diff --git a/libnfc_crypto1_crack.c b/libnfc_crypto1_crack.c
index 839b37b..404447c 100644
--- a/libnfc_crypto1_crack.c
+++ b/libnfc_crypto1_crack.c
@@ -649,8 +649,8 @@ int main (int argc, const char * argv[]) {
        target_key = MC_AUTH_B;
     }
     
-    char filename[20];
-    sprintf(filename, "0x%04x_%03u.txt", uid, target_block);
+    char filename[21];
+    sprintf(filename, "0x%04x_%03u%s.txt", uid, target_block, ab_key == MC_AUTH_A ? "A" : "B");
     fp = fopen(filename, "wb");
 
     printf("Found tag with uid %04x, collecting nonces for key %s of block %u using known key %s %012"PRIx64" for block %u\n", uid, target_key == MC_AUTH_A ? "A" : "B", target_block, ab_key == MC_AUTH_A ? "A" : "B", known_key, for_block);