refactor (MD5): Remove custom MD5, use inbuilt framework version.

Since conditionals were removed for silverlight, can leverage framework MD5 now.

Author: jafin

PdfSharpCore.Test/Security/PdfSecurity.cs

@@ -0,0 +1,35 @@
+using System.IO;
+using FluentAssertions;
+using PdfSharpCore.Drawing;
+using PdfSharpCore.Pdf;
+using PdfSharpCore.Pdf.IO;
+using PdfSharpCore.Pdf.Security;
+using Xunit;
+
+namespace PdfSharpCore.Test.Security
+{
+    public class PdfSecurity
+    {
+        [Theory]
+        [InlineData(PdfDocumentSecurityLevel.Encrypted40Bit, "hunter1")]
+        [InlineData(PdfDocumentSecurityLevel.Encrypted128Bit, "hunter1")]
+        public void CreateAndReadPasswordProtectedPdf(PdfDocumentSecurityLevel securityLevel, string password)
+        {
+            var document = new PdfDocument();
+            var pageNewRenderer = document.AddPage();
+            var renderer = XGraphics.FromPdfPage(pageNewRenderer);
+            renderer.DrawString("Test Test Test", new XFont("Arial", 12), XBrushes.Black, new XPoint(12, 12));
+            document.SecuritySettings.DocumentSecurityLevel = securityLevel;
+            document.SecuritySettings.UserPassword = password;
+
+            using var ms = new MemoryStream();
+            document.Save(ms);
+            ms.Position = 0;
+
+            var loadDocument = Pdf.IO.PdfReader.Open(ms, PdfDocumentOpenMode.Modify,
+                delegate(PdfPasswordProviderArgs args) { args.Password = password; });
+
+            loadDocument.PageCount.Should().Be(1);
+        }
+    }
+}