diff --git a/libfreefare/freefare.h b/libfreefare/freefare.h
index c6bae69..e0cc22c 100644
--- a/libfreefare/freefare.h
+++ b/libfreefare/freefare.h
@@ -92,7 +92,17 @@ int		 mifare_classic_get_data_block_permission (MifareTag tag, const MifareClass
 int		 mifare_classic_format_sector (MifareTag tag, const MifareClassicBlockNumber block);
 char		*mifare_classic_get_uid (MifareTag tag);
 
-void		 mifare_classic_trailer_block (MifareClassicBlock *block, const MifareClassicKey key_a, const uint8_t ab_0, const uint8_t ab_1, const uint8_t ab_2, const uint8_t ab_tb, const uint8_t gpb, const MifareClassicKey key_b);
+void		 mifare_classic_trailer_block (MifareClassicBlock *block, const MifareClassicKey key_a, uint8_t ab_0, uint8_t ab_1, uint8_t ab_2, uint8_t ab_tb, const uint8_t gpb, const MifareClassicKey key_b);
+
+#define C_000 0
+#define C_001 1
+#define C_010 2
+#define C_011 3
+#define C_100 4
+#define C_101 5
+#define C_110 6
+#define C_111 7
+#define C_DEFAULT 255
 
 /* MIFARE Classic Access Bits */
 #define MCAB_R 0x8
diff --git a/libfreefare/freefare_internal.h b/libfreefare/freefare_internal.h
index b205d94..a845641 100644
--- a/libfreefare/freefare_internal.h
+++ b/libfreefare/freefare_internal.h
@@ -118,4 +118,10 @@ struct mifare_ultralight_tag {
 #define MIFARE_CLASSIC(tag) ((struct mifare_classic_tag *) tag)
 #define MIFARE_ULTRALIGHT(tag) ((struct mifare_ultralight_tag *) tag)
 
+/*
+ * Access bits manipulation macros
+ */
+#define DB_AB(ab) ((ab == C_DEFAULT) ? C_000 : ab)
+#define TB_AB(ab) ((ab == C_DEFAULT) ? C_100 : ab)
+
 #endif /* !__FREEFARE_INTERNAL_H__ */
diff --git a/libfreefare/mifare_classic.c b/libfreefare/mifare_classic.c
index b624439..8590ee9 100644
--- a/libfreefare/mifare_classic.c
+++ b/libfreefare/mifare_classic.c
@@ -709,10 +709,15 @@ mifare_classic_last_sector_block (MifareClassicBlockNumber block)
  * Generates a MIFARE trailer block.
  */
 void
-mifare_classic_trailer_block (MifareClassicBlock *block, const MifareClassicKey key_a, const uint8_t ab_0, const uint8_t ab_1, const uint8_t ab_2, const uint8_t ab_tb, const uint8_t gpb, const MifareClassicKey key_b)
+mifare_classic_trailer_block (MifareClassicBlock *block, const MifareClassicKey key_a, uint8_t ab_0, uint8_t ab_1, uint8_t ab_2, uint8_t ab_tb, const uint8_t gpb, const MifareClassicKey key_b)
 {
     union mifare_classic_block *b = (union mifare_classic_block *)block; // *((union mifare_classic_block *)(&block));
 
+    ab_0 = DB_AB(ab_0);
+    ab_1 = DB_AB(ab_1);
+    ab_2 = DB_AB(ab_2);
+    ab_tb = TB_AB(ab_tb);
+
     memcpy (b->trailer.key_a, key_a, sizeof (MifareClassicKey));
 
     uint32_t access_bits = ((((( ab_0  & 0x4) >> 2) << 8) | (((ab_0  & 0x2) >> 1) << 4) | (ab_0  & 0x1)) |