diff --git a/programs/pkey/rsa_encrypt.c b/programs/pkey/rsa_encrypt.c
index aa6ca0e..83dcef2 100644
--- a/programs/pkey/rsa_encrypt.c
+++ b/programs/pkey/rsa_encrypt.c
@@ -60,7 +60,7 @@
     ctr_drbg_context ctr_drbg;
     unsigned char input[1024];
     unsigned char buf[512];
-    char *pers = "rsa_encrypt";
+    const char *pers = "rsa_encrypt";
 
     ret = 1;
 
@@ -80,7 +80,8 @@
 
     entropy_init( &entropy );
     if( ( ret = ctr_drbg_init( &ctr_drbg, entropy_func, &entropy,
-                               (unsigned char *) pers, strlen( pers ) ) ) != 0 )
+                               (const unsigned char *) pers,
+                               strlen( pers ) ) ) != 0 )
     {
         printf( " failed\n  ! ctr_drbg_init returned %d\n", ret );
         goto exit;