Introduction

In some cases, you only have the base values for RSA at your disposal and want to use those within Mbed TLS. Mbed TLS, specifically the mbedtls_rsa_context structure, requires more than the base values to perform optimized operations for RSA.

You can deduce the other values if you have access to your E, P and Q. In most cases, you already have your N, so you can skip the next section.

You can import these values to your mbedtls_rsa_context structure with their raw value, in Big Endian, using the mbedtls_rsa_import_raw() function, or as mbedtls_mpi structures, using the mbedtls_rsa_import() function. If you do not wish to set a component to the context, whether you do not know it or it is already set, you can just set it as NULL as the function parameter.

The following example shows you how to correctly initialize the RSA context named ctx with the values for P, Q and E into mbedtls_rsa_context.

Getting the modulus (N)

If the modulus (N) is known, you should send it as parameter to mbedtls_rsa_import() (or mbedtls_rsa_import_raw()). However if it is not known, it is calculated within the mbedtls_rsa_complete() function, if P and Q have been imported to the context.

Fill the context

To fill the context you should use the following sequence of function calls. You will have to define and initialize the appropriate variables for E, P and Q.

ret = mbedtls_rsa_import( &ctx, NULL, &P, &Q, NULL, &E );
if( ret != 0 )
{
    mbedtls_printf( " failed\n  ! mbedtls_rsa_import returned %d\n\n",
                    ret );
    goto exit;
}
if( ( ret = mbedtls_rsa_complete( &ctx ) ) != 0 )
{
    mbedtls_printf( " failed\n  ! mbedtls_rsa_complete returned %d\n\n",
                    ret );
    goto exit;
}

The function mbedtls_rsa_complete() deduces all the other components in the RSA context. Of course, if all components are known in advance and imported using the mbedtls_rsa_import() or mbedtls_rsa_import_raw(), the computation time of mbedtls_rsa_complete() will be shorter.

Check RSA key consistency

If you want to check correctness of all values in your context, use mbedtls_rsa_check_privkey().

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