diff --git a/include/polarssl/base64.h b/include/polarssl/base64.h
index 1c6498c..27ba685 100644
--- a/include/polarssl/base64.h
+++ b/include/polarssl/base64.h
@@ -27,6 +27,8 @@
 #ifndef POLARSSL_BASE64_H
 #define POLARSSL_BASE64_H
 
+#include <string.h>
+
 #define POLARSSL_ERR_BASE64_BUFFER_TOO_SMALL               0x0010
 #define POLARSSL_ERR_BASE64_INVALID_CHARACTER              0x0012
 
@@ -49,8 +51,8 @@
  * \note           Call this function with *dlen = 0 to obtain the
  *                 required buffer size in *dlen
  */
-int base64_encode( unsigned char *dst, int *dlen,
-                   const unsigned char *src, int  slen );
+int base64_encode( unsigned char *dst, size_t *dlen,
+                   const unsigned char *src, size_t slen );
 
 /**
  * \brief          Decode a base64-formatted buffer
@@ -68,8 +70,8 @@
  * \note           Call this function with *dlen = 0 to obtain the
  *                 required buffer size in *dlen
  */
-int base64_decode( unsigned char *dst, int *dlen,
-                   const unsigned char *src, int  slen );
+int base64_decode( unsigned char *dst, size_t *dlen,
+                   const unsigned char *src, size_t slen );
 
 /**
  * \brief          Checkup routine