diff --git a/lib/hcrypto/rc4.c b/lib/hcrypto/rc4.c
index 81cf093e4..988c42424 100644
--- a/lib/hcrypto/rc4.c
+++ b/lib/hcrypto/rc4.c
@@ -45,7 +45,7 @@
 }
 
 void
-RC4_set_key(RC4_KEY *key, const int len, unsigned char *data)
+RC4_set_key(RC4_KEY *key, const int len, const unsigned char *data)
 {
     int i, j;
 
diff --git a/lib/hcrypto/rc4.h b/lib/hcrypto/rc4.h
index 463365578..f93482f4d 100644
--- a/lib/hcrypto/rc4.h
+++ b/lib/hcrypto/rc4.h
@@ -42,5 +42,5 @@ typedef struct rc4_key {
     unsigned int state[256];
 } RC4_KEY;
 
-void RC4_set_key(RC4_KEY *, const int, unsigned char *);
+void RC4_set_key(RC4_KEY *, const int, const unsigned char *);
 void RC4(RC4_KEY *, const int, const unsigned char *, unsigned char *);