BLE - security

[Roei]

Hi,

I have a question regarding diffie-hellman.

What exactly are we doing with diffie-hellman key? Do we use it to create another key? Or is it the key itself?

Thanks!

[Dmitry]

Hi,

Roei said:

Why do need another step after generating diffie-hellman key?

Short answer - direct use of shared secret is insecure. The theory is somewhat complicated but we can trust OpenSSL wiki that clearly says: "Never use a derived secret directly".

Roei said:

In addition - what about a connection which isn't bonded?

Do we still have diffie-hellman and LTK?

As Einar pointed out, LTK is the only key that we have after pairing process in case of LESC - we just have an option to forget it after disconnection when bonding is not required.

[Roei]

Thanks!

I understand that we are generating the LTK, using both sides addresses and random numbers which are sent to both sides. Can you please tell me if those numbers are sent before or after diffie-hellman? Does it happen over secured connection?

[Dmitry]

First, public keys are exchanged. Second, confirmation and random values are sent. The channel is not encrypted at this stage, but these values are not secret.

[Roei]

Thanks!

Regarding phases - does key generation occur during the second phase? Because I know that third phase is key distribution.

[Roei]

In other words - why do we need another phase for key distribution? After Diffie-hellman - both sides can generate the LTK.

Thanks!