I had the below code in a SharePoint Webpart and it worked properly:

using (var wb = new WebClient()) {     wb.UseDefaultCredentials = true;     var data = wb.DownloadData("http://sharepointSite/_layouts/15/Resources/Stamps/image.png"); } 

If I try to use https in browser like:

https://sharepointSite/_layouts/15/Resources/Stamps/image.png 

After entering the credential I can download it:

but the problem is where I try to use https in my code:

 using (var wb = new WebClient()) {     wb.UseDefaultCredentials = true;     var data = wb.DownloadData("https://sharepointSite/_layouts/15/Resources/Stamps/image.png"); } 

and I’ve got error:

The remote server returned an error: (401) Unauthorized.