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/DTO/JsonWebKey.cs

@@ -0,0 +1,49 @@
+using System.Text.Json.Serialization;
+using IdentityShroud.Core.Helpers;
+
+namespace IdentityShroud.Core.Messages;
+
+// https://www.rfc-editor.org/rfc/rfc7517.html
+
+
+public class JsonWebKey
+{
+    [JsonPropertyName("kty")]
+    public string KeyType { get; set; } = "RSA";
+    
+    // Common values sig(nature) enc(ryption)
+    [JsonPropertyName("use")]
+    public string? Use { get; set; } = "sig"; // "sig" for signature, "enc" for encryption
+    
+    // Per standard this field is optional, commented out for now as it seems not 
+    // have any good use in an identity server. Anyone validating tokens should use
+    // the algorithm specified in the header of the token.
+    // [JsonPropertyName("alg")]
+    // public string? Algorithm { get; set; } = "RS256";
+    
+    [JsonPropertyName("kid")]
+    public required string KeyId { get; set; }
+    
+    // RSA Public Key Components
+    [JsonPropertyName("n")]
+    public string? Modulus { get; set; }
+    
+    [JsonPropertyName("e")]
+    public string? Exponent { get; set; }
+    
+    // ECdsa
+    public string? Curve { get; set; }
+    [JsonConverter(typeof(Base64UrlConverter))]
+    public byte[]? X { get; set; }
+    [JsonConverter(typeof(Base64UrlConverter))]
+    public byte[]? Y { get; set; }
+    
+    // Optional fields
+    // [JsonPropertyName("x5c")]
+    // [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    // public List<string>? X509CertificateChain { get; set; }
+    //
+    // [JsonPropertyName("x5t")]
+    // [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    // public string? X509CertificateThumbprint { get; set; }    
+}