irk as the only security perimeter

[Stanislav Silnitskiy]

Hello, Nordic!

In our entertainment project, nrf51 based devices exclusively utilizes either central or periph. role. All of them are headless (body mounted markers, etc). The environment is hostile (in the sense, that some cheater could spoof device address, advertising it as valid periph.) Besides, we don't need a connection at all, and wish to relay upon pre-shared set of IRKs, belonging to peripherals. What we need is just RSSI and some extra info in advert.

Is it right case to use private addresses for? If so, which API does sdk provide to hash/rehash the address in central and how IRK is being set in peripheral?

I tend to use S130 and if it is redudant for scenario explained, I'd appreciated an advice.

Stan

[Hung Bui]

Hi Stan,

It's possible to distribute the IRKs in advance and use the IRK to generate Random resolvable private address on each of the advertiser. It's supported by our softdevices (S110, S120, S130), please have a look in the description of the sd_ble_gap_address_set() function to know how you can tell the softdevice to use Random resolvable address, when to generate new one. And to the sd_ble_opt_set() ( and ble_gap_opt_t ->ble_gap_opt_privacy_t) to know how to set the IRK.

I just want to know how you would deal with the attacker that can spoof the device address ? Even if you use resolvable random address and can change it periodically, the attacker can still simply copy the new address and spoof it when you update your address.

Some little more on the security requirement and the implementation of your application would give better understanding.

[Stanislav Silnitskiy]

right question! I think to replace a random salt by incremental space, keeping track of lastly used address on central device. As soon as the solution is intended to be used limited time (2-3 hours session), it may work... I realize, that this decreases the security, though.

Again, to balance drawback of above, an array of per-peripherial IRK could be used, those changed every 1000 address or so...

Do I understand it right, that to check received address in central, I will use consequent calls of sd_ble_gap_address_set and sd_ble_gap_address_get for every pre-shared IPK?

[Hung Bui]

@Stan: No, you can get the device address when you receive the advertising packet (BLE_GAP_EVT_ADV_REPORT event) . It's in peer_addr in ble_gap_evt_adv_report_t struct.

I still don't know how you can avoid address spoofing with the IRK. Note that 2 devices with same address can still advertise at the same time without causing interference.

[Stanislav Silnitskiy]

Yes, I understand that I need to extract the address from received advert. packet. I mean how to check this address?

As for spoofing I see the following infrastructure:

Each peripheral has an array of IRKs, those shared with all clients (centrals). All IRKs are uniq across all perihps. When periph. starts advertising, it generates next address (the hash, as per spec.) from its first IRK and 0x1 in place of random part, so the broadcasted address has a form (24bit hash) || (0x1). The next address will be generated with 0x2 as a random part and so on... The new address will be generated for every Nth second of operation or, if not a performance issue, for every advert. packet.

[Stanislav Silnitskiy]

... So, from client's point of view, it will receive broadcasts with incremental "random" part and will keep track of expired addresses pretty straightforward. Every 1000th address periph. takes the next IRK from available (and pre-shared with clients) list and generates next 1000 addresses using a new IRK. The client sees, that "random" part of address has reached 1000 and expires previously used IRK.