Please replace the below function :
public string DecryptData(string encryptedtext)
{
encryptedtext = encryptedtext.Replace(" ", "+");
int mod4 = encryptedtext.Length % 4;
if (mod4 > 0 )
{
encryptedtext += new string('=', 4 - mod4);
}
byte[] encryptedBytes = Convert.FromBase64String(encryptedtext);
MemoryStream ms = new MemoryStream();
CryptoStream decStream = new CryptoStream(ms,
tripleDESCryptoServiceProvider.CreateDecryptor(), CryptoStreamMode.Write);
decStream.Write(encryptedBytes, 0, encryptedBytes.Length);
decStream.FlushFinalBlock();
return System.Text.Encoding.Unicode.GetString(ms.ToArray());
}