mbedtls how to generate a random data

mbedtls how to generate a random data

The path of the source code base the newest mbedtls:
mbedtls/programs/random/gen_random_ctr_drbg.c

The source code of how to gernarate a random data as below show:

#if !defined(MBEDTLS_CONFIG_FILE)
#include "mbedtls/config.h"
#else
#include MBEDTLS_CONFIG_FILE
#endif

#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
#else
#include <stdio.h>
#include <stdlib.h>
#define mbedtls_fprintf         fprintf
#define mbedtls_printf          printf
#define mbedtls_exit            exit
#define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
#define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
#endif /* MBEDTLS_PLATFORM_C */

#if defined(MBEDTLS_CTR_DRBG_C) && defined(MBEDTLS_ENTROPY_C) && \
 defined(MBEDTLS_FS_IO)
#include "mbedtls/entropy.h"
#include "mbedtls/ctr_drbg.h"

#include <stdio.h>
#endif

#if !defined(MBEDTLS_CTR_DRBG_C) || !defined(MBEDTLS_ENTROPY_C) || \
 !defined(MBEDTLS_FS_IO)
int main( void )
{
    mbedtls_printf("MBEDTLS_CTR_DRBG_C and/or MBEDTLS_ENTROPY_C and/or MBEDTLS_FS_IO not defined.\n");
    return( 0 );
}
#else


int main( int argc, char *argv[] )
{
    FILE *f;
    int i, k, ret = 1;
    int exit_code = MBEDTLS_EXIT_FAILURE;
    mbedtls_ctr_drbg_context ctr_drbg;
    mbedtls_entropy_context entropy;
    unsigned char buf[1024];

    mbedtls_ctr_drbg_init( &ctr_drbg );

    if( argc < 2 )
    {
        mbedtls_fprintf( stderr, "usage: %s <output filename>\n", argv[0] );
        return( exit_code );
    }

    if( ( f = fopen( argv[1], "wb+" ) ) == NULL )
    {
        mbedtls_printf( "failed to open '%s' for writing.\n", argv[1] );
        return( exit_code );
    }

    mbedtls_entropy_init( &entropy );
    ret = mbedtls_ctr_drbg_seed( &ctr_drbg, mbedtls_entropy_func, &entropy, (const unsigned char *) "RANDOM_GEN", 10 );
    if( ret != 0 )
    {
        mbedtls_printf( "failed in mbedtls_ctr_drbg_seed: %d\n", ret );
        goto cleanup;
    }
    mbedtls_ctr_drbg_set_prediction_resistance( &ctr_drbg, MBEDTLS_CTR_DRBG_PR_OFF );

#if defined(MBEDTLS_FS_IO)
    ret = mbedtls_ctr_drbg_update_seed_file( &ctr_drbg, "seedfile" );

    if( ret == MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR )
    {
        mbedtls_printf( "Failed to open seedfile. Generating one.\n" );
        ret = mbedtls_ctr_drbg_write_seed_file( &ctr_drbg, "seedfile" );
        if( ret != 0 )
        {
            mbedtls_printf( "failed in mbedtls_ctr_drbg_write_seed_file: %d\n", ret );
            goto cleanup;
        }
    }
    else if( ret != 0 )
    {
        mbedtls_printf( "failed in mbedtls_ctr_drbg_update_seed_file: %d\n", ret );
        goto cleanup;
    }
#endif

    for( i = 0, k = 768; i < k; i++ )
    {
        ret = mbedtls_ctr_drbg_random( &ctr_drbg, buf, sizeof( buf ) );
        if( ret != 0 )
        {
            mbedtls_printf("failed!\n");
            goto cleanup;
        }

        fwrite( buf, 1, sizeof( buf ), f );

        mbedtls_printf( "Generating %ldkb of data in file '%s'... %04.1f" \
                "%% done\r", (long)(sizeof(buf) * k / 1024), argv[1], (100 * (float) (i + 1)) / k );
        fflush( stdout );
    }

    exit_code = MBEDTLS_EXIT_SUCCESS;

cleanup:
    mbedtls_printf("\n");

    fclose( f );
    mbedtls_ctr_drbg_free( &ctr_drbg );
    mbedtls_entropy_free( &entropy );

    return( exit_code );
}
#endif /* MBEDTLS_CTR_DRBG_C && MBEDTLS_ENTROPY_C */

If you just want to generate a fixed length of random data, you can use the below code:

  int ret = -1;
  mbedtls_entropy_context entropy;
  mbedtls_ctr_drbg_context ctr_drbg;
  const uint8_t *pers = "ecdsa";

  mbedtls_entropy_init(&entropy);
  mbedtls_ctr_drbg_init(&ctr_drbg);

  ret = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func, &entropy,
                              (const unsigned char *) pers, strlen(pers));
  if (ret != 0)
    {
      goto cleanup;
    }

  ret = mbedtls_ctr_drbg_random(&ctr_drbg, random, random_request_length);
  if (ret != 0)
    {
      goto cleanup;
    }

cleanup:
  mbedtls_entropy_free(&entropy);
  mbedtls_ctr_drbg_free(&ctr_drbg);

  return ret;