diff --git a/include/polarssl/md4.h b/include/polarssl/md4.h
index 6a94752..c590736 100644
--- a/include/polarssl/md4.h
+++ b/include/polarssl/md4.h
@@ -55,7 +55,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void md4_update( md4_context *ctx, unsigned char *input, int ilen );
+void md4_update( md4_context *ctx, const unsigned char *input, int ilen );
 
 /**
  * \brief          MD4 final digest
@@ -72,7 +72,7 @@
  * \param ilen     length of the input data
  * \param output   MD4 checksum result
  */
-void md4( unsigned char *input, int ilen, unsigned char output[16] );
+void md4( const unsigned char *input, int ilen, unsigned char output[16] );
 
 /**
  * \brief          Output = MD4( file contents )
@@ -83,7 +83,7 @@
  * \return         0 if successful, 1 if fopen failed,
  *                 or 2 if fread failed
  */
-int md4_file( char *path, unsigned char output[16] );
+int md4_file( const char *path, unsigned char output[16] );
 
 /**
  * \brief          MD4 HMAC context setup
@@ -92,7 +92,7 @@
  * \param key      HMAC secret key
  * \param keylen   length of the HMAC key
  */
-void md4_hmac_starts( md4_context *ctx, unsigned char *key, int keylen );
+void md4_hmac_starts( md4_context *ctx, const unsigned char *key, int keylen );
 
 /**
  * \brief          MD4 HMAC process buffer
@@ -101,7 +101,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void md4_hmac_update( md4_context *ctx, unsigned char *input, int ilen );
+void md4_hmac_update( md4_context *ctx, const unsigned char *input, int ilen );
 
 /**
  * \brief          MD4 HMAC final digest
@@ -120,8 +120,8 @@
  * \param ilen     length of the input data
  * \param output   HMAC-MD4 result
  */
-void md4_hmac( unsigned char *key, int keylen,
-               unsigned char *input, int ilen,
+void md4_hmac( const unsigned char *key, int keylen,
+               const unsigned char *input, int ilen,
                unsigned char output[16] );
 
 /**