diff --git a/include/polarssl/md4.h b/include/polarssl/md4.h
index c3c5d7e..5a796ae 100644
--- a/include/polarssl/md4.h
+++ b/include/polarssl/md4.h
@@ -27,6 +27,8 @@
 #ifndef POLARSSL_MD4_H
 #define POLARSSL_MD4_H
 
+#include <string.h>
+
 /**
  * \brief          MD4 context structure
  */
@@ -59,7 +61,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void md4_update( md4_context *ctx, const unsigned char *input, int ilen );
+void md4_update( md4_context *ctx, const unsigned char *input, size_t ilen );
 
 /**
  * \brief          MD4 final digest
@@ -76,7 +78,7 @@
  * \param ilen     length of the input data
  * \param output   MD4 checksum result
  */
-void md4( const unsigned char *input, int ilen, unsigned char output[16] );
+void md4( const unsigned char *input, size_t ilen, unsigned char output[16] );
 
 /**
  * \brief          Output = MD4( file contents )
@@ -96,7 +98,7 @@
  * \param key      HMAC secret key
  * \param keylen   length of the HMAC key
  */
-void md4_hmac_starts( md4_context *ctx, const unsigned char *key, int keylen );
+void md4_hmac_starts( md4_context *ctx, const unsigned char *key, size_t keylen );
 
 /**
  * \brief          MD4 HMAC process buffer
@@ -105,7 +107,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void md4_hmac_update( md4_context *ctx, const unsigned char *input, int ilen );
+void md4_hmac_update( md4_context *ctx, const unsigned char *input, size_t ilen );
 
 /**
  * \brief          MD4 HMAC final digest
@@ -131,8 +133,8 @@
  * \param ilen     length of the input data
  * \param output   HMAC-MD4 result
  */
-void md4_hmac( const unsigned char *key, int keylen,
-               const unsigned char *input, int ilen,
+void md4_hmac( const unsigned char *key, size_t keylen,
+               const unsigned char *input, size_t ilen,
                unsigned char output[16] );
 
 /**