diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 86b4034..15a1194 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -81,12 +81,14 @@
     ctx->issuer_key = key;
 }
 
-int x509write_crt_set_subject_name( x509write_cert *ctx, char *subject_name )
+int x509write_crt_set_subject_name( x509write_cert *ctx,
+                                    const char *subject_name )
 {
     return x509_string_to_names( &ctx->subject, subject_name );
 }
 
-int x509write_crt_set_issuer_name( x509write_cert *ctx, char *issuer_name )
+int x509write_crt_set_issuer_name( x509write_cert *ctx,
+                                   const char *issuer_name )
 {
     return x509_string_to_names( &ctx->issuer, issuer_name );
 }
@@ -101,8 +103,8 @@
     return( 0 );
 }
 
-int x509write_crt_set_validity( x509write_cert *ctx, char *not_before,
-                                char *not_after )
+int x509write_crt_set_validity( x509write_cert *ctx, const char *not_before,
+                                const char *not_after )
 {
     if( strlen(not_before) != X509_RFC5280_UTC_TIME_LEN - 1 ||
         strlen(not_after)  != X509_RFC5280_UTC_TIME_LEN - 1 )