diff --git a/library/md4.c b/library/md4.c
index 4be1939..251e63f 100644
--- a/library/md4.c
+++ b/library/md4.c
@@ -73,7 +73,7 @@
     ctx->state[3] = 0x10325476;
 }
 
-static void md4_process( md4_context *ctx, unsigned char data[64] )
+static void md4_process( md4_context *ctx, const unsigned char data[64] )
 {
     unsigned long X[16], A, B, C, D;
 
@@ -179,7 +179,7 @@
 /*
  * MD4 process buffer
  */
-void md4_update( md4_context *ctx, unsigned char *input, int ilen )
+void md4_update( md4_context *ctx, const unsigned char *input, int ilen )
 {
     int fill;
     unsigned long left;
@@ -259,7 +259,7 @@
 /*
  * output = MD4( input buffer )
  */
-void md4( unsigned char *input, int ilen, unsigned char output[16] )
+void md4( const unsigned char *input, int ilen, unsigned char output[16] )
 {
     md4_context ctx;
 
@@ -273,7 +273,7 @@
 /*
  * output = MD4( file contents )
  */
-int md4_file( char *path, unsigned char output[16] )
+int md4_file( const char *path, unsigned char output[16] )
 {
     FILE *f;
     size_t n;
@@ -305,7 +305,7 @@
 /*
  * MD4 HMAC context setup
  */
-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 )
 {
     int i;
     unsigned char sum[16];
@@ -335,7 +335,7 @@
 /*
  * MD4 HMAC process buffer
  */
-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 )
 {
     md4_update( ctx, input, ilen );
 }
@@ -359,7 +359,8 @@
 /*
  * output = HMAC-MD4( hmac key, input buffer )
  */
-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] )
 {
     md4_context ctx;