diff --git a/include/polarssl/base64.h b/include/polarssl/base64.h
index 4858ef4..6144184 100644
--- a/include/polarssl/base64.h
+++ b/include/polarssl/base64.h
@@ -46,7 +46,7 @@
  *                 required buffer size in *dlen
  */
 int base64_encode( unsigned char *dst, int *dlen,
-                   unsigned char *src, int  slen );
+                   const unsigned char *src, int  slen );
 
 /**
  * \brief          Decode a base64-formatted buffer
@@ -65,7 +65,7 @@
  *                 required buffer size in *dlen
  */
 int base64_decode( unsigned char *dst, int *dlen,
-                   unsigned char *src, int  slen );
+                   const unsigned char *src, int  slen );
 
 /**
  * \brief          Checkup routine