diff --git a/GCryptLib/include/GCrypt/Block.h b/GCryptLib/include/GCrypt/Block.h
index ae029d0..2ebbb39 100644
--- a/GCryptLib/include/GCrypt/Block.h
+++ b/GCryptLib/include/GCrypt/Block.h
@@ -1,4 +1,6 @@
-#pragma once
+#ifndef GCRYPT_CONFIG_H
+#define GCRYPT_CONFIG_H
+
 #include "GCrypt/SecureBitset.h"
 #include "GCrypt/Config.h"
 
@@ -6,3 +8,5 @@ namespace Leonetienne::GCrypt {
   typedef SecureBitset<BLOCK_SIZE> Block;
 }
 
+#endif
+
diff --git a/GCryptLib/include/GCrypt/Config.h b/GCryptLib/include/GCrypt/Config.h
index 0560a19..c5ec236 100644
--- a/GCryptLib/include/GCrypt/Config.h
+++ b/GCryptLib/include/GCrypt/Config.h
@@ -1,4 +1,6 @@
-#pragma once
+#ifndef GCRYPT_CONFIG_H
+#define GCRYPT_CONFIG_H
+
 #include <cstddef>
 
 namespace Leonetienne::GCrypt {
@@ -8,3 +10,6 @@ namespace Leonetienne::GCrypt {
   // MUST BE > 2
   constexpr std::size_t N_ROUNDS = 64;
 }
+
+#endif
+
diff --git a/GCryptLib/include/GCrypt/Feistel.h b/GCryptLib/include/GCrypt/Feistel.h
index 586d131..50bd4dd 100644
--- a/GCryptLib/include/GCrypt/Feistel.h
+++ b/GCryptLib/include/GCrypt/Feistel.h
@@ -1,4 +1,6 @@
-#pragma once
+#ifndef GCRYPT_FEISTEL_H
+#define GCRYPT_FEISTEL_H
+
 #include "GCrypt/Keyset.h"
 #include "GCrypt/Block.h"
 #include "GCrypt/Key.h"
@@ -60,3 +62,6 @@ namespace Leonetienne::GCrypt {
     Keyset roundKeys;
   };
 }
+
+#endif
+
diff --git a/GCryptLib/include/GCrypt/Flexblock.h b/GCryptLib/include/GCrypt/Flexblock.h
index 2873f74..80f9174 100644
--- a/GCryptLib/include/GCrypt/Flexblock.h
+++ b/GCryptLib/include/GCrypt/Flexblock.h
@@ -1,7 +1,12 @@
-#pragma once
+#ifndef GCRYPT_FLEXBLOCK_H
+#define GCRYPT_FLEXBLOCK_H
+
 #include <string>
 
 namespace Leonetienne::GCrypt {
   //! A "bitset" of variable length
   typedef std::string Flexblock;
 }
+
+#endif
+
diff --git a/GCryptLib/include/GCrypt/GCipher.h b/GCryptLib/include/GCrypt/GCipher.h
index 5228ecf..f17a277 100644
--- a/GCryptLib/include/GCrypt/GCipher.h
+++ b/GCryptLib/include/GCrypt/GCipher.h
@@ -1,4 +1,6 @@
-#pragma once
+#ifndef GCRYPT_GCIPHER_H
+#define GCRYPT_GCIPHER_H
+
 #include "GCrypt/Feistel.h"
 #include "GCrypt/Flexblock.h"
 
@@ -35,3 +37,6 @@ namespace Leonetienne::GCrypt {
     Block lastBlock;
   };
 }
+
+#endif
+
diff --git a/GCryptLib/include/GCrypt/Keyset.h b/GCryptLib/include/GCrypt/Keyset.h
index ab331f6..9871d23 100644
--- a/GCryptLib/include/GCrypt/Keyset.h
+++ b/GCryptLib/include/GCrypt/Keyset.h
@@ -1,4 +1,6 @@
-#pragma once
+#ifndef GCRYPT_KEYSET_H
+#define GCRYPT_KEYSET_H
+
 #include <array>
 #include "GCrypt/Key.h"
 #include "GCrypt/Config.h"
@@ -7,3 +9,5 @@ namespace Leonetienne::GCrypt {
   typedef std::array<Key, N_ROUNDS> Keyset;
 }
 
+#endif
+
diff --git a/GCryptLib/include/GCrypt/SecureBitset.h b/GCryptLib/include/GCrypt/SecureBitset.h
index b4b1a9d..e288b13 100644
--- a/GCryptLib/include/GCrypt/SecureBitset.h
+++ b/GCryptLib/include/GCrypt/SecureBitset.h
@@ -1,4 +1,6 @@
-#pragma once
+#ifndef GCRYPT_SECUREBITSET_H
+#define GCRYPT_SECUREBITSET_H
+
 #include <bitset>
 #include <ostream>
 #include <istream>
@@ -285,3 +287,6 @@ namespace Leonetienne::GCrypt {
     return ifs >> bs.Get();
   }
 }
+
+#endif
+
diff --git a/GCryptLib/include/GCrypt/Version.h b/GCryptLib/include/GCrypt/Version.h
index a536c89..801185e 100644
--- a/GCryptLib/include/GCrypt/Version.h
+++ b/GCryptLib/include/GCrypt/Version.h
@@ -1,3 +1,7 @@
-#pragma once
+#ifndef GCRYPT_VERSION_H
+#define GCRYPT_VERSION_H
+
 #define GHETTOCRYPT_VERSION 0.22
 
+#endif
+