diff --git a/lib/ykpiv.c b/lib/ykpiv.c
index a6f5572..d588e19 100644
--- a/lib/ykpiv.c
+++ b/lib/ykpiv.c
@@ -159,10 +159,10 @@ ykpiv_rc ykpiv_connect(ykpiv_state *state, const char *wanted) {
   return YKPIV_OK;
 }
 
-ykpiv_rc ykpiv_transfer_data(ykpiv_state *state, unsigned char *templ,
-    unsigned char *in_data, long in_len,
+ykpiv_rc ykpiv_transfer_data(ykpiv_state *state, const unsigned char *templ,
+    const unsigned char *in_data, long in_len,
     unsigned char *out_data, unsigned long *out_len, int *sw) {
-  unsigned char *in_ptr = in_data;
+  const unsigned char *in_ptr = in_data;
   unsigned long max_out = *out_len;
   ykpiv_rc res;
   *out_len = 0;
diff --git a/lib/ykpiv.h b/lib/ykpiv.h
index 7253198..662cb6d 100644
--- a/lib/ykpiv.h
+++ b/lib/ykpiv.h
@@ -55,8 +55,8 @@ extern "C"
 	ykpiv_rc ykpiv_init(ykpiv_state **state, int verbose);
 	ykpiv_rc ykpiv_done(ykpiv_state *state);
 	ykpiv_rc ykpiv_connect(ykpiv_state *state, const char *wanted);
-	ykpiv_rc ykpiv_transfer_data(ykpiv_state *state, unsigned char *templ,
-			unsigned char *in_data, long in_len,
+	ykpiv_rc ykpiv_transfer_data(ykpiv_state *state, const unsigned char *templ,
+			const unsigned char *in_data, long in_len,
 			unsigned char *out_data, unsigned long *out_len, int *sw);
 	ykpiv_rc ykpiv_send_data(ykpiv_state *state, unsigned char *apdu,
 			unsigned char *data, unsigned long *recv_len, int *sw);