What happens when Bluetooth devices pair?

Bluetooth devices use radio signals to exchange digital messages. Both Bluetooth Classic and Bluetooth Low Energy use the 2.4-gigahertz radio band, although their communication details differ. A device can announce its presence so another device can discover it. Receiving that announcement does not by itself prove which device has answered.

Pairing is a separate process that establishes shared secret keys. A key is numerical information used by a security system. The devices exchange information and use a pairing method suited to their capabilities. Authentication checks help establish that the intended devices are taking part, while encryption uses keys to protect the contents of their radio link.

The picture shows one Bluetooth Low Energy method called numeric comparison. Both devices have displays and a way to confirm a choice. They show six-digit values produced during the security process. When matching values are accepted, pairing can complete; a rejected mismatch stops it. The displayed number helps confirm the connection. It is not the secret key used to encrypt the data.

Bonding means storing key information for later connections. Bonded devices can use retained keys when they reconnect, rather than starting a new pairing process every time. Pairing, bonding and radio communication therefore name different parts of the system. Other pairing methods work with different device capabilities, and some provide less protection against an impostor. Radio range or a familiar device name alone does not establish authentication.