BLE Security Mode 2, Level 1: Unauthenticated pairing with data signing

[grosdode]

Hello,

we are developing a verry small sensor node. Since we like to know if the sensor we are connected to, is one of our sensors instead of an attacker fake sensor we need a method to check this. Because the sensor node is so small (3mm x 27mm) we need to use the nrf52811 and have no room for nfc, a display or keys. So “Numeric Comparison” and “Out of Band (OOB) Pairing” as pairing method is no option and “Passkey” is only possible when we hard coded the key into all the devices. Since 6 digits are possible to brute force if the attacker just has/bought one (or x) of the sensors this is no real security. Therefore, signing the data with a hard coded 128/256Bit private key (BLE Security Mode 2, Level 1: Unauthenticated pairing with data signing) looks like a practical option. The S112 SoftDevice (https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.s112.api.v7.2.0%2Fstructble__gap__conn__sec__mode__t.html&cp=4_7_0_1_2_1_6_3) should support this, but as far as I know the private key to sign the data is generated while connecting instead of a fix private key. Is this right? Can this be changed or is this just a stupid idea?

[Hung Bui]

Hi Dominik, 

Data signing is not supported by the softdevice. I don't think many phones on the market support this feature also. 

What you can do is either encrypt your data using your own key on application level. There is a discussion about that here, it's pretty old but should give you some idea. 

Another option is to use some challenge response authentication: https://en.wikipedia.org/wiki/Challenge%E2%80%93response_authentication

You can disable the functionality on the central side and disconnect if you don't receive a correct response to your random generated challenge.