diff --git a/U2FMessage.hpp b/U2FMessage.hpp
index 1283df2..488ddad 100644
--- a/U2FMessage.hpp
+++ b/U2FMessage.hpp
@@ -22,7 +22,6 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include <vector>
 
 struct U2FMessage {
-public:
 	uint32_t cid;
 	uint8_t cmd;
 	std::vector<uint8_t> data;
diff --git a/U2F_Authenticate_APDU.hpp b/U2F_Authenticate_APDU.hpp
index e540682..a388255 100644
--- a/U2F_Authenticate_APDU.hpp
+++ b/U2F_Authenticate_APDU.hpp
@@ -20,7 +20,8 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include "Storage.hpp"
 #include "U2F_Msg_CMD.hpp"
 
-struct U2F_Authenticate_APDU : U2F_Msg_CMD {
+class U2F_Authenticate_APDU : public U2F_Msg_CMD {
+protected:
 	uint8_t controlByte;
 	std::array<uint8_t, 32> challengeP;
 	Storage::AppParam appParam;
@@ -31,6 +32,7 @@ public:
 
 	virtual void respond(const uint32_t channelID) const override;
 
+protected:
 	enum ControlCode {
 		CheckOnly = 0x07,
 		EnforcePresenceSign = 0x03,
diff --git a/U2F_CMD.hpp b/U2F_CMD.hpp
index d255a52..929d932 100644
--- a/U2F_CMD.hpp
+++ b/U2F_CMD.hpp
@@ -20,7 +20,7 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include "U2FMessage.hpp"
 #include <memory>
 
-struct U2F_CMD {
+class U2F_CMD {
 protected:
 	U2F_CMD() = default;
 
diff --git a/U2F_Init_CMD.hpp b/U2F_Init_CMD.hpp
index 5d600d9..4f1fc5d 100644
--- a/U2F_Init_CMD.hpp
+++ b/U2F_Init_CMD.hpp
@@ -22,7 +22,8 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include <cstdint>
 #include <memory>
 
-struct U2F_Init_CMD : U2F_CMD {
+class U2F_Init_CMD : public U2F_CMD {
+protected:
 	uint64_t nonce;
 
 public:
diff --git a/U2F_Msg_CMD.hpp b/U2F_Msg_CMD.hpp
index f0daae0..d8e60fd 100644
--- a/U2F_Msg_CMD.hpp
+++ b/U2F_Msg_CMD.hpp
@@ -23,7 +23,7 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include <memory>
 #include <vector>
 
-struct U2F_Msg_CMD : U2F_CMD {
+struct U2F_Msg_CMD : public U2F_CMD {
 	uint8_t cla;
 	uint8_t ins;
 	uint8_t p1;
diff --git a/U2F_Ping_CMD.hpp b/U2F_Ping_CMD.hpp
index 204d3a3..4f2d99b 100644
--- a/U2F_Ping_CMD.hpp
+++ b/U2F_Ping_CMD.hpp
@@ -22,7 +22,8 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include <cstdint>
 #include <memory>
 
-struct U2F_Ping_CMD : U2F_CMD {
+class U2F_Ping_CMD : public U2F_CMD {
+protected:
 	std::vector<uint8_t> nonce;
 
 public:
diff --git a/U2F_Register_APDU.hpp b/U2F_Register_APDU.hpp
index db6ca39..19aaa83 100644
--- a/U2F_Register_APDU.hpp
+++ b/U2F_Register_APDU.hpp
@@ -20,7 +20,8 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include "Storage.hpp"
 #include "U2F_Msg_CMD.hpp"
 
-struct U2F_Register_APDU : U2F_Msg_CMD {
+class U2F_Register_APDU : public U2F_Msg_CMD {
+protected:
 	std::array<uint8_t, 32> challengeP;
 	Storage::AppParam appP;
 	Storage::KeyHandle keyH;
diff --git a/U2F_Version_APDU.hpp b/U2F_Version_APDU.hpp
index eb907d5..02da04e 100644
--- a/U2F_Version_APDU.hpp
+++ b/U2F_Version_APDU.hpp
@@ -19,7 +19,7 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #pragma once
 #include "U2F_Msg_CMD.hpp"
 
-struct U2F_Version_APDU : U2F_Msg_CMD {
+class U2F_Version_APDU : public U2F_Msg_CMD {
 public:
 	U2F_Version_APDU(const U2F_Msg_CMD& msg, const std::vector<uint8_t>& data);
 	void respond(const uint32_t channelID) const override;