diff --git a/modes.cpp b/modes.cpp
index b88208f9..9ff761ca 100644
--- a/modes.cpp
+++ b/modes.cpp
@@ -125,8 +125,8 @@ void CTR_ModePolicy::SeekToIteration(lword iterationCount)
 	int carry=0;
 	for (int i=BlockSize()-1; i>=0; i--)
 	{
-		unsigned int sum = m_register[i] + byte(iterationCount) + carry;
-		m_counterArray[i] = static_cast<byte>(sum);
+		unsigned int sum = m_register[i] + (iterationCount & 0xff) + carry;
+		m_counterArray[i] = byte(sum & 0xff);
 		carry = sum >> 8;
 		iterationCount >>= 8;
 	}
@@ -147,14 +147,15 @@ void CTR_ModePolicy::OperateKeystream(KeystreamOperation /*operation*/, byte *ou
 
 	while (iterationCount)
 	{
-		byte lsb = m_counterArray[s-1];
-		size_t blocks = UnsignedMin(iterationCount, 256U-lsb);
+		const byte lsb = m_counterArray[s-1];
+		const size_t blocks = UnsignedMin(iterationCount, 256U-lsb);
+
 		m_cipher->AdvancedProcessBlocks(m_counterArray, input, output, blocks*s, BlockTransformation::BT_InBlockIsCounter|BlockTransformation::BT_AllowParallel);
-		if ((m_counterArray[s-1] = static_cast<byte>(lsb + blocks)) == 0)
+		if ((m_counterArray[s-1] = byte(lsb + blocks)) == 0)
 			IncrementCounterBy256();
 
 		output = PtrAdd(output, blocks*s);
-		input  = PtrAdd(input, blocks*inputIncrement);
+		input = PtrAdd(input, blocks*inputIncrement);
 		iterationCount -= blocks;
 	}
 }
@@ -166,7 +167,7 @@ void CTR_ModePolicy::CipherResynchronize(byte *keystreamBuffer, const byte *iv,
 	CRYPTOPP_ASSERT(length == BlockSize());
 
 	CopyOrZero(m_register, m_register.size(), iv, length);
-	m_counterArray = m_register;
+	m_counterArray.Assign(m_register.begin(), m_register.size());
 }
 
 void BlockOrientedCipherModeBase::UncheckedSetKey(const byte *key, unsigned int length, const NameValuePairs &params)
diff --git a/modes.h b/modes.h
index 0299e865..67d8dc1a 100644
--- a/modes.h
+++ b/modes.h
@@ -242,7 +242,8 @@ protected:
 	void CipherResynchronize(byte *keystreamBuffer, const byte *iv, size_t length);
 	void SeekToIteration(lword iterationCount);
 
-	SecByteBlock m_counterArray;
+	// adv-simd.h increments the counter
+	mutable SecByteBlock m_counterArray;
 };
 
 /// \brief Block cipher mode of operation default implementation