OSSL_DRBG_PARAM_MAC
drbg_params[1] = OSSL_PARAM_construct_utf8_string(OSSL_DRBG_PARAM_MAC, "HMAC", 0);
p = OSSL_PARAM_locate(params, OSSL_DRBG_PARAM_MAC);
OSSL_PARAM_utf8_string(OSSL_DRBG_PARAM_MAC, NULL, 0),
OSSL_PARAM_utf8_string(OSSL_DRBG_PARAM_MAC, NULL, 0),
*p++ = OSSL_PARAM_construct_utf8_string(OSSL_DRBG_PARAM_MAC, "HMAC", 0);