[mac] add RxFrame::IsSecuredWith() helper method (#13064)

This commit introduces a new helper method, `RxFrame::IsSecuredWith()`,
which allows callers to cleanly verify if a received MAC frame has
security enabled and uses a specific set of allowed Key ID Modes.

This eliminates redundant logic in `ThreadLinkInfo::SetFrom()`, where
the code previously had to manually check `GetSecurityEnabled()`,
extract the Key ID Mode, and validate it against `kKeyIdMode0` or
`kKeyIdMode1`. Mac::ProcessCsl()` is updated to use this new method
to cleanly enforce that CSL IE processing only occurs on frames
secured with Key ID Mode 1

Crucially, this commit also updates `DataPollHandler::HandleDataPoll()`
to use this new helper. Previously, it only checked if the frame
was secured (`GetSecurityEnabled()`), which would accept frames
using any Key ID Mode (including mode 2 with fixed/known keys). By
restricting the data poll handling to only accept Key ID Mode 1, we
ensure that data polls are only processed if they are secured with
a valid Thread network key.

src/core/mac/data_poll_handler.cpp

@@ -89,7 +89,8 @@ void DataPollHandler::HandleDataPoll(Mac::RxFrame &aFrame)
     Child       *child;
     uint16_t     indirectMsgCount;
 
-    VerifyOrExit(aFrame.GetSecurityEnabled());
+    VerifyOrExit(aFrame.IsSecuredWith(Mac::RxFrame::kAllowKeyIdMode1));
+
     VerifyOrExit(!Get<Mle::Mle>().IsDetached());
 
     SuccessOrExit(aFrame.GetSrcAddr(macSource));

src/core/mac/mac.cpp

@@ -2491,12 +2491,8 @@ void Mac::ProcessCsl(const RxFrame &aFrame, const Address &aSrcAddr)
     CslNeighbor *neighbor = nullptr;
     const CslIe *csl;
 
-    uint8_t keyIdMode;
-
-    VerifyOrExit(aFrame.IsVersion2015() && aFrame.GetSecurityEnabled());
-
-    IgnoreError(aFrame.GetKeyIdMode(keyIdMode));
-    VerifyOrExit(keyIdMode == Frame::kKeyIdMode1);
+    VerifyOrExit(aFrame.IsVersion2015());
+    VerifyOrExit(aFrame.IsSecuredWith(RxFrame::kAllowKeyIdMode1));
 
     csl = aFrame.GetCslIe();
     VerifyOrExit(csl != nullptr);

src/core/mac/mac_frame.cpp

@@ -1561,6 +1561,32 @@ exit:
 }
 #endif // OPENTHREAD_CONFIG_WAKEUP_COORDINATOR_ENABLE
 
+bool RxFrame::IsSecuredWith(KeyIdModeFlags aFlags) const
+{
+    bool    isSecure = false;
+    uint8_t keyIdMode;
+
+    VerifyOrExit(GetSecurityEnabled());
+    SuccessOrExit(GetKeyIdMode(keyIdMode));
+
+    switch (keyIdMode)
+    {
+    case kKeyIdMode0:
+        VerifyOrExit(aFlags & kAllowKeyIdMode0);
+        break;
+    case kKeyIdMode1:
+        VerifyOrExit(aFlags & kAllowKeyIdMode1);
+        break;
+    default:
+        ExitNow();
+    }
+
+    isSecure = true;
+
+exit:
+    return isSecure;
+}
+
 Error RxFrame::ProcessReceiveAesCcm(const ExtAddress &aExtAddress, const KeyMaterial &aMacKey)
 {
 #if OPENTHREAD_FTD || OPENTHREAD_MTD

src/core/mac/mac_frame.hpp

@@ -933,6 +933,30 @@ class RxFrame : public Frame
 public:
     friend class TxFrame;
 
+    /**
+     * Defines flags to indicate allowed Key ID Modes, used in `IsSecuredWith()`.
+     */
+    enum KeyIdModeFlag : uint8_t
+    {
+        kAllowKeyIdMode0 = (1 << 0), ///< Allow Key ID Mode 0.
+        kAllowKeyIdMode1 = (1 << 1), ///< Allow Key ID Mode 1.
+    };
+
+    /**
+     * Represents a set of `KeyIdModeFlag`s.
+     */
+    typedef uint8_t KeyIdModeFlags;
+
+    /**
+     * Indicates whether the frame is secured with a given set of allowed Key ID Modes.
+     *
+     * @param[in] aFlags  A bitmask of `KeyIdModeFlags` specifying the allowed modes.
+     *
+     * @retval TRUE   The frame has security enabled and uses one of the allowed Key ID Modes.
+     * @retval FALSE  The frame does not have security enabled, or its Key ID Mode is not allowed.
+     */
+    bool IsSecuredWith(KeyIdModeFlags aFlags) const;
+
     /**
      * Returns the RSSI in dBm used for reception.
      *

src/core/thread/thread_link_info.cpp

@@ -55,24 +55,10 @@ void ThreadLinkInfo::SetFrom(const Mac::RxFrame &aFrame)
         mIsDstPanIdBroadcast = (dstPanId == Mac::kPanIdBroadcast);
     }
 
-    if (aFrame.GetSecurityEnabled())
-    {
-        uint8_t keyIdMode;
-
-        // MAC Frame Security was already validated at the MAC
-        // layer. As a result, `GetKeyIdMode()` will never return
-        // failure here.
-        IgnoreError(aFrame.GetKeyIdMode(keyIdMode));
-
-        mLinkSecurity = (keyIdMode == Mac::Frame::kKeyIdMode0) || (keyIdMode == Mac::Frame::kKeyIdMode1);
-    }
-    else
-    {
-        mLinkSecurity = false;
-    }
-    mChannel = aFrame.GetChannel();
-    mRss     = aFrame.GetRssi();
-    mLqi     = aFrame.GetLqi();
+    mLinkSecurity = aFrame.IsSecuredWith(Mac::RxFrame::kAllowKeyIdMode0 | Mac::RxFrame::kAllowKeyIdMode1);
+    mChannel      = aFrame.GetChannel();
+    mRss          = aFrame.GetRssi();
+    mLqi          = aFrame.GetLqi();
 #if OPENTHREAD_CONFIG_TIME_SYNC_ENABLE
     if (aFrame.GetTimeIe() != nullptr)
     {