diff --git a/library/arc4.c b/library/arc4.c
index 0dccd53..fb4542f 100644
--- a/library/arc4.c
+++ b/library/arc4.c
@@ -35,7 +35,7 @@
 /*
  * ARC4 key schedule
  */
-void arc4_setup( arc4_context *ctx, unsigned char *key, int keylen )
+void arc4_setup( arc4_context *ctx, const unsigned char *key, int keylen )
 {
     int i, j, k, a;
     unsigned char *m;