Encryption Key Generation and Distribution
Encryption Request from Master, Example (ComProbe Frame Display, BPA 600 low energy capture
To distribute the LTK, EDIV, and Rand values an encrypted session needs to be set up. The initiator will use STK to enable encryption on the link. Once an encrypted link is set up, the LTK is distributed. LTK is a 128-bit random number that the slave device will generate along with EDIV and Rand. Both the master and slave devices can distribute these numbers, but Bluetooth low energy is designed to conserve energy, so the slave device is often resource constrained and does not have the database storage resources for holding LTKs. Therefore the slave will distribute LTK, EDIV, and Rand to the master device for storage. When a slave begins a new encrypted session with a previously linked master device, it will request distribution of EDIV and Rand and will regenerate LTK.
Encryption Response from Slave, Example (ComProbe Frame Display, BPA 600 low energy capture)