cryptographicexception was unhandled length of the data to decrypt is invalid

I have an app to Encrypt and Decrypt a string. I am new using the functions and I don't really know why I get a cryptographicexception telling me that length of the data to decrypt is invalid with some strings but the app works great with other stirngs. Attached is the code that I use for the Encrypt, Decrypt and I call the funtion on Windows with the following code:

private void button2_Click(object sender, EventArgs e)
        {
            Crypto cr = new Crypto(Crypto.SymmProvEnum.DES);
            txt2.Text = cr.Encrypting(txt1.Text.Trim());
        }

        private void button3_Click(object sender, EventArgs e)
        {
            Crypto cr = new Crypto(Crypto.SymmProvEnum.DES);
            txt3.Text = cr.Decrypting(txt2.Text.Trim());

        }
Everything works fine until I put gonzo as the input string the I get the exception. Like I said I am new using System.Cryptography and don't know where to start. Any help will be appreciated.

namespace WindowsFormsApplication1
{
    public class Crypto
    {
        string Key = "12345678";

        public Crypto()
        {
            //
            // TODO: Add constructor logic here
            //
        }



        /// <remarks>
        /// Supported .Net intrinsic SymmetricAlgorithm classes.
        /// </remarks>
        public enum SymmProvEnum : int
        {
            DES, RC2, RC4, Rijndael
        }

        private SymmetricAlgorithm mobjCryptoService;

        /// <remarks>
        /// Constructor for using an intrinsic .Net SymmetricAlgorithm class.
        /// </remarks>
        public Crypto(SymmProvEnum NetSelected)
        {
            switch (NetSelected)
            {
                case SymmProvEnum.DES:
                    mobjCryptoService = new DESCryptoServiceProvider();
                    break;
                case SymmProvEnum.RC2:
                    mobjCryptoService = new RC2CryptoServiceProvider();
                    break;
                case SymmProvEnum.Rijndael:
                    mobjCryptoService = new RijndaelManaged();
                    break;
                case SymmProvEnum.RC4:
                    mobjCryptoService = new RijndaelManaged();
                    break;
            }
        }

        /// <remarks>
        /// Constructor for using a customized SymmetricAlgorithm class.
        /// </remarks>
        public Crypto(SymmetricAlgorithm ServiceProvider)
        {
            mobjCryptoService = ServiceProvider;
        }

        /// <remarks>
        /// Depending on the legal key size limitations of a specific CryptoService provider
        /// and length of the private key provided, padding the secret key with space character
        /// to meet the legal size of the algorithm.
        /// </remarks>
        private byte[] GetLegalKey(string Key)
        {
            string sTemp = Key;
            if (mobjCryptoService.LegalKeySizes.Length > 0)
            {
                int moreSize = mobjCryptoService.LegalKeySizes[0].MinSize;
                // key sizes are in bits
                if (sTemp.Length * 8 > mobjCryptoService.LegalKeySizes[0].MaxSize)
                    // get the left of the key up to the max size allowed
                    sTemp = sTemp.Substring(0, mobjCryptoService.LegalKeySizes[0].MaxSize / 8);
                else if (sTemp.Length * 8 < moreSize)
                    if (mobjCryptoService.LegalKeySizes[0].SkipSize == 0)
                        // simply pad the key with spaces up to the min size allowed
                        sTemp = sTemp.PadRight(moreSize / 8, ' ');
                    else
                    {
                        while (sTemp.Length * 8 > moreSize)
                            moreSize += mobjCryptoService.LegalKeySizes[0].SkipSize;

                        sTemp = sTemp.PadRight(moreSize / 8, ' ');
                    }
            }

            // convert the secret key to byte array
            return ASCIIEncoding.ASCII.GetBytes(sTemp);
        }

        public string Encrypting(string Source)
        {
            byte[] bytIn = System.Text.ASCIIEncoding.ASCII.GetBytes(System.Web.HttpUtility.UrlEncode(Source));

            // create a MemoryStream so that the process can be done without I/O files
            System.IO.MemoryStream ms = new System.IO.MemoryStream();

            byte[] bytKey = GetLegalKey(Key);

            // set the private key
            mobjCryptoService.Key = bytKey;
            mobjCryptoService.IV = bytKey;

            // create an Encryptor from the Provider Service instance
            ICryptoTransform encrypto = mobjCryptoService.CreateEncryptor();

            // create Crypto Stream that transforms a stream using the encryption
            CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Write);

            // write out encrypted content into MemoryStream
            cs.Write(bytIn, 0, bytIn.Length);
            cs.FlushFinalBlock();

            // get the output and trim the '\0' bytes
            byte[] bytOut = ms.GetBuffer();
            int i = 0;
            for (i = 0; i < bytOut.Length; i++)
                if (bytOut[i] == 0)
                    break;

            // convert into Base64 so that the result can be used in xml
            return System.Convert.ToBase64String(bytOut, 0, i);
        }

        public string Decrypting(string Source)
        {
            // convert from Base64 to binary
            byte[] bytIn = System.Convert.FromBase64String(Source);
            // create a MemoryStream with the input
            System.IO.MemoryStream ms = new System.IO.MemoryStream(bytIn, 0, bytIn.Length);

            byte[] bytKey = GetLegalKey(Key);

            // set the private key
            mobjCryptoService.Key = bytKey;
            mobjCryptoService.IV = bytKey;

            // create a Decryptor from the Provider Service instance
            ICryptoTransform encrypto = mobjCryptoService.CreateDecryptor();

            // create Crypto Stream that transforms a stream using the decryption
            CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Read);

            // read out the result from the Crypto Stream
            System.IO.StreamReader sr = new System.IO.StreamReader(cs);
            string sEncoded = sr.ReadToEnd();
            return System.Web.HttpUtility.UrlDecode(sEncoded);
        }


    }
}

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