diff --git a/cryptlib.cpp b/cryptlib.cpp
index 55df7134..ddc829db 100644
--- a/cryptlib.cpp
+++ b/cryptlib.cpp
@@ -148,16 +148,16 @@ size_t BlockTransformation::AdvancedProcessBlocks(const byte *inBlocks, const by
 	CRYPTOPP_ASSERT(outBlocks);
 	CRYPTOPP_ASSERT(length);
 
-	ptrdiff_t blockSize = static_cast<ptrdiff_t>(BlockSize());
+	const ptrdiff_t blockSize = static_cast<ptrdiff_t>(BlockSize());
 	ptrdiff_t inIncrement = (flags & (BT_InBlockIsCounter|BT_DontIncrementInOutPointers)) ? 0 : blockSize;
 	ptrdiff_t xorIncrement = xorBlocks ? blockSize : 0;
 	ptrdiff_t outIncrement = (flags & BT_DontIncrementInOutPointers) ? 0 : blockSize;
 
 	if (flags & BT_ReverseDirection)
 	{
-		inBlocks += length - blockSize;
-		xorBlocks += length - blockSize;
-		outBlocks += length - blockSize;
+		inBlocks += static_cast<ptrdiff_t>(length) - blockSize;
+		xorBlocks += static_cast<ptrdiff_t>(length) - blockSize;
+		outBlocks += static_cast<ptrdiff_t>(length) - blockSize;
 		inIncrement = 0-inIncrement;
 		xorIncrement = 0-xorIncrement;
 		outIncrement = 0-outIncrement;