TRNG support for nRF5x devices

[HITMAnsOFT]

• Type: Enhancement

I am working on a BLE-based car alarm using nRF52832, and I need secure random key generation for a custom communication protocol. The nRF51x and nRF52x devices all contain a hardware RNG core, and the HAL and drivers are available in the Nordic SDK. However, the mbed OS doesn't seem to support trng_api for Nordic targets. I tried to directly call the Nordic HAL methods, but I found out that the corresponding files are missing from the TARGET_SDK11 folder. Those files (drivers_nrf/hal/nrf_rng.h, drivers_nrf/rng/nrf_drv_rng.h, drivers_nrf/rng/nrf_drv_rng.c) do exist in the TARGET_SDK13 folder, but for some reason it is not used with the nRF52832 device.

It would be nice to support TRNG HAL for nRF51 and nRF52 devices.

[pan-]

@nvlsianpu

[HITMAnsOFT]

Update: I downloaded the Nordic SDK version 11 and copied the HAL and driver files mentioned above into my project source dir and was able to use the RNG module directly.

[nvlsianpu]

@nvlsianpu have You a POC for look at?

[HITMAnsOFT]

@nvlsianpu I have this small C++ wrapper. It may not be robust and may cause an infinite loop, but it's working for me on nRF52832. The buffer depth of the RNG seems to be 32 bytes:

#include "nrf_drv_rng.h"

class Random {
public:
    Random ()
    {
        nrf_drv_rng_init(nullptr);
    }

    template <class T, size_t size> void getArray(std::array<T, size>& out)
    {
        for (T& a : out) {
            uint8_t available = 0;
            do {
                nrf_drv_rng_bytes_available(&available);
            } while (available < sizeof(T));
            nrf_drv_rng_rand(&a, sizeof(T)); 
        }
    }
};

[nvlsianpu]

Thanks for the snipped. I wanted to know whether rng driver work out-off-the-box for nRF52. So answer is yes.

[HITMAnsOFT]

Yes, it works. Chip code is QFAA B0. Unfortunately I was not able to run my applicaiton on nRF51. Do you happen to know if mbed works on Rev. 2 nRF51822 chips?

[nvlsianpu]

mbed-os apps are supposed to work since rev. 3 of nRF51822. I never tested it with rev. 2

[HITMAnsOFT]

I've ordered a couple of what seems to be Rev 3 nRF51822 modules. Will test this code on them when I get them.

[dlfryar-zz]

Mbed OS SDK updates happening here

[marcuschangarm]

New TRNG driver has been merged to master: https://github.com/ARMmbed/mbed-os/pull/6116