settings.h, random.c: warn when MICRIUM seeds from Math_Rand

IDE/ECLIPSE/MICRIUM/README.md

@@ -68,6 +68,15 @@ static  void  App_TaskStart (void *p_arg)
 
 10. Now you are ready to download and debug your image on the board.
 
+## Random seed
+
+Micrium provides no entropy source. Unless you configure one, wolfSSL seeds its DRBG from uC/LIB `Math_Rand()` and the build emits a warning saying so. Keys and nonces generated this way are predictable. Before production, supply a hardware RNG in `user_settings.h`:
+
+```
+extern int my_trng_seed(unsigned char* output, unsigned int sz); /* return 0 on success */
+#define CUSTOM_RAND_GENERATE_SEED my_trng_seed
+```
+
 The test results below were collected from the NXP Kinetis K70 (Freescale TWR-K70F120M MCU) tower system board with the following software and tool chains:
 
 - IAR Embedded Workbench IDE - ARM 8.32.1 (IAR ELF Linker V8.32.1.169/W32 for ARM)

IDE/ECLIPSE/MICRIUM/user_settings.h

@@ -28,6 +28,9 @@
 
 #define MICRIUM
 
+/* Supply a hardware RNG seed via CUSTOM_RAND_GENERATE_SEED for production
+ * use. See IDE/ECLIPSE/MICRIUM/README.md */
+
 /* You can select one or all of the following tests */
 #define WOLFSSL_WOLFCRYPT_TEST
 #define WOLFSSL_BENCHMARK_TEST

wolfcrypt/src/random.c

@@ -10598,6 +10598,34 @@ int wc_GenerateSeed(OS_Seed* os, byte* output, word32 sz)
             }
         }
 
+        return 0;
+    }
+#elif defined(MICRIUM) && !defined(CUSTOM_RAND_GENERATE_BLOCK)
+    #if !defined(_MSC_VER) && !defined(__TASKING__)
+        #warning "MICRIUM: seeding the DRBG from Math_Rand(), not an entropy source. Define CUSTOM_RAND_GENERATE_SEED to a hardware RNG."
+    #else
+        #pragma message("Warning: MICRIUM: seeding the DRBG from Math_Rand(), not an entropy source. Define CUSTOM_RAND_GENERATE_SEED to a hardware RNG.")
+    #endif
+    int wc_GenerateSeed(OS_Seed* os, byte* output, word32 sz)
+    {
+        word32 i = 0;
+        word32 len;
+        RAND_NBR rnd;
+
+        (void)os;
+
+        if (output == NULL)
+            return BAD_FUNC_ARG;
+
+        while (i < sz) {
+            rnd = Math_Rand();
+            len = sz - i;
+            if (len > (word32)sizeof(rnd))
+                len = (word32)sizeof(rnd);
+            XMEMCPY(&output[i], &rnd, len);
+            i += len;
+        }
+
         return 0;
     }
 #elif defined(WOLFSSL_VXWORKS)

wolfssl/wolfcrypt/settings.h

@@ -2969,11 +2969,6 @@
     #define NO_WOLFSSL_DIR
     #define NO_WRITEV
 
-    #if !defined(WOLFSSL_SILABS_SE_ACCEL) && !defined(STM32_RNG) && \
-        !defined(CUSTOM_RAND_GENERATE)
-        #define CUSTOM_RAND_TYPE     RAND_NBR
-        #define CUSTOM_RAND_GENERATE Math_Rand
-    #endif
     #define STRING_USER
     #define XSTRCASECMP(s1,s2) strcasecmp((s1),(s2))
     #define XSTRCMP(s1,s2) strcmp((s1),(s2))