esp32: Add esp_fill_random() function

Convenience function to fill a buffer with random bytes.

Add some unit tests (only sanity checks, really.)

components/esp32/test/test_random.c

@@ -0,0 +1,67 @@
+#include <stdio.h>
+#include <string.h>
+#include "unity.h"
+#include "esp_system.h"
+
+/* Note: these are just sanity tests, not the same as
+   entropy tests
+*/
+
+TEST_CASE("call esp_random()", "[random]")
+{
+    const size_t NUM_RANDOM = 128; /* in most cases this is massive overkill */
+
+    uint32_t zeroes = UINT32_MAX;
+    uint32_t ones = 0;
+    for (int i = 0; i < NUM_RANDOM - 1; i++) {
+        uint32_t r = esp_random();
+        ones |= r;
+        zeroes &= ~r;
+    }
+
+    /* assuming a 'white' random distribution, we can expect
+       usually at least one time each bit will be zero and at
+       least one time each will be one. Statistically this
+       can still fail, just *very* unlikely to. */
+    TEST_ASSERT_EQUAL_HEX32(0, zeroes);
+    TEST_ASSERT_EQUAL_HEX32(UINT32_MAX, ones);
+}
+
+TEST_CASE("call esp_fill_random()", "[random]")
+{
+    const size_t NUM_BUF = 200;
+    const size_t BUF_SZ = 16;
+    uint8_t buf[NUM_BUF][BUF_SZ];
+    uint8_t zero_buf[BUF_SZ];
+    uint8_t one_buf[BUF_SZ];
+
+    bzero(buf, sizeof(buf));
+    bzero(one_buf, sizeof(zero_buf));
+    memset(zero_buf, 0xFF, sizeof(one_buf));
+
+    for (int i = 0; i < NUM_BUF; i++) {
+        esp_fill_random(buf[i], BUF_SZ);
+    }
+    /* No two 128-bit buffers should be the same
+       (again, statistically this could happen but it's very unlikely) */
+    for (int i = 0; i < NUM_BUF; i++) {
+        for (int j = 0; j < NUM_BUF; j++) {
+            if (i != j) {
+                TEST_ASSERT_NOT_EQUAL(0, memcmp(buf[i], buf[j], BUF_SZ));
+            }
+        }
+    }
+
+    /* Do the same all bits are zero and one at least once test across the buffers */
+    for (int i = 0; i < NUM_BUF; i++) {
+        for (int x = 0; x < BUF_SZ; x++) {
+            zero_buf[x] &= ~buf[i][x];
+            one_buf[x] |= buf[i][x];
+        }
+    }
+    for (int x = 0; x < BUF_SZ; x++) {
+        TEST_ASSERT_EQUAL_HEX8(0, zero_buf[x]);
+        TEST_ASSERT_EQUAL_HEX8(0xFF, one_buf[x]);
+    }
+}
+