求助：加密问题

我的代码是这样的
/// <summary>
        /// 加密文件
        /// </summary>
        /// <param name="sInputFilename"></param>
        /// <param name="sOutputFilename"></param>
        /// <param name="sKey"></param>
public static void EncryptFile(string sInputFilename,string sOutputFilename, string sKey) 
{
FileStream fsInput = new FileStream(sInputFilename, FileMode.Open, FileAccess.Read); FileStream fsEncrypted = new FileStream(sOutputFilename, FileMode.Create, FileAccess.Write);
DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
ICryptoTransform desencrypt = DES.CreateEncryptor();
CryptoStream cryptostream = new CryptoStream(fsEncrypted, desencrypt, CryptoStreamMode.Write); 
byte[] bytearrayinput = new byte[fsInput.Length];
fsInput.Read(bytearrayinput, 0, bytearrayinput.Length);
cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length);
cryptostream.Close();
fsInput.Close();
fsEncrypted.Close();
}
/// <summary>
        /// 把一个加密文件解密成另一个文件
        /// </summary>
        /// <param name="sInputFilename"></param>
        /// <param name="sOutputFilename"></param>
        /// <param name="sKey"></param>
public static void DecryptFile(string sInputFilename, string sOutputFilename,string sKey)
{   
            FileStream fsread = new FileStream(sInputFilename, FileMode.Open, FileAccess.Read);
            StreamWriter fsDecrypted = new StreamWriter(sOutputFilename);
try
{
DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
//A 64 bit key and IV is required for this provider.
//Set secret key For DES algorithm.
DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
//Set initialization vector.
DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey); //Create a file stream to read the encrypted file back.

//Create a DES decryptor from the DES instance.
ICryptoTransform desdecrypt = DES.CreateDecryptor();
//Create crypto stream set to read and do a 
//DES decryption transform on incoming bytes.
CryptoStream cryptostreamDecr = new CryptoStream(fsread, desdecrypt,CryptoStreamMode.Read);
//Print the contents of the decrypted file.
fsDecrypted.Write(new StreamReader(cryptostreamDecr).ReadToEnd());
fsDecrypted.Flush();
cryptostreamDecr.Close();
fsDecrypted.Close();
fsread.Close(); }
catch(Exception e1)
{
                fsread.Close();
fsDecrypted.Close();
}
} 
要加解密的是含有全角，半角的excel文件，或图片文件