diff --git a/library/common.h b/library/common.h
index 943f84c..c175064 100644
--- a/library/common.h
+++ b/library/common.h
@@ -89,7 +89,7 @@
 inline void mbedtls_xor( unsigned char *r, unsigned char const *a, unsigned char const *b, size_t n )
 {
     size_t i;
-    for ( i = 0; ( i + 4 ) < n; i+= 4 )
+    for ( i = 0; ( i + 4 ) <= n; i += 4 )
     {
         uint32_t x = mbedtls_get_unaligned_uint32( a + i ) ^ mbedtls_get_unaligned_uint32( b + i );
         mbedtls_put_unaligned_uint32( r + i, x );