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]

Roei said:

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

They do. In case of LESC, third phase is only needed to distribute IRK and CSRK (if requested in pairing request/response). As these values are confidential, this step is performed over an encrypted channel.

[Roei]

OK, just to be sure everything is cleared:

Diffie-Hellman is used to generate shared secret. In the end - both sides have the shared secret.

After that - we use that shared secret key and together with both sides's addresses and rand numbers, we create the LTK? Do we know the exact generation function?

Thanks!!

[Dmitry]

Roei said:

After that - we use that shared secret key and together with both sides's addresses and rand numbers, we create the LTK?

Exactly.

Roei said:

Do we know the exact generation function?

Yes, see Bluetooth Core specification Vol 3 Part H, "LE Secure Connections key generation function f5".

[Roei]

Thanks!

Can you please tell me in which phase does authentication (passkey etc...) occur?

[Dmitry]

Phase 2, Authentication Stage 1.

If you're curious about such low-level things, please read Bluetooth Core spec (Vol 3 Part H) - the whole pairing process is explained in details.