Although less flexible, this mode is well suited to enforcing a common security level, and it is easier to implement than mode 2. All link-level security functions are based on the concept of link keys. A secret link key is a 128 bit random number stored individually for each pair of devices. The most secure type of link key is a combination key, derived from the input of both devices. For devices with low storage capabilities, there is also the option of choosing a unit key, which may be used for several remote devices.
Authentication requires no user input.
For the first time only that two devices communicate, an initialization procedure is needed to create a common link key in a safe manner. This procedure is called pairing. The standard way of doing this assumes that the user has access to both devices at the same time. For firsttime connection, pairing requires the user to key in a Bluetooth security code of up to 16 bytes or 128 bits into paired devices. However, when this is done manually, this code will usually be shorter.