[pskc] set random PSKc when generating a new Active Dataset (#3769)

When generating a new Active Dataset, the current PSKc value is used. Because
the PSKc is initialized to all zeros, new Active Datasets have a default
PSKc value of all zeros.

This commit checks if the PSKc has been set (i.e. not all zeros) and, if not
set, generates a random PSKc when creating a new Active Dataset.

src/core/meshcop/dataset_manager_ftd.cpp

@@ -403,8 +403,22 @@ otError ActiveDataset::GenerateLocal(void)
     if (dataset.Get(Tlv::kPSKc) == NULL)
     {
         PSKcTlv tlv;
+
         tlv.Init();
-        tlv.SetPSKc(Get<KeyManager>().GetPSKc());
+
+        if (Get<KeyManager>().IsPSKcSet())
+        {
+            // use configured PSKc
+            tlv.SetPSKc(Get<KeyManager>().GetPSKc());
+        }
+        else
+        {
+            // PSKc has not yet been configured, generate new PSKc at random
+            otPSKc pskc;
+            SuccessOrExit(error = otPlatRandomGetTrue(pskc.m8, sizeof(pskc)));
+            tlv.SetPSKc(pskc);
+        }
+
         dataset.Set(tlv);
     }
 

src/core/thread/key_manager.cpp

@@ -81,7 +81,9 @@ KeyManager::KeyManager(Instance &aInstance)
     , mKeyRotationTimer(aInstance, &KeyManager::HandleKeyRotationTimer, this)
     , mKekFrameCounter(0)
     , mSecurityPolicyFlags(0xff)
+    , mIsPSKcSet(false)
 {
+    memset(&mPSKc, 0, sizeof(mPSKc));
     ComputeKey(mKeySequence, mKey);
 }
 
@@ -104,7 +106,7 @@ void KeyManager::SetPSKc(const otPSKc &aPSKc)
     Get<Notifier>().Signal(OT_CHANGED_PSKC);
 
 exit:
-    return;
+    mIsPSKcSet = true;
 }
 #endif // OPENTHREAD_MTD || OPENTHREAD_FTD
 

src/core/thread/key_manager.hpp

@@ -103,6 +103,17 @@ public:
     otError SetMasterKey(const otMasterKey &aKey);
 
 #if OPENTHREAD_FTD || OPENTHREAD_MTD
+    /**
+     * This method indicates whether the PSKc is configured.
+     *
+     * A value of all zeros indicates that the PSKc is not configured.
+     *
+     * @retval TRUE  if the PSKc is configured.
+     * @retval FALSE if the PSKc is not not configured.
+     *
+     */
+    bool IsPSKcSet(void) const { return mIsPSKcSet; }
+
     /**
      * This method returns a pointer to the PSKc.
      *
@@ -368,6 +379,7 @@ private:
     uint32_t mKekFrameCounter;
 
     uint8_t mSecurityPolicyFlags;
+    bool    mIsPSKcSet : 1;
 };
 
 /**