Random data generation

When making a security application, you may require a random data generator. Arm Mbed TLS includes the for random generation.

Setting up CTR-DRBG in your code requires an entropy source and a personalization string.

Setting up the entropy source

Mbed TLS includes the to provide a central pool of entropy from which to extract entropy.

To use the entropy collector in your code, include the header file:

#include "mbedtls/entropy.h"

Add the following somewhere in your main():

mbedtls_entropy_context entropy;
mbedtls_entropy_init( &entropy );

If your platform has a hardware TRNG or PRNG in the processor or TPM, you can hook it up to the entropy collector with entropy_add_source() to enhance the entropy even further. Please see our article on for more information.

The random generator

To use the CTR-DRBG module in your code, you need to include the header file:

#include "mbedtls/ctr_drbg.h"

Add:

mbedtls_ctr_drbg_context ctr_drbg;
char *personalization = "my_app_specific_string";

mbedtls_ctr_drbg_init( &ctr_drbg );

ret = mbedtls_ctr_drbg_seed( &ctr_drbg , mbedtls_entropy_func, &entropy,
                 (const unsigned char *) personalization,
                 strlen( personalization ) );
if( ret != 0 )
{
    // ERROR HANDLING CODE FOR YOUR APP
}

The personalization string is a small protection against a lack of startup entropy and ensures each application has at least a different starting point.

Enabling prediction resistance

To prevent an adversary from reading your random data, you can enable prediction resistance:

c mbedtls_ctr_drbg_set_prediction_resistance( &ctr_drbg, MBEDTLS_CTR_DRBG_PR_ON );

Note: If enabled, entropy is gathered before each call. Only use this if you have ample supply of good entropy.

Loading a seed file

Another way to add entropy at the start of your application is to use a seed file. The Mbed TLS random generator can read and update a seed file with mbedtls_ctr_drbg_update_seed_file() to increase entropy.

Multithreaded use

If you intend to use the CTR-DRBG module in multiple threads, please read our article on entropy collection, random generation with threads.

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