diff --git a/include/polarssl/arc4.h b/include/polarssl/arc4.h
index b012299..20f142a 100644
--- a/include/polarssl/arc4.h
+++ b/include/polarssl/arc4.h
@@ -45,7 +45,7 @@
  * \param key      the secret key
  * \param keylen   length of the key
  */
-void arc4_setup( arc4_context *ctx, unsigned char *key, int keylen );
+void arc4_setup( arc4_context *ctx, const unsigned char *key, int keylen );
 
 /**
  * \brief          ARC4 cipher function