Mbedtls entropy generation runs forever - stm32f7

Question

I'm trying to write an test function for mbedtls which randomly generates a key for AES encryption.
I use the original tutorial Code from mbedtls.
My Programm always stops when executing "mbedtls_ctr_drbg_seed()".
About my environmet: Basic Sourcefiles from STM_CUBEmx, Board: ST32F767 Nucleo, Compiling based on Makefile from STM_Cube
mbedtls_ctr_drbg_context ctr_drbg;
mbedtls_entropy_context entropy;
char *pers="anything";
int ret;
//Start
mbedtls_entropy_init(&entropy);
debugPrintln("Init entropy done");
mbedtls_ctr_drbg_init(&ctr_drbg);
debugPrintln("Init ctr_drbg done");
if((ret=mbedtls_ctr_drbg_seed(&ctr_drbg,mbedtls_entropy_func,&entropy,(unsigned char *) pers,strlen(pers)))!=0){
//Error info
debugPrintln("ERROR ctr_drbg_seed ");
return -1;
}
debugPrintln("Init ctr_drbg_seed done");
if((ret=mbedtls_ctr_drbg_random(&ctr_drbg,key,32))!=0){
return -1;
}
Thank you in advance

Answer 1

From your description,
I am assuming that your application is stuck in the call to mbedtls_ctr_drbg_seed().
Most probable reason, IMHO, is in the functionmbedtls_entropy_func():
do
{
if( count++ > ENTROPY_MAX_LOOP )
{
ret = MBEDTLS_ERR_ENTROPY_SOURCE_FAILED;
goto exit;
}
if( ( ret = entropy_gather_internal( ctx ) ) != 0 )
goto exit;
done = 1;
for( i = 0; i < ctx->source_count; i++ )
if( ctx->source[i].size < ctx->source[i].threshold )
done = 0;
}
while( ! done );
You should check that your entropy collection increases the collected size, that the threshold is not MAX_INT or something of the sort, and that your hw entropy collector actually returns entropy data.

Answer 2

I have found the reason
STM32 Cube MX places the HAL Init function for the RNG after the mbedtls init
So when I call mbedtls_ctr_drbg_seed() inside mbedtls_init() the RNG hasn't yet been initialized and it iterates forever inside:
do
{
if( count++ > ENTROPY_MAX_LOOP )
{
ret = MBEDTLS_ERR_ENTROPY_SOURCE_FAILED;
goto exit;
}
if( ( ret = entropy_gather_internal( ctx ) ) != 0 )
goto exit;
done = 1;
for( i = 0; i < ctx->source_count; i++ )
if( ctx->source[i].size < ctx->source[i].threshold )
done = 0;
}
while( ! done );
Solution
swap the lines