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/Security/ConfigurationSecretProviderTests.cs

@@ -0,0 +1,63 @@
+using System.Text;
+using IdentityShroud.Core.Security;
+using Microsoft.Extensions.Configuration;
+
+namespace IdentityShroud.Core.Tests.Security;
+
+public class ConfigurationSecretProviderTests
+{
+    private static IConfiguration BuildConfigFromJson(string json)
+    {
+        // Convert the JSON string into a stream that the config builder can read.
+        var jsonBytes = Encoding.UTF8.GetBytes(json);
+        using var stream = new MemoryStream(jsonBytes);
+
+        // Build the configuration just like the real app does, but from the stream.
+        var config = new ConfigurationBuilder()
+            .AddJsonStream(stream)   // <-- reads from the in‑memory JSON
+            .Build();
+
+        return config;
+    }
+
+    [Fact]
+    public void Test()
+    {
+        string jsonConfig = """
+            {
+                "secrets": {
+                    "master": [
+                        {
+                            "Id": "5676d159-5495-4945-aa84-59ee694aa8a2",
+                            "Active": true,
+                            "Algorithm": "AES",
+                            "Key": "yoQ4W7EaNjo7s3FBYkWo5BLyX1BnLyWd7BlSaDIrkzo="
+                        },
+                        {
+                            "Id": "b82489e7-a05a-4d64-b9a5-58d2f2c0dc39",
+                            "Active": false,
+                            "Algorithm": "AES",
+                            "Key": "YSWK6vTJXCJOGLpCo+TtZ6anKNzvA1VT2xXLHbmq4M0="
+                        }
+                    ]    
+                }
+            }
+            """;
+        
+
+        ConfigurationSecretProvider sut = new(BuildConfigFromJson(jsonConfig));
+
+        // act
+        var keys = sut.GetKeys("master");
+
+        // verify
+        Assert.Equal(2, keys.Length);
+        var active = keys.Single(k => k.Active);
+        Assert.Equal(new Guid("5676d159-5495-4945-aa84-59ee694aa8a2"), active.Id.Id);
+        Assert.Equal("AES", active.Algorithm);
+        Assert.Equal(Convert.FromBase64String("yoQ4W7EaNjo7s3FBYkWo5BLyX1BnLyWd7BlSaDIrkzo="), active.Key);
+
+        var inactive = keys.Single(k => !k.Active);
+        Assert.Equal(new Guid("b82489e7-a05a-4d64-b9a5-58d2f2c0dc39"), inactive.Id.Id);
+    }
+}