diff --git a/include/polarssl/arc4.h b/include/polarssl/arc4.h
index 2344c74..48ad60b 100644
--- a/include/polarssl/arc4.h
+++ b/include/polarssl/arc4.h
@@ -27,6 +27,8 @@
 #ifndef POLARSSL_ARC4_H
 #define POLARSSL_ARC4_H
 
+#include <string.h>
+
 /**
  * \brief          ARC4 context structure
  */
@@ -49,7 +51,7 @@
  * \param key      the secret key
  * \param keylen   length of the key
  */
-void arc4_setup( arc4_context *ctx, const unsigned char *key, int keylen );
+void arc4_setup( arc4_context *ctx, const unsigned char *key, unsigned int keylen );
 
 /**
  * \brief          ARC4 cipher function
@@ -61,7 +63,7 @@
  *
  * \return         0 if successful
  */
-int arc4_crypt( arc4_context *ctx, int length, const unsigned char *input,
+int arc4_crypt( arc4_context *ctx, size_t length, const unsigned char *input,
                 unsigned char *output );
 
 /*