IRK distribution when devices connected with public address

Question

I have a question, for BLE experts, regarding the IRK exchanged when bonding occurs between two BLE devices with public address. I don't remember where I saw the statement that if during exchange keys a device "A" got IRK from a peer device "B" that has public address, the device "A" keep the received IRK as zeros. Could somebody confirm or deny this statement?

Thank you in advance

Searching in the spec5.3 I found nothing about keeping the IRK that received from device with public address.

Answer 1

Every device must have either a Public Address or a Static Random Address that is used as the Identity Address.

When a device advertises, connects or sends a scan request, it can either choose to use this Identity address as its own address or use a Resolvable Private Address derived from its own IRK (if the device has an IRK). If the device does not have an IRK, it must use its Identity Address (or possibly a non-resolvable private address).

Whenever the IdKey bit is set in the Pairing Request and Pairing Response messages for the central or the peripheral, respectively, the IRK and Identity Address shall be distributed. If a device does not have an IRK, it sets the IRK field to 16 zero bytes in the SMP Identity Information packet during the key distribution phase during the pairing, per Bluetooth Core specification v5.3, Vol 3, Part H (SMP), section 3.6.4:

Note: An all zero Identity Resolving Key data field indicates that a device does not have a valid resolvable private address.

I think you have misunderstood something, because there is nothing that says that a device receiving a valid IRK or no IRK at all shall "set it to zeros" if the same remote device happens to have a Public Address (either as the address used during the connection or as its Identity Address). In fact, an all-zero IRK is not even valid, since such a one could not be sent over the SMP protocol (since an all-zero IRK would mean a non-existing IRK).

Note that it's perfectly fine for a device that "has a public address" to also have an IRK. It might for one connection use the public address and for another connection use a Resolvable Private Address derived from its IRK.

It is however common that devices have a public address as Identity Address and no IRK. In that case the bonding storage representing this device will store the public address and no IRK, (since it receives an all-zero IRK during key distribution).