Switch the DRBGs from AES-128-CTR to AES-256-CTR

Reviewed-by: NDr. Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
GH: #5401

include/internal/rand.h

@@ -15,14 +15,22 @@
 
 /*
  * Default security strength (in the sense of [NIST SP 800-90Ar1])
- * of the default OpenSSL DRBG, and the corresponding NID.
  *
- * Currently supported values: 128, 192, 256
+ * NIST SP 800-90Ar1 supports the strength of the DRBG being smaller than that
+ * of the cipher by collecting less entropy. The current DRBG implemantion does
+ * not take RAND_DRBG_STRENGTH into account and sets the strength of the DRBG
+ * to that of the cipher.
  *
- * TODO(DRBG): would be nice to have the strength configurable
+ * RAND_DRBG_STRENGTH is currently only used for the legacy RAND
+ * implementation.
+ *
+ * Currently supported ciphers are: NID_aes_128_ctr, NID_aes_192_ctr and
+ * NID_aes_256_ctr
+ *
+ * TODO(DRBG): would be nice to have the NID and strength configurable
  */
-# define RAND_DRBG_STRENGTH             128
-# define RAND_DRBG_NID                  NID_aes_128_ctr
+# define RAND_DRBG_STRENGTH             256
+# define RAND_DRBG_NID                  NID_aes_256_ctr
 
 /*
  * Object lifetime functions.