diff --git a/include/polarssl/md2.h b/include/polarssl/md2.h
index 2629039..37eef0a 100644
--- a/include/polarssl/md2.h
+++ b/include/polarssl/md2.h
@@ -56,7 +56,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void md2_update( md2_context *ctx, unsigned char *input, int ilen );
+void md2_update( md2_context *ctx, const unsigned char *input, int ilen );
 
 /**
  * \brief          MD2 final digest
@@ -73,7 +73,7 @@
  * \param ilen     length of the input data
  * \param output   MD2 checksum result
  */
-void md2( unsigned char *input, int ilen, unsigned char output[16] );
+void md2( const unsigned char *input, int ilen, unsigned char output[16] );
 
 /**
  * \brief          Output = MD2( file contents )
@@ -84,7 +84,7 @@
  * \return         0 if successful, 1 if fopen failed,
  *                 or 2 if fread failed
  */
-int md2_file( char *path, unsigned char output[16] );
+int md2_file( const char *path, unsigned char output[16] );
 
 /**
  * \brief          MD2 HMAC context setup
@@ -93,7 +93,7 @@
  * \param key      HMAC secret key
  * \param keylen   length of the HMAC key
  */
-void md2_hmac_starts( md2_context *ctx, unsigned char *key, int keylen );
+void md2_hmac_starts( md2_context *ctx, const unsigned char *key, int keylen );
 
 /**
  * \brief          MD2 HMAC process buffer
@@ -102,7 +102,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void md2_hmac_update( md2_context *ctx, unsigned char *input, int ilen );
+void md2_hmac_update( md2_context *ctx, const unsigned char *input, int ilen );
 
 /**
  * \brief          MD2 HMAC final digest
@@ -121,8 +121,8 @@
  * \param ilen     length of the input data
  * \param output   HMAC-MD2 result
  */
-void md2_hmac( unsigned char *key, int keylen,
-               unsigned char *input, int ilen,
+void md2_hmac( const unsigned char *key, int keylen,
+               const unsigned char *input, int ilen,
                unsigned char output[16] );
 
 /**