我想做一个加密文件传输的程序，传输部分已经做好了，但是加密这块一点头绪没有，请各位大侠能否赐教，谢谢啊

using System.Security.Cryptography;

        /// <summary>
        /// 加密文件
        /// </summary>
        /// <param name="file_in">待加密文件</param>
        /// <param name="file_out">输出文件</param>
        /// <param name="password">密码</param>
        private void Encrypt(string file_in, string file_out,string password)
        {
            if (File.Exists(file_in) && file_out != "")
            {
                if (File.Exists(file_out))
                {
                    if (MessageBox.Show("文件已存在，是否覆盖?", "警告", MessageBoxButtons.YesNo, MessageBoxIcon.Warning) != DialogResult.Yes)
                    {
                        return;
                    }
                }
                DESCryptoServiceProvider des = new DESCryptoServiceProvider();
                FileStream fs = File.OpenRead(file_in);
                byte[] inputByteArray = new byte[fs.Length];
                fs.Read(inputByteArray, 0, (int)fs.Length);
                fs.Close();
                byte[] mypasswordbyte = Encoding.Default.GetBytes(password);
                System.Security.Cryptography.MD5CryptoServiceProvider get_md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
                byte[] hash_byte = get_(mypasswordbyte);
                string resule = System.BitConverter.ToString(hash_byte);
                resule = resule.Replace("-", "");
                byte[] keyByteArray = Encoding.Default.GetBytes(resule.Substring(0, 8));
                des.Key = keyByteArray;
                des.IV = keyByteArray;
                MemoryStream ms = new MemoryStream();
                CryptoStream cs = new CryptoStream(ms, des.CreateEncryptor(), CryptoStreamMode.Write);
                cs.Write(inputByteArray, 0, inputByteArray.Length);
                cs.FlushFinalBlock();
                fs = File.OpenWrite(file_out);
                foreach (byte b in ms.ToArray())
                {
                    fs.WriteByte(b);
                }
                fs.Close();
                cs.Close();
                ms.Close();
                if (File.Exists(file_out))
                {
                    MessageBox.Show("加密成功!", "成功", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                else
                {
                    MessageBox.Show("加密失败!", "失败", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
            }
            else
            {
                MessageBox.Show("未能找到文件 " + file_in + "或者输出文件路径错误！", "错误", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        /// <summary>
        /// 解密文件
        /// </summary>
        /// <param name="file_in">待解密文件</param>
        /// <param name="file_out">输出文件</param>
        /// <param name="password">密码</param>
        private void Decrypt(string file_in, string file_out, string password)
        {
            if (File.Exists(file_in) && file_out != "")
            {
                byte[] info;
                FileInfo MyFileInfo = new FileInfo(file_in);
                StreamReader mp3info = new StreamReader(file_in);
                info = new Byte[MyFileInfo.Length];
                info = File.ReadAllBytes(file_in);
                byte[] newjiamifile = new byte[MyFileInfo.Length - 1000];
                if (File.Exists(file_out))
                {
                    if (MessageBox.Show("文件已存在，是否覆盖", "警告", MessageBoxButtons.YesNo, MessageBoxIcon.Question) != DialogResult.Yes)
                    {
                        return;
                    }
                }
                DESCryptoServiceProvider des = new DESCryptoServiceProvider();
                FileStream fs = File.OpenRead(file_in);
                byte[] inputByteArray = new byte[fs.Length];
                fs.Read(inputByteArray, 0, (int)fs.Length);
                fs.Close();
                byte[] mypasswordbyte = Encoding.Default.GetBytes(password);
                System.Security.Cryptography.MD5CryptoServiceProvider get_md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
                byte[] hash_byte = get_(mypasswordbyte);
                string resule = System.BitConverter.ToString(hash_byte);
                resule = resule.Replace("-", "");
                byte[] keyByteArray = Encoding.Default.GetBytes(resule.Substring(0, 8));
                des.Key = keyByteArray;
                des.IV = keyByteArray;
                MemoryStream ms = new MemoryStream();
                CryptoStream cs = new CryptoStream(ms, des.CreateDecryptor(), CryptoStreamMode.Write);
                cs.Write(inputByteArray, 0, inputByteArray.Length);
                cs.FlushFinalBlock();
                fs = File.OpenWrite(file_out);
                foreach (byte b in ms.ToArray())
                {
                    fs.WriteByte(b);
                }
                fs.Close();
                cs.Close();
                ms.Close();
                if (File.Exists(file_out))
                {
                    MessageBox.Show("解密成功!", "成功", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                else
                {
                    MessageBox.Show("解密失败!", "失败", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
            }
            else
            {
                MessageBox.Show("未能找到文件 " + file_in + "或者输出文件错误！", "错误", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

  byte[] newjiamifile = new byte[MyFileInfo.Length - 1000];出现错误。如果将1000删去，那么cs.FlushFinalBlock();会出现“要解密的数据长度无效”的错误、、、求解。。