diff --git a/library/md2.c b/library/md2.c
index e942ff3..061ebbe 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -114,7 +114,7 @@
 /*
  * MD2 process buffer
  */
-void md2_update( md2_context *ctx, unsigned char *input, int ilen )
+void md2_update( md2_context *ctx, const unsigned char *input, int ilen )
 {
     int fill;
 
@@ -163,7 +163,7 @@
 /*
  * output = MD2( input buffer )
  */
-void md2( unsigned char *input, int ilen, unsigned char output[16] )
+void md2( const unsigned char *input, int ilen, unsigned char output[16] )
 {
     md2_context ctx;
 
@@ -177,7 +177,7 @@
 /*
  * output = MD2( file contents )
  */
-int md2_file( char *path, unsigned char output[16] )
+int md2_file( const char *path, unsigned char output[16] )
 {
     FILE *f;
     size_t n;
@@ -209,7 +209,7 @@
 /*
  * MD2 HMAC context setup
  */
-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 )
 {
     int i;
     unsigned char sum[16];
@@ -239,7 +239,7 @@
 /*
  * MD2 HMAC process buffer
  */
-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 )
 {
     md2_update( ctx, input, ilen );
 }
@@ -263,7 +263,8 @@
 /*
  * output = HMAC-MD2( hmac key, input buffer )
  */
-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] )
 {
     md2_context ctx;