diff --git a/library/cmac.c b/library/cmac.c
index 035ad07..6df56aa 100644
--- a/library/cmac.c
+++ b/library/cmac.c
@@ -169,10 +169,10 @@
                             const unsigned char *input2,
                             const size_t block_size )
 {
-    size_t index;
+    size_t idx;
 
-    for( index = 0; index < block_size; index++ )
-        output[ index ] = input1[ index ] ^ input2[ index ];
+    for( idx = 0; idx < block_size; idx++ )
+        output[ idx ] = input1[ idx ] ^ input2[ idx ];
 }
 
 /*