diff --git a/library/sha2.c b/library/sha2.c
index 8afe73c..87f02ea 100644
--- a/library/sha2.c
+++ b/library/sha2.c
@@ -94,7 +94,7 @@
     ctx->is224 = is224;
 }
 
-static void sha2_process( sha2_context *ctx, unsigned char data[64] )
+static void sha2_process( sha2_context *ctx, const unsigned char data[64] )
 {
     unsigned long temp1, temp2, W[64];
     unsigned long A, B, C, D, E, F, G, H;
@@ -228,7 +228,7 @@
 /*
  * SHA-256 process buffer
  */
-void sha2_update( sha2_context *ctx, unsigned char *input, int ilen )
+void sha2_update( sha2_context *ctx, const unsigned char *input, int ilen )
 {
     int fill;
     unsigned long left;
@@ -314,7 +314,7 @@
 /*
  * output = SHA-256( input buffer )
  */
-void sha2( unsigned char *input, int ilen,
+void sha2( const unsigned char *input, int ilen,
            unsigned char output[32], int is224 )
 {
     sha2_context ctx;
@@ -329,7 +329,7 @@
 /*
  * output = SHA-256( file contents )
  */
-int sha2_file( char *path, unsigned char output[32], int is224 )
+int sha2_file( const char *path, unsigned char output[32], int is224 )
 {
     FILE *f;
     size_t n;
@@ -361,7 +361,7 @@
 /*
  * SHA-256 HMAC context setup
  */
-void sha2_hmac_starts( sha2_context *ctx, unsigned char *key, int keylen,
+void sha2_hmac_starts( sha2_context *ctx, const unsigned char *key, int keylen,
                        int is224 )
 {
     int i;
@@ -392,7 +392,7 @@
 /*
  * SHA-256 HMAC process buffer
  */
-void sha2_hmac_update( sha2_context *ctx, unsigned char *input, int ilen )
+void sha2_hmac_update( sha2_context *ctx, const unsigned char *input, int ilen )
 {
     sha2_update( ctx, input, ilen );
 }
@@ -420,8 +420,8 @@
 /*
  * output = HMAC-SHA-256( hmac key, input buffer )
  */
-void sha2_hmac( unsigned char *key, int keylen,
-                unsigned char *input, int ilen,
+void sha2_hmac( const unsigned char *key, int keylen,
+                const unsigned char *input, int ilen,
                 unsigned char output[32], int is224 )
 {
     sha2_context ctx;