求改正~ C# DES加密和解密后不一样


public class EncryptString
{
    private static byte[] Key64 = { 42, 16, 93, 156, 78, 4, 218, 32 };
    private static byte[] IV64 = { 55, 103, 246, 79, 36, 99, 167, 3 };
   
  
    public static String Encrypt(String valueString)                                  //DES 加密
    {
        if (valueString != "")
        {   //定义DES的Provider
            DESCryptoServiceProvider desprovider =
            new DESCryptoServiceProvider();
            //定义内存流
            MemoryStream memoryStream = new MemoryStream();
            //定义加密流
            CryptoStream cryptoStream = new CryptoStream(memoryStream,
            desprovider.CreateEncryptor(Key64, IV64),
            CryptoStreamMode.Write);
            //定义写IO流
            StreamWriter writerStream = new StreamWriter(cryptoStream);
            //写入加密后的字符流
            writerStream.Write(valueString);
            writerStream.Flush();
            cryptoStream.FlushFinalBlock();
            memoryStream.Flush();
            //返回加密后的字符串
            return (Convert.ToBase64String(memoryStream.GetBuffer(), 0,
            (int)memoryStream.Length));
        }
        return (null);
    }
    public static String Decrypt(String valueString)                                 //DES解密
    {
        if (valueString != "")
        {   //定义DES的Provider
            DESCryptoServiceProvider desprovider =
            new DESCryptoServiceProvider();
            //转换解密的字符串为二进制
            byte[] buffer = Convert.FromBase64String(valueString);
            //定义内存流
            MemoryStream memoryStream = new MemoryStream();
            //定义加密流
            CryptoStream cryptoStream = new CryptoStream(memoryStream,
            desprovider.CreateDecryptor(Key64,IV64),
            CryptoStreamMode.Read);
            //定义读IO流
            StreamReader readerStream = new StreamReader(cryptoStream);
            //返回解密后的字符串
            return (readerStream.ReadToEnd());
        }
        return (null);
    }
   
   
}