sys/luid: shuffle bytes

The CPU ID can have large parts that are identical between devices.
To add some more entropy to the generated IDs, shuffle the bytes.

sys/luid/luid.c

@@ -35,12 +35,33 @@ void __attribute__((weak)) luid_base(void *buf, size_t len)
     memset(buf, LUID_BACKUP_SEED, len);
 
 #if CPUID_LEN
-    uint8_t *out = (uint8_t *)buf;
+    uint8_t *out = buf;
     uint8_t cid[CPUID_LEN];
+    uint8_t sum = 0;
 
     cpuid_get(cid);
+
+    /* CPU ID is XORed with LUID_BACKUP_SEED.
+     * Then Fisher–Yates shuffle is performed to mix
+     * the bytes, as often large continuous chunks of
+     * CPU ID are equal on machines with the same CPU.
+     */
     for (size_t i = 0; i < MAX(len, CPUID_LEN); i++) {
-        out[i % len] ^= cid[i % CPUID_LEN];
+        uint8_t tmp, j, k;
+        j = i % len;
+        k = i % CPUID_LEN;
+
+        /* xor with CPU ID */
+        out[j] ^= cid[k];
+
+        /* get 'random' position */
+        sum += out[j];
+        k = sum % (j + 1);
+
+        /* shuffle bytes */
+        tmp    = out[j];
+        out[j] = out[k];
+        out[k] = tmp;
     }
 #endif
 }