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478 From GSM to LTE-Advanced Pro and 5G

Link manager (LM) Link manager (LM)
device A device B

LMP_IN_RAND
LMP_ACCEPTED

LMP_COMB_KEY
LMP_COMB_KEY

LMP_AU_RAND
LMP_SRES

LMP_AU_RAND
LMP_SRES

Figure 7.15 Pairing procedure between two Bluetooth devices.

generate a link key on both sides. The link key is then saved in the Bluetooth device
database of both devices and can be used in the future for authentication and activation
of ciphering. The different steps of the pairing procedure are shown in Figure 7.15 and
are performed as follows.
To invoke the pairing procedure, an LMP_IN_RAND message is sent by the initiating
device over an established ACL connection to the remote device. The message contains
a random number. The random number is used together with the PIN and the device
address to generate an initialization key, which is called K init . As the PIN is not exchanged
between the two devices, a third device is not able to calculate K init with an intercepted
LMP_IN_RAND message.
By using K init , which is identical in both devices, each side then creates a different part
of a combination key. The combination key is based on K init , the device address of one
of the devices and an additional random number, which is not exchanged over the air
interface. Then, the two combination key halves are XOR combined with K init and are
exchanged over the air interface by sending LMP_COMB_KEY messages. The XOR
combination is necessary in order to avoid exchanging the two combination key halves
in clear text over the still unencrypted connection.
As K init is known to both sides, the XOR combination can be reversed and thus the
complete combination key is then available on both devices to form the final link key.
The link key forms the basis for the authentication and ciphering of future connections
between the two devices.
As the link key is saved in both devices, a pairing procedure and the input of a PIN by
the user are only necessary during the first connection attempt. If the link key together