5-improve-encrypted-storage (#6)

Added the use of DEK's for encryption of secrets. Both the KEK's and DEK's are stored in a way that you can have multiple key of which one is active. But the others are still available for decrypting. This allows for implementing key rotation.

Co-authored-by: eelke <eelke@eelkeklein.nl>
Co-authored-by: Eelke76 <31384324+Eelke76@users.noreply.github.com>
Reviewed-on: #6

IdentityShroud.Core.Tests/Helpers/Base64UrlConverterTests.cs

@@ -0,0 +1,36 @@
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using IdentityShroud.Core.Helpers;
+
+namespace IdentityShroud.Core.Tests.Helpers;
+
+public class Base64UrlConverterTests
+{
+    internal class Data
+    {
+        [JsonConverter(typeof(Base64UrlConverter))]
+        public byte[]? X { get; set; }
+    }
+
+    [Fact]
+    public void Serialize()
+    {
+        Data d = new() { X = ">>>???"u8.ToArray() };
+        string s = JsonSerializer.Serialize(d);
+
+        Assert.Contains("\"Pj4-Pz8_\"", s);
+    }
+    
+    [Fact]
+    public void Deerialize()
+    {
+        var jsonstring = """
+            { "X": "Pj4-Pz8_" }
+            """;
+        var d = JsonSerializer.Deserialize<Data>(jsonstring);
+
+        Assert.Equal(">>>???", Encoding.UTF8.GetString(d.X)); 
+    }
+    
+}