diff --git a/src/protocolP2P/ProtocolP2PDatagram.java b/src/protocolP2P/ProtocolP2PDatagram.java
index 11db2d5..079c0fd 100644
--- a/src/protocolP2P/ProtocolP2PDatagram.java
+++ b/src/protocolP2P/ProtocolP2PDatagram.java
@@ -243,28 +243,32 @@ public class ProtocolP2PDatagram {
 		this.hostR = hostR;
 	}
 
-	private int computeCheckSum(byte [] datagram){
+	private int computeCheckSum(byte [] datagram) throws SizeError {
 		int checksum = 0;
-		for (int i=CHECKSUM_POSITION+2; i<datagram.length; ++i){
+		for (int i=CHECKSUM_POSITION+2; i<CHECKSUM_POSITION+2+4+Payload.getPayloadSize(datagram); ++i){
 			checksum += BytesArrayTools.readInt16Bits(datagram,i);
 			checksum &= 0xffff;
 		}
 		return checksum ^ 0xffff;
 	}
 
-	private void setCheckSum(byte [] datagram) throws InternalError{
-		int checksum = computeCheckSum(datagram);
-		try{
+	private void setCheckSum(byte [] datagram) throws InternalError {
+		try {
+			int checksum = computeCheckSum(datagram);
 			BytesArrayTools.write16Bits(datagram,CHECKSUM_POSITION,checksum);
 		} catch (SizeError e) {
 			throw new InternalError();
 		}
 	}
 
-	private void checkCheckSum(byte [] datagram) throws TransmissionError{
-		int checksum = BytesArrayTools.readInt16Bits(datagram,CHECKSUM_POSITION);
-		int computedCheckSum = computeCheckSum(datagram);
-		if (computedCheckSum != checksum){
+	private void checkCheckSum(byte [] datagram) throws TransmissionError {
+		try {
+			int checksum = BytesArrayTools.readInt16Bits(datagram,CHECKSUM_POSITION);
+			int computedCheckSum = computeCheckSum(datagram);
+			if (computedCheckSum != checksum){
+				throw new TransmissionError();
+			}
+		} catch(SizeError e) {
 			throw new TransmissionError();
 		}
 	}
diff --git a/src/tools/BytesArrayTools.java b/src/tools/BytesArrayTools.java
index 5c99051..3642bd8 100644
--- a/src/tools/BytesArrayTools.java
+++ b/src/tools/BytesArrayTools.java
@@ -53,7 +53,7 @@ public class BytesArrayTools {
 	public static int readInt(byte[] array, int start) throws SizeError {
 		int size = 0;
 		for(int i=0;i<4;i++) {
-			size |= ((int)array[start + i]) << (8* (3 -i));
+			size |= (0xFF & (int)array[start + i]) << (8* (3 -i));
 		}
 		if (size < 0) {
 			// Size in array is probably correct
@@ -72,11 +72,12 @@ public class BytesArrayTools {
 		int size = 0;
 		for(int i=0;i<2;i++) {
 			try {
-				size |= ((int)array[start + i]) << (8* (1 -i));
+				size |= (0xFF & (int)array[start + i]) << (8* (1 -i));
 			} catch (ArrayIndexOutOfBoundsException e){
 				// 0 padding
 			}
 		}
+		assert (size & 0xFFFF0000) == 0 : "Read on more than 16 bits";
 		return size;
 	}
 
@@ -89,7 +90,7 @@ public class BytesArrayTools {
 	public static long readLong(byte[] array, int start) throws SizeError {
 		long size = 0;
 		for(int i=0;i<8;i++) {
-			size |= ((int)array[start + i]) << (8* (7 - i));
+			size |= (0xFF & (int)array[start + i]) << (8* (7 - i));
 		}
 		if (size < 0) {
 			// Size in array is probably correct