mbedtls RSA trouble, mbedtls_rsa_import/mbedtls_rsa_complete fails

basvkesteren said:

Because I don't really have a clear picture about how and when the whole nrfxlib crypto stuff comes into play, the documentation (https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrfxlib/crypto/README.html) is sort of useless; a changelog and an api-list really doesn't do...

 It is not meant that you should use the crypto libraries directly, instead, you should use the Nordic Security Module: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrfxlib/nrf_security/README.html

I have also made a very small sample that seems to be working correctly, though I had to use Zephyr's sys_csrand_get() instead of mbedTLS's hardware_poll():

Hi Didrik,

Spend the day doing some testing. When I do the same entropy-trick (using the local f_rng function instead of mbedtls_entropy_func) stuff starts to work.

BUT, if I'm correct, I'm not using the cryptocell hardware. What do I need to do to get that working?

I've created a test-project, just doing the encryption, nothing else. I need AES-128 and RSA-1024. To get AES working, I need CONFIG_OBERON_BACKEND=y, otherwise the output is all zero's.

For RSA, i need to set CONFIG_CC3XX_BACKEND=n, or mbedtls_rsa_complete() fails with an input error.

If you could take a look at my code, i've uploaded it here

and the included testdata here

prj.conf is here

(think i don't have the option of uploading stuff to the ticket, like you did?)

Hi Didrik,

Thanks for the reply! About that upload, dunno how I've missed that button... A well.

But, any word from the developers?

Since I've uploaded my initial testcode I've been tweaking and fixing the mbedtls code; it's working stable now, but rather slow (especially the RSA calls). Would like to fix that

Hi, and sorry for the slow response.

The reason for why it fails when using the cryptocell is because the library doesn't calculate N based on P and Q.

However, if you calculate N and provide it to the library, it will work.

We will also add this missing detail in future versions of the crypto libraries.

This is the code snippet that should have been in the crypto library:

   if( !have_N && have_P && have_Q )
    {
        if( ( ret = mbedtls_mpi_mul_mpi( &ctx->N, &ctx->P,
                                         &ctx->Q ) ) != 0 )
        {
            return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
        }

        ctx->len = mbedtls_mpi_size( &ctx->N );
    }

Hi Didrik,

Thanks for the response. So, do I understand it right that this missing functionality will be added to a future NCS release? Looks to me the crypto-library is binary only, I can't modify it myself, right?

Also, I did a quick test this morning. In the following code the test_RSA_N/P/Q/E contain a valid RSA key. This sequence works with CONFIG_CC3XX_BACKEND=n, but fails with CONFIG_CC3XX_BACKEND=y (I get a 'mbedtls_rsa_complete failed: -16512' printout).

What am I missing? I'm using NCS 1.4.0

    LOG_DBG("RSA import, again:");
    mbedtls_rsa_init(&rsa, MBEDTLS_RSA_PKCS_V21, MBEDTLS_MD_SHA256);
    if((r = mbedtls_rsa_import_raw(&rsa, test_RSA_N, sizeof(test_RSA_N),
                                         test_RSA_P, sizeof(test_RSA_P), 
                                         test_RSA_Q, sizeof(test_RSA_Q),
                                         NULL, 0, /* D */
                                         test_RSA_E, sizeof(test_RSA_E)))) {
        mbedtls_strerror(r, errbuf, sizeof(errbuf));
        LOG_DBG("mbedtls_rsa_import_raw failed: %d (%s)", r, errbuf);
    }
    LOG_DBG("ok");

    LOG_DBG("mbedtls_rsa_complete");
    if((r = mbedtls_rsa_complete(&rsa))) {
        mbedtls_strerror(r, errbuf, sizeof(errbuf));
        LOG_DBG("mbedtls_rsa_complete failed: %d (%s)\n", r, errbuf);
    }
    LOG_DBG("ok");

Hi, and sorry for the late reply.

Since my last comment, the developers has found even more problems with the library.

It seems like you at least also have to provide E, although that is unpractical for the application.

However, I have been promised that a Pull Request with an updated version of the library will be available within the next few days.

Sorry for the inconvenience, and the time this is taking.

Best regards,

Didrik

The pull request which should fix your issues has now been published:https://github.com/nrfconnect/sdk-nrfxlib/pull/390

I don't have the opportunity to test it myself today, but it should solve all the RSA problems in the cryptocell library.

The pull request which should fix your issues has now been published:https://github.com/nrfconnect/sdk-nrfxlib/pull/390

I don't have the opportunity to test it myself today, but it should solve all the RSA problems in the cryptocell library.