connection is established is always secure. The only possible protection with Just-Works is to activate BLE only when reading the client, thus ensuring physical access to the device.
SSP defines three ways to secure a connection. These methods can be used depending on the capabilities of the devices.
The procedures are:
– Numeric comparison: 6-digit codes are calculated and displayed on both devices. If they match, the connection must be confirmed on both devices.
– Passkey Entry: A code is displayed on one of the devices, which must be entered by the connection partner.
– Out-of-Band (OOB): Here, the keys required to establish a secure connection are transmitted over an alternative channel, in this example, NFC. Due to the short range, communication is protected against eavesdropping.
Thus, the use of an NFC chip not only increases convenience but also increases the security of the connection. Due to the low cost of the components and the existing functionality of the BLE modules, the possibility of NFC pairing should always be considered when using Bluetooth (the procedure described is also possible with classic Bluetooth).
If you have any questions, comments or need help with BLE and pairing, please feel free to contact me at any time.
Note: The technical processes are specified by the Bluetooth Special Interest Group (SIG) and can be found in the Bluetooth Core specifications from version 4.0 onwards.