diff --git a/APDU.hpp b/APDU.hpp
index c489b06..7229dbc 100644
--- a/APDU.hpp
+++ b/APDU.hpp
@@ -9,10 +9,9 @@ enum APDU : uint8_t
 
 enum APDU_STATUS : uint16_t
 {
-	SW_NO_ERROR = 0x9000,
+	SW_NO_ERROR                 = 0x9000,
+	SW_WRONG_DATA               = 0x6A80,
 	SW_CONDITIONS_NOT_SATISFIED = 0x6985,
-	SW_WRONG_DATA = 0x6A80,
-	SW_WRONG_LENGTH = 0x6700,
-	SW_CLA_NOT_SUPPORTED = 0x6E00,
-	SW_INS_NOT_SUPPORTED = 0x6D00
+	SW_COMMAND_NOT_ALLOWED      = 0x6986,
+	SW_INS_NOT_SUPPORTED        = 0x6D00
 };
diff --git a/U2F_Authenticate_APDU.cpp b/U2F_Authenticate_APDU.cpp
index ba1f9d6..3a66f41 100644
--- a/U2F_Authenticate_APDU.cpp
+++ b/U2F_Authenticate_APDU.cpp
@@ -14,8 +14,16 @@ using namespace std;
 U2F_Authenticate_APDU::U2F_Authenticate_APDU(const U2F_Msg_CMD &msg, const vector<uint8_t> &data)
 	: U2F_Msg_CMD{ msg }
 {
-	if (data.size() < 66)
-		throw runtime_error{ "Invalid authentication request" };
+	if (p2 != 0)
+	{
+		//Invalid U2F (APDU) parameter detected
+		throw APDU_STATUS::SW_COMMAND_NOT_ALLOWED;
+	}
+	else if (data.size() < 66)
+	{
+		//Invalid authentication request
+		throw APDU_STATUS::SW_WRONG_LENGTH;
+	}
 
 	copy(data.begin() + 0,  data.begin() + 32, challengeP.begin());
 	copy(data.begin() + 32, data.begin() + 64, appParam.begin());
@@ -27,20 +35,11 @@ U2F_Authenticate_APDU::U2F_Authenticate_APDU(const U2F_Msg_CMD &msg, const vecto
 
 void U2F_Authenticate_APDU::respond(const uint32_t channelID) const
 {
-	U2FMessage msg{};
-	msg.cid = channelID;
-	msg.cmd = U2FHID_MSG;
-	auto statusCode = APDU_STATUS::SW_NO_ERROR;
-
-	auto &response = msg.data;
-
 	if (keyH.size() != sizeof(Storage::KeyHandle))
 	{
 		//Respond with error code - key handle is of wrong size
 		cerr << "Invalid key handle length" << endl;
-		statusCode = APDU_STATUS::SW_WRONG_DATA;
-		response.insert(response.end(), FIELD_BE(statusCode));
-		msg.write();
+		this->error(channelID, APDU_STATUS::SW_WRONG_DATA);
 		return;
 	}
 
@@ -50,14 +49,19 @@ void U2F_Authenticate_APDU::respond(const uint32_t channelID) const
 	{
 		//Respond with error code - key handle doesn't exist in storage
 		cerr << "Invalid key handle" << endl;
-		statusCode = APDU_STATUS::SW_WRONG_DATA;
-		response.insert(response.end(), FIELD_BE(statusCode));
-		msg.write();
+		this->error(channelID, SW_WRONG_DATA);
 		return;
 	}
 
 	auto appMatches = (Storage::appParams.at(keyHB) == appParam);
 
+	U2FMessage msg{};
+	msg.cid = channelID;
+	msg.cmd = U2FHID_MSG;
+
+	auto &response = msg.data;
+	APDU_STATUS statusCode = APDU_STATUS::SW_NO_ERROR;
+
 	switch (p1)
 	{
 		case ControlCode::CheckOnly:
@@ -76,10 +80,8 @@ void U2F_Authenticate_APDU::respond(const uint32_t channelID) const
 			break;
 
 		default:
-			cerr << "Unknown APDU command" << endl;
-			statusCode = APDU_STATUS::SW_WRONG_DATA;
-			response.insert(response.end(), FIELD_BE(statusCode));
-			msg.write();
+			cerr << "Unknown APDU authentication command" << endl;
+			this->error(channelID, APDU_STATUS::SW_COMMAND_NOT_ALLOWED);
 			return;
 	}
 
diff --git a/U2F_CMD.cpp b/U2F_CMD.cpp
index b50ca80..75c2a59 100644
--- a/U2F_CMD.cpp
+++ b/U2F_CMD.cpp
@@ -10,7 +10,14 @@ shared_ptr<U2F_CMD> U2F_CMD::get(const shared_ptr<U2FMessage> uMsg)
 	switch (uMsg->cmd)
 	{
 		case U2FHID_MSG:
-			return U2F_Msg_CMD::generate(uMsg);
+			try
+			{
+				return U2F_Msg_CMD::generate(uMsg);
+			}
+			catch (runtime_error)
+			{
+				return {};
+			}
 		case U2FHID_INIT:
 			return make_shared<U2F_Init_CMD>(uMsg);
 		default:
diff --git a/U2F_Msg_CMD.cpp b/U2F_Msg_CMD.cpp
index 68bc375..19084da 100644
--- a/U2F_Msg_CMD.cpp
+++ b/U2F_Msg_CMD.cpp
@@ -47,12 +47,22 @@ shared_ptr<U2F_Msg_CMD> U2F_Msg_CMD::generate(const shared_ptr<U2FMessage> uMsg)
 	if (uMsg->cmd != U2FHID_MSG)
 		throw runtime_error{ "Failed to get U2F Msg uMsg" };
 	else if (uMsg->data.size() < 4)
+	{
+		U2F_Msg_CMD::error(uMsg->cid, APDU_STATUS::SW_WRONG_LENGTH);
 		throw runtime_error{ "Msg data is incorrect size" };
+	}
 
 	U2F_Msg_CMD cmd;
 	auto &dat = uMsg->data;
 
 	cmd.cla = dat[0];
+
+	if (cmd.cla != 0)
+	{
+		U2F_Msg_CMD::error(uMsg->cid, APDU_STATUS::SW_COMMAND_NOT_ALLOWED);
+		throw runtime_error{ "Invalid CLA value in U2F Message" };
+	}
+
 	cmd.ins = dat[1];
 	cmd.p1  = dat[2];
 	cmd.p2  = dat[3];
@@ -64,7 +74,10 @@ shared_ptr<U2F_Msg_CMD> U2F_Msg_CMD::generate(const shared_ptr<U2FMessage> uMsg)
 	if (usesData.at(cmd.ins) || data.size() > 3)
 	{
 		if (cBCount == 0)
+		{
+			U2F_Msg_CMD::error(uMsg->cid, APDU_STATUS::SW_WRONG_LENGTH);
 			throw runtime_error{ "Invalid command - should have attached data" };
+		}
 
 		if (data[0] != 0) //1 byte length
 		{
@@ -79,14 +92,30 @@ shared_ptr<U2F_Msg_CMD> U2F_Msg_CMD::generate(const shared_ptr<U2FMessage> uMsg)
 
 		endPtr = startPtr + cmd.lc;
 
-		cmd.le = getLe(data.end() - endPtr, vector<uint8_t>(endPtr, data.end()));
+		try
+		{
+			cmd.le = getLe(data.end() - endPtr, vector<uint8_t>(endPtr, data.end()));
+		}
+		catch (runtime_error)
+		{
+			U2F_Msg_CMD::error(uMsg->cid, APDU_STATUS::SW_WRONG_LENGTH);
+			throw;
+		}
 	}
 	else
 	{
 		cmd.lc = 0;
 		endPtr = startPtr;
 
-		cmd.le = getLe(cBCount, data);
+		try
+		{
+			cmd.le = getLe(cBCount, data);
+		}
+		catch (runtime_error)
+		{
+			U2F_Msg_CMD::error(uMsg->cid, APDU_STATUS::SW_WRONG_LENGTH);
+			throw;
+		}
 	}
 
 	const auto dBytes = vector<uint8_t>(startPtr, endPtr);
@@ -124,16 +153,25 @@ shared_ptr<U2F_Msg_CMD> U2F_Msg_CMD::generate(const shared_ptr<U2FMessage> uMsg)
 			"\t\t</table>\n"
 			"\t\t<br />", cmd.le);
 
-	switch (cmd.ins)
+	try
 	{
-		case APDU::U2F_REG:
-			return make_shared<U2F_Register_APDU>(cmd, dBytes);
-		case APDU::U2F_AUTH:
-			return make_shared<U2F_Authenticate_APDU>(cmd, dBytes);
-		case APDU::U2F_VER:
-			return make_shared<U2F_Version_APDU>(cmd);
-		default:
-			throw runtime_error{ "Unknown APDU command issued" };
+		switch (cmd.ins)
+		{
+			case APDU::U2F_REG:
+				return make_shared<U2F_Register_APDU>(cmd, dBytes);
+			case APDU::U2F_AUTH:
+				return make_shared<U2F_Authenticate_APDU>(cmd, dBytes);
+			case APDU::U2F_VER:
+				return make_shared<U2F_Version_APDU>(cmd);
+			default:
+				cerr << "Invalid command used" << endl;
+				throw APDU_STATUS::SW_INS_NOT_SUPPORTED;
+		}
+	}
+	catch (const APDU_STATUS e)
+	{
+		U2F_Msg_CMD::error(uMsg->cid, e);
+		throw runtime_error{ "APDU construction error" };
 	}
 }
 
diff --git a/U2F_Register_APDU.cpp b/U2F_Register_APDU.cpp
index 63d7308..dd095fb 100644
--- a/U2F_Register_APDU.cpp
+++ b/U2F_Register_APDU.cpp
@@ -16,11 +16,14 @@ U2F_Register_APDU::U2F_Register_APDU(const U2F_Msg_CMD &msg, const vector<uint8_
 	: U2F_Msg_CMD{ msg }
 {
 	if (data.size() != 64)
-		throw runtime_error{ "Incorrect registration size" };
+	{
+		//Incorrect registration size
+		throw APDU_STATUS::SW_WRONG_LENGTH;
+	}
 	else if (p1 != 0x00 || p2 != 0x00)
 	{
-		cerr << "Ins: " << static_cast<uint32_t>(ins) << ", p1: " << static_cast<uint32_t>(p1) << ", p2: " << static_cast<uint32_t>(p2) << endl;
-		cerr << "Invalid APDU parameters detected" << endl;
+		//Invalid U2F Message (APDU) parameters detected
+		throw APDU_STATUS::SW_COMMAND_NOT_ALLOWED;
 	}
 
 	copy(data.data() + 0,  data.data() + 32, challengeP.begin());
@@ -30,7 +33,7 @@ U2F_Register_APDU::U2F_Register_APDU(const U2F_Msg_CMD &msg, const vector<uint8_
 	Storage::PrivKey privKey{};
 	Storage::PubKey pubKey{};
 	
-	//Unsure if necessary
+	//First byte must be 0x04 for some reason
 	pubKey[0] = 0x04;
 
 	uECC_make_key(pubKey.data() + 1, privKey.data(), uECC_secp256r1());
diff --git a/U2F_Version_APDU.cpp b/U2F_Version_APDU.cpp
index b5af8dc..5702ee0 100644
--- a/U2F_Version_APDU.cpp
+++ b/U2F_Version_APDU.cpp
@@ -9,7 +9,11 @@ using namespace std;
 
 U2F_Version_APDU::U2F_Version_APDU(const U2F_Msg_CMD &msg)
 {
-	//Don't actually respond yet
+	//Don't actually respond yet unless invalid
+	if (msg.p1 != 0 || msg.p2 != 0)
+		throw APDU_STATUS::SW_COMMAND_NOT_ALLOWED;
+	else if (msg.data.size() != 0)
+		throw APDU_STATUS::SW_WRONG_LENGTH;
 }
 
 void U2F_Version_APDU::respond(const uint32_t channelID) const