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!

[Einar Thorsrud]

Hi,

I assume you are referring to LE Secure Connections (LESC), which is where a Diffie–Hellman key exchange is used in Bluetooth. That is used so that the two peers can exchange their public keys with each other to generate a shared secret that no one else can know, even an attacker that has intercepted all of the communication. This in turn is used to generate a encryption key.

You can read more about LESC here, and refer to the Bluetooth specification for all details.

[Roei]

Hi,

I know that with diffie-hellman, both sides generates a key, without sharing private keys.

After it's done - we are we doing with that key? Do we use it to generate another key?

[Einar Thorsrud]

Yes. In the end, the only key that remains after the pairing procedure is the LTK. This is briefly described in the second link I provided.

[Roei]

Thanks!

Why do need another step after generating diffie-hellman key? What is the difference between this key and LTK?

[Roei]

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

Do we still have diffie-hellman and LTK?

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.