C#上传到ftp文件加密问题

上传功能：private void Upload(string filename) //上传功能
{
FileInfo fileInf = new FileInfo(filename);
string uri = "ftp://" + ftpServerIP + "/" + fileInf.Name; //uri登录方式
FtpWebRequest reqFTP;
reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri("ftp://" + ftpServerIP + "/" + fileInf.Name));

reqFTP.Credentials = new NetworkCredential(ftpUserID, ftpPassword);
reqFTP.KeepAlive = false;
reqFTP.Method = WebRequestMethods.Ftp.UploadFile;
reqFTP.UseBinary = true;
reqFTP.ContentLength = fileInf.Length;
// 缓冲区大小设置成2kb
int buffLength = 2048;
byte[] buff = new byte[buffLength];
int contentLen;
//打开一个文件流来读入上传的文件
FileStream fs = fileInf.OpenRead();

        try
        {

            // 把要上传的文件写入流


            Stream strm=reqFTP.GetRequestStream();



            //从文件流中读取数据，一次读2kb大小的数据 
            contentLen = fs.Read(buff, 0, buffLength);

            // Till Stream content ends
            while (contentLen != 0)
            {
                //把文件的内容从文件流写到FTP上传流中 
                strm.Write(buff, 0, contentLen);
                contentLen = fs.Read(buff, 0, buffLength);
            }

            //关闭文件流和请求流 
            strm.Close();
            fs.Close();

        }
        catch(Exception ex)
        {
            MessageBox.Show(ex.Message, "上传出错");
        }
    }

加密功能：
public static String Encrypt(String Key, String str)
{
byte[] bKey = Encoding.UTF8.GetBytes(Key.Substring(0, 8));
byte[] bIV = IV;
byte[] bStr = Encoding.UTF8.GetBytes(str);
try
{
DESCryptoServiceProvider desc = new DESCryptoServiceProvider();
MemoryStream mStream = new MemoryStream();
CryptoStream cStream = new CryptoStream(mStream, desc.CreateEncryptor(bKey, bIV), CryptoStreamMode.Write);
cStream.Write(bStr, 0, bStr.Length);
cStream.FlushFinalBlock();
return Convert.ToBase64String(mStream.ToArray());
}
catch
{
return string.Empty;
}
}
我调用加密函数显示cannot 'string' to 'System.IO.Stream'，求大神指点