[SL-UP] Adds support for PKCS7 padding in OTA flow (#511)

Co-authored-by: Restyled.io <[email protected]>

Author: rosahay-silabs

scripts/tools/silabs/ota/crypto_utils.py

@@ -380,10 +380,11 @@ def decrypt(self, ciphertext):
 
 def encryptFlashData(nonce, key, data, imageLen):
     encyptedBlock = ''
-    if (imageLen % 16) != 0:
-        for x in range(16 - (imageLen % 16)):
-            data = data + bytes([255])
-        imageLen = len(data)
+    block_size = 16
+    # pad using PKCS#7 padding
+    pad_len = block_size - (imageLen % block_size)
+    data = data + bytes([pad_len] * pad_len)
+    imageLen = len(data)
 
     r = rijndael(key, block_size=16)
 

src/platform/silabs/multi-ota/OTAFirmwareProcessor.cpp

@@ -72,6 +72,12 @@ CHIP_ERROR OTAFirmwareProcessor::ProcessInternal(ByteSpan & block)
     }
 
     OTATlvProcessor::vOtaProcessInternalEncryption(byteBlock);
+    if (IsLastBlock() == true)
+    {
+        // Remove padding from the last block since if the file was padded, last block will contain padding bytes.
+        VerifyOrReturnError(OTATlvProcessor::RemovePadding(byteBlock) == CHIP_NO_ERROR, CHIP_ERROR_WRONG_ENCRYPTION_TYPE,
+                            ChipLogError(SoftwareUpdate, "Failed to remove padding"));
+    }
     block = byteBlock;
 #endif
 

src/platform/silabs/multi-ota/OTAMultiImageProcessorImpl.cpp

@@ -427,14 +427,13 @@ void OTAMultiImageProcessorImpl::HandleApply(intptr_t context)
     imageProcessor->mAccumulator.Clear();
 
     ChipLogProgress(SoftwareUpdate, "HandleApply: Finished and Soft Reset initiated");
-
+#if (defined(_SILICON_LABS_32B_SERIES_3) || defined(SLI_SI91X_MCU_INTERFACE)) && CHIP_PROGRESS_LOGGING
+    osDelay(500); // sl-temp: delay for uart print before reboot
+#endif
     // This reboots the device
 #ifdef SLI_SI91X_MCU_INTERFACE // 917 SoC reboot
     chip::DeviceLayer::Silabs::GetPlatform().SoftwareReset();
 #else // EFR reboot
-#if defined(_SILICON_LABS_32B_SERIES_3) && CHIP_PROGRESS_LOGGING
-    osDelay(100); // sl-temp: delay for uart print before reboot
-#endif
     CORE_CRITICAL_SECTION(bootloader_rebootAndInstall();)
 #endif
 }

src/platform/silabs/multi-ota/OTATlvProcessor.cpp

@@ -59,7 +59,10 @@ CHIP_ERROR OTATlvProcessor::Process(ByteSpan & block)
     CHIP_ERROR status     = CHIP_NO_ERROR;
     uint32_t bytes        = chip::min(mLength - mProcessedLength, static_cast<uint32_t>(block.size()));
     ByteSpan relevantData = block.SubSpan(0, bytes);
-
+    if (mProcessedLength + bytes >= mLength)
+    {
+        mLastBlock = true;
+    }
     status = ProcessInternal(relevantData);
     if (!IsError(status))
     {
@@ -86,6 +89,7 @@ void OTATlvProcessor::ClearInternal()
     mLength          = 0;
     mProcessedLength = 0;
     mWasSelected     = false;
+    mLastBlock       = false;
 #if OTA_ENCRYPTION_ENABLE
     mIVOffset = 0;
 #endif
@@ -148,5 +152,34 @@ CHIP_ERROR OTATlvProcessor::vOtaProcessInternalEncryption(MutableByteSpan & bloc
 
     return CHIP_NO_ERROR;
 }
+
+CHIP_ERROR OTATlvProcessor::RemovePadding(MutableByteSpan & block)
+{
+    if (block.size() == 0)
+    {
+        ChipLogError(DeviceLayer, "Block size is zero, cannot unpad");
+        return CHIP_ERROR_INVALID_ARGUMENT;
+    }
+
+    uint8_t padLength = block.data()[block.size() - 1];
+    if (padLength == 0 || padLength > block.size())
+    {
+        ChipLogError(DeviceLayer, "Invalid PKCS7 padding");
+        return CHIP_ERROR_INVALID_ARGUMENT;
+    }
+
+    // Verify padding bytes
+    for (size_t i = 0; i < padLength; ++i)
+    {
+        if (block.data()[block.size() - 1 - i] != padLength)
+        {
+            ChipLogError(DeviceLayer, "PKCS7 padding verification failed");
+            return CHIP_ERROR_INVALID_ARGUMENT;
+        }
+    }
+
+    block.reduce_size(block.size() - padLength);
+    return CHIP_NO_ERROR;
+}
 #endif // OTA_ENCRYPTION_ENABLE
 } // namespace chip

src/platform/silabs/multi-ota/OTATlvProcessor.h

@@ -152,9 +152,19 @@ class OTATlvProcessor
     void SetWasSelected(bool selected) { mWasSelected = selected; }
     bool WasSelected() { return mWasSelected; }
     bool IsValidTag(OTAProcessorTag tag);
+    bool IsLastBlock() const { return mLastBlock; }
 
 #ifdef OTA_ENCRYPTION_ENABLE
     CHIP_ERROR vOtaProcessInternalEncryption(MutableByteSpan & block);
+    /**
+     * @brief Remove padding from the given block.
+     * This is necessary to remove any padding that might have been added during encryption.
+     *
+     * @param block The block of data to remove padding from.
+     * @return CHIP_NO_ERROR on success, or an error code if padding removal fails.
+     * @note The method assumes the block is padded using PKCS7 padding.
+     */
+    CHIP_ERROR RemovePadding(MutableByteSpan & block);
 #endif
 
 protected:
@@ -199,8 +209,9 @@ class OTATlvProcessor
     uint32_t mProcessedLength                    = 0;
     bool mWasSelected                            = false;
     ProcessDescriptor mCallbackProcessDescriptor = nullptr;
+    bool mDescriptorProcessed                    = false;
+    bool mLastBlock                              = false;
     OTADataAccumulator mAccumulator;
-    bool mDescriptorProcessed = false;
 };
 
 } // namespace chip

src/platform/silabs/multi-ota/SiWx917/OTAWiFiFirmwareProcessor.cpp

@@ -63,6 +63,12 @@ CHIP_ERROR OTAWiFiFirmwareProcessor::ProcessInternal(ByteSpan & block)
     }
 
     OTATlvProcessor::vOtaProcessInternalEncryption(byteBlock);
+    if (IsLastBlock() == true)
+    {
+        // Remove padding from the last block since if the file was padded, last block will contain padding bytes.
+        VerifyOrReturnError(OTATlvProcessor::RemovePadding(byteBlock) == CHIP_NO_ERROR, CHIP_ERROR_WRONG_ENCRYPTION_TYPE,
+                            ChipLogError(SoftwareUpdate, "Failed to remove padding"));
+    }
     block = byteBlock;
 #endif // OTA_ENCRYPTION_ENABLE