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static class Cryption
    {
        public enum CryptionBitMode { Bit128, Bit192, Bit256 }
        
        static public string Encrypt(string text, CryptionBitMode mode, byte[] key, byte[] iv)
        {
            int size;
            switch (mode)
            {
                case CryptionBitMode.Bit128: size = 128; break;
                case CryptionBitMode.Bit192: size = 192; break;
                case CryptionBitMode.Bit256: size = 256; break;
                default: throw new Exception("Invalid CryptionBitMode.");
            }

            //Implementing a new Rijndael-instance
            Rijndael rijndael = Rijndael.Create();
            rijndael.BlockSize = size;
            rijndael.IV = iv;
            rijndael.Key = key;

            //Creating streams for cryption...
            MemoryStream stream = new MemoryStream();
            CryptoStream crypter = new CryptoStream(stream, rijndael.CreateEncryptor(), CryptoStreamMode.Write );

            //Encrypting string and write it to memory...
            byte[] buffer = Encoding.Unicode.GetBytes(text);
            crypter.Write(buffer, 0, buffer.Length);

            //Read the encrypted string form memory...
            buffer = new byte[stream.Length];
            stream.Seek(0, SeekOrigin.Begin);
            stream.Read(buffer, 0, buffer.Length);

            crypter.FlushFinalBlock();
            crypter.Close();
            stream.Close();

            //Return with the encrypted string...
            return Encoding.Unicode.GetString(buffer);
        }

        static public string Decrypt(string text, CryptionBitMode mode, byte[] key, byte[] iv)
        {
            int size;
            switch (mode)
            {
                case CryptionBitMode.Bit128: size = 128; break;
                case CryptionBitMode.Bit192: size = 192; break;
                case CryptionBitMode.Bit256: size = 256; break;
                default: throw new Exception("Invalid CryptionBitMode.");
            }



            //Implementing a new Rijndael-instance
            Rijndael rijndael = Rijndael.Create();
            rijndael.BlockSize = size;
            rijndael.IV = iv;
            rijndael.Key = key;

            //Creating streams for cryption...
            MemoryStream stream = new MemoryStream();
            CryptoStream crypter = new CryptoStream(stream, rijndael.CreateDecryptor(), CryptoStreamMode.Read);

            //Write the encrypted text to memory
            byte[] buffer = Encoding.Unicode.GetBytes(text);
            stream.Write(buffer, 0, buffer.Length);

            //Read the encrypted stirng and decrypt it...
            buffer = new byte[stream.Length];
            stream.Seek(0, SeekOrigin.Begin);
            crypter.Read(buffer, 0, buffer.Length); //Hier tritt die Exception bei byte 32 auf
            
            crypter.FlushFinalBlock();
            crypter.Close();
            stream.Close();

            //Return with the encrypted string...
            return Encoding.Unicode.GetString(buffer);
        }
    }                    

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            byte[] key = encoder.GetBytes("TestingTheKey123"), IV = new byte[16]; //key erstellen
            for(int i = 0; i<IV.Length; i++) IV[i] = Convert.ToByte(i+5);             //IV erstellen
            string s = Cryption.Encrypt("Hallo Das hier ist Ein Test",Cryption.CryptionBitMode.Bit128, key, IV);
            string x = Cryption.Decrypt(s, Cryption.CryptionBitMode.Bit128, key, IV);