diff --git a/include/polarssl/x509_crt.h b/include/polarssl/x509_crt.h
index a556676..a5ab178 100644
--- a/include/polarssl/x509_crt.h
+++ b/include/polarssl/x509_crt.h
@@ -312,8 +312,8 @@
  * \return          0 if timestamp was parsed successfully, or
  *                  a specific error code
  */
-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 );
 
 /**
  * \brief           Set the issuer name for a Certificate
@@ -327,7 +327,8 @@
  * \return          0 if issuer name was parsed successfully, or
  *                  a specific error code
  */
-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 );
 
 /**
  * \brief           Set the subject name for a Certificate
@@ -341,7 +342,8 @@
  * \return          0 if subject name was parsed successfully, or
  *                  a specific error code
  */
-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 );
 
 /**
  * \brief           Set the subject public key for the certificate