Refaktorisierung: Absicherung von DB-Operationen und Verbesserung der Geschäftslogik

- Implementierung von LINQ-Abfragen innerhalb der Core-Bibliothek zur Minderung von SQL-Injection-Anfälligkeiten für DB-Operationen von Umschlägen und Empfängern.
- Aktualisierung der Geschäftslogik in der Service-Schicht für verbessertes Transaktionshandling.
- Erweiterung der ServiceMessage um eine neue Flag-Funktion zum Verfolgen von Cybersecurity- und Datenintegritätsproblemen.
- Hinzufügen spezifischer Benutzerverhaltensflags zur besseren Erkennung und Behandlung potenzieller Datenverletzungen.

EnvelopeGenerator.Application/Services/EnvelopeGeneratorExtensions.cs

@@ -5,13 +5,76 @@
     /// </summary>
     public static class EnvelopeGeneratorExtensions
     {
+        /// <summary>
+        /// Validates whether a given string is a correctly formatted Base-64 encoded string.
+        /// </summary>
+        /// <remarks>
+        /// This method checks the string for proper Base-64 formatting, which includes validating
+        /// the length of the string (must be divisible by 4). It also checks each character to ensure
+        /// it belongs to the Base-64 character set (A-Z, a-z, 0-9, '+', '/', and '=' for padding).
+        /// The method ensures that padding characters ('=') only appear at the end of the string and
+        /// are in a valid configuration (either one '=' at the end if the string's length % 4 is 3,
+        /// or two '==' if the length % 4 is 2).
+        /// </remarks>
+        /// <param name="input">The Base-64 encoded string to validate.</param>
+        /// <returns>
+        /// <c>true</c> if the string is a valid Base-64 encoded string; otherwise, <c>false</c>.
+        /// </returns>
+        /// <example>
+        /// <code>
+        /// string testString = "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnk=";
+        /// bool isValid = IsValidBase64String(testString);
+        /// Console.WriteLine(isValid); // Output: true
+        /// </code>
+        /// </example>
+        public static bool IsBase64String(this string input)
+        {
+            // Check if the string is null or empty
+            if (string.IsNullOrEmpty(input))
+            {
+                return false;
+            }
+
+            // Replace valid base-64 padding
+            input = input.Trim();
+            int mod4 = input.Length % 4;
+            if (mod4 > 0)
+            {
+                // Base-64 string lengths should be divisible by 4
+                return false;
+            }
+
+            // Check each character to ensure it is valid base-64
+            foreach (char c in input)
+            {
+                if (!char.IsLetterOrDigit(c) && c != '+' && c != '/' && c != '=')
+                {
+                    // Invalid character detected
+                    return false;
+                }
+            }
+
+            // Ensure no invalid padding scenarios exist
+            if (input.EndsWith("==") && (input.Length % 4 == 0) ||
+                input.EndsWith("=") && (input.Length % 4 == 3))
+            {
+                return true;
+            }
+
+            return input.IndexOf('=') == -1; // No padding allowed except at the end
+        }
+
         /// <summary>
         /// Decodes the envelope receiver ID and extracts the envelope UUID and receiver signature.
         /// </summary>
         /// <param name="envelopeReceiverId">The base64 encoded string containing the envelope UUID and receiver signature.</param>
         /// <returns>A tuple containing the envelope UUID and receiver signature.</returns>
-        public static (string EnvelopeUuid, string ReceiverSignature) DecodeEnvelopeReceiverId(this string envelopeReceiverId)
+        public static (string? EnvelopeUuid, string? ReceiverSignature) DecodeEnvelopeReceiverId(this string envelopeReceiverId)
         {
+            if (!envelopeReceiverId.IsBase64String())
+            {
+                return (null, null);
+            }
             byte[] bytes = Convert.FromBase64String(envelopeReceiverId);
             string decodedString = System.Text.Encoding.UTF8.GetString(bytes);
             string[] parts = decodedString.Split(new string[] { "::" }, StringSplitOptions.None);