Enhance otPlatRandomGet API to provide random stream (#471)

* Enhance otPlatRandomGet API to provide random stream

examples/platforms/cc2538/random.c

@@ -34,6 +34,9 @@
  *   This implementation is not a true random number generator and does @em satisfy the Thread requirements.
  */
 
+#include <openthread-types.h>
+
+#include <common/code_utils.hpp>
 #include <platform/random.h>
 #include "platform-cc2538.h"
 
@@ -66,3 +69,20 @@ uint32_t otPlatRandomGet(void)
 
     return mlcg;
 }
+
+ThreadError otPlatSecureRandomGet(uint16_t aInputLength, uint8_t *aOutput, uint16_t *aOutputLength)
+{
+    ThreadError error = kThreadError_None;
+
+    VerifyOrExit(aOutput && aOutputLength, error = kThreadError_InvalidArgs);
+
+    for (uint16_t length = 0; length < aInputLength; length++)
+    {
+        aOutput[length] = (uint8_t)otPlatRandomGet();
+    }
+
+    *aOutputLength = aInputLength;
+
+exit:
+    return error;
+}

examples/platforms/posix/random.c

@@ -34,6 +34,9 @@
  *   This implementation is not a true random number generator and does @em satisfy the Thread requirements.
  */
 
+#include <openthread-types.h>
+
+#include <common/code_utils.hpp>
 #include <platform/random.h>
 #include "platform-posix.h"
 
@@ -65,3 +68,20 @@ uint32_t otPlatRandomGet(void)
 
     return mlcg;
 }
+
+ThreadError otPlatSecureRandomGet(uint16_t aInputLength, uint8_t *aOutput, uint16_t *aOutputLength)
+{
+    ThreadError error = kThreadError_None;
+
+    VerifyOrExit(aOutput && aOutputLength, error = kThreadError_InvalidArgs);
+
+    for (uint16_t length = 0; length < aInputLength; length++)
+    {
+        aOutput[length] = (uint8_t)otPlatRandomGet();
+    }
+
+    *aOutputLength = aInputLength;
+
+exit:
+    return error;
+}

include/platform/random.h

@@ -52,6 +52,21 @@ extern "C" {
  *
  */
 
+/**
+ * Get random stream.
+ *
+ * @param[in]   aInputLength      The expected size of random values.
+ * @param[out]  aOutput           A pointer to the buffer for the generated random stream. The pointer should never be NULL.
+ * @param[out]  aOutputLength     A pointer to the generated size of random stream.
+ *                                It is supposed to be the same as aInputLength, but maybe less than aInputLength.
+ *                                The pointer should never be NULL.
+ *
+ * @retval kThreadError_None         Generate random successfully.
+ * @retval kThreadError_Fail         Generate random fail.
+ * @retval kThreadError_InvalidArgs  Invalid args.
+ */
+ThreadError otPlatSecureRandomGet(uint16_t aInputLength, uint8_t *aOutput, uint16_t *aOutputLength);
+
 /**
  * Get a 32-bit true random value.
  *