Use RtlGenRandom() directly instead of going through rand_s().

base/rand_util_win.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/rand_util.h"
 
-#include <stdlib.h>
+#include <windows.h>
 
-#include "base/basictypes.h"
+// #define needed to link in RtlGenRandom(), a.k.a. SystemFunction036.  See the
+// "Community Additions" comment on MSDN here:
+// http://msdn.microsoft.com/en-us/library/windows/desktop/aa387694.aspx
+#define SystemFunction036 NTAPI SystemFunction036
+#include <NTSecAPI.h>
+#undef SystemFunction036
+
+#include <algorithm>
+#include <limits>
+
 #include "base/logging.h"
 
-namespace {
-
-uint32 RandUint32() {
-  uint32 number;
-  CHECK_EQ(rand_s(&number), 0);
-  return number;
-}
-
-}  // namespace
-
 namespace base {
 
 // NOTE: This function must be cryptographically secure. http://crbug.com/140076
 uint64 RandUint64() {
-  uint32 first_half = RandUint32();
-  uint32 second_half = RandUint32();
-  return (static_cast<uint64>(first_half) << 32) + second_half;
+  uint64 number;
+  RandBytes(&number, sizeof(number));
+  return number;
 }
 
 void RandBytes(void* output, size_t output_length) {
-  uint64 random_int;
-  const size_t random_int_size = sizeof(random_int);
-  for (size_t i = 0; i < output_length; i += random_int_size) {
-    random_int = base::RandUint64();
-    const size_t copy_count = std::min(output_length - i, random_int_size);
-    memcpy(((uint8*)output) + i, &random_int, copy_count);
+  char* output_ptr = static_cast<char*>(output);
+  while (output_length > 0) {
+    const ULONG output_bytes_this_pass = static_cast<ULONG>(std::min(
+        output_length, static_cast<size_t>(std::numeric_limits<ULONG>::max())));
+    const bool success =
+        RtlGenRandom(output_ptr, output_bytes_this_pass) != FALSE;
+    CHECK(success);
+    output_length -= output_bytes_this_pass;
+    output_ptr += output_bytes_this_pass;
   }
 }
 
 }  // namespace base