refactor(.Extensions): Extensions nach Gruppen aufgeteilt.

- erstellt dekodierende Erweiterungen.
 - Kodierungserweiterungen erstellt.
 - XSS-Erweiterungen in das Extensions-Paket verschoben.
 - EncodeTypes vom Paket Common in das Paket Constants verschoben.

EnvelopeGenerator.Extensions/DecodingExtensions.cs

@@ -0,0 +1,148 @@
+using System.Text;
+using static EnvelopeGenerator.Common.Constants;
+
+namespace EnvelopeGenerator.Extensions
+{
+    public static class DecodingExtensions
+    {
+        /// <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
+        }
+
+        public static bool TryDecode(this string encoded, out string[] decoded)
+        {
+            if (!encoded.IsBase64String())
+            {
+                decoded = Array.Empty<string>();
+                return false;
+            }
+            byte[] bytes = Convert.FromBase64String(encoded);
+            string decodedString = Encoding.UTF8.GetString(bytes);
+            decoded = decodedString.Split(new string[] { "::" }, StringSplitOptions.None);
+            return true;
+        }
+
+        public static EncodeType GetEncodeType(this string[] decoded) => decoded.Length switch
+        {
+            2 => EncodeType.EnvelopeReceiver,
+            3 => long.TryParse(decoded[1], out var _) ? EncodeType.EnvelopeReceiverReadOnly : EncodeType.Undefined,
+            _ => EncodeType.Undefined,
+        };
+
+        public static (string? EnvelopeUuid, string? ReceiverSignature) ToEnvelopeReceiverId(this string[] decoded)
+            => decoded.GetEncodeType() == EncodeType.EnvelopeReceiver
+            ? (EnvelopeUuid: decoded[0], ReceiverSignature: decoded[1])
+            : throw new InvalidOperationException("Attempted to convert a decoded other than type EnvelopeReceiver to EnvelopeReceiver. ");
+
+        public static long ToReadOnlyId(this string[] decoded)
+            => decoded.GetEncodeType() == EncodeType.EnvelopeReceiverReadOnly
+            ? long.Parse(decoded[1])
+            : throw new InvalidOperationException("Attempted to convert a decoded other than type EnvelopeReceiver to EnvelopeReceiver. ");
+
+        /// <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)
+        {
+            if (!envelopeReceiverId.IsBase64String())
+            {
+                return (null, null);
+            }
+            byte[] bytes = Convert.FromBase64String(envelopeReceiverId);
+            string decodedString = Encoding.UTF8.GetString(bytes);
+            string[] parts = decodedString.Split(new string[] { "::" }, StringSplitOptions.None);
+
+            if (parts.Length > 1)
+                return (EnvelopeUuid: parts[0], ReceiverSignature: parts[1]);
+            else
+                return (string.Empty, string.Empty);
+        }
+
+        public static long? DecodeEnvelopeReceiverReadOnlyId(this string envelopeReceiverReadOnlyId)
+        {
+            if (!envelopeReceiverReadOnlyId.IsBase64String())
+            {
+                return null;
+            }
+            byte[] bytes = Convert.FromBase64String(envelopeReceiverReadOnlyId);
+            string decodedString = System.Text.Encoding.UTF8.GetString(bytes);
+            string[] parts = decodedString.Split(new string[] { "::" }, StringSplitOptions.None);
+
+            if (parts.Length > 2)
+                return long.TryParse(parts[1], out long readOnlyId) ? readOnlyId : null;
+            else
+                return null;
+        }
+
+        /// <summary>
+        /// Gets the envelope UUID from the decoded envelope receiver ID.
+        /// </summary>
+        /// <param name="envelopeReceiverId">The base64 encoded string to decode.</param>
+        /// <returns>The envelope UUID.</returns>
+        public static string? GetEnvelopeUuid(this string envelopeReceiverId) => envelopeReceiverId.DecodeEnvelopeReceiverId().EnvelopeUuid;
+
+        /// <summary>
+        /// Gets the receiver signature from the decoded envelope receiver ID.
+        /// </summary>
+        /// <param name="envelopeReceiverId">The base64 encoded string to decode.</param>
+        /// <returns>The receiver signature.</returns>
+        public static string? GetReceiverSignature(this string envelopeReceiverId) => envelopeReceiverId.DecodeEnvelopeReceiverId().ReceiverSignature;
+    }
+}