需求是媒体文件，保留头部，加密文件的BODY，所以要定位文件头，并且要有由buffer进行大文件的处理。

[C#]代码

using System.Text;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.IO;
using System.Security.Cryptography;

public class AESEncryption
    {

   static string strKey="dongbinhuiasxiny";//密钥,128位，也可以改为192位（24字节）或256位（32字节）。
        static void Main(string[] args)
        {
            RijndaelManaged rij = new RijndaelManaged();
            rij.KeySize = 128;//指定密钥长度

            string fp = @"...";//待加密文件
            string sPhysicalFilePath = @"...";//加密后的文件
            string fw = @"...";//解密后的文件
            Console.WriteLine("Encrypting begin...");
            encryption(rij, fp, sPhysicalFilePath);
            decryption(rij,sPhysicalFilePath,fw);

        }
        //用于加密的函数
        public static void encryption(RijndaelManaged rij,string readfile, string writefile)
        {
            try
            {
                    //byte[] key = rij.Key;
      byte[] key=Encoding.UTF8.GetBytes (strKey);
                    byte[] iv = rij.IV;
                    byte[] buffer = new byte[4096];
                    Rijndael crypt = Rijndael.Create();
                    ICryptoTransform transform = crypt.CreateEncryptor(key, iv);
                    //写进文件
                    FileStream fswrite = new FileStream(writefile, FileMode.Create);
                    CryptoStream cs = new CryptoStream(fswrite, transform, CryptoStreamMode.Write);
                    //打开文件
                    FileStream fsread = new FileStream(readfile, FileMode.Open);

      /*------------------定位要加密的部分-----------------*/
      long _file_size=fsread.Length;
      byte[] _header = new byte[8];
      //定位GUID
      fsread.Seek(16, SeekOrigin.Begin);
      //读取header size
      fsread.Read(_header, 0, _header.Length);
      //头部长度
      long _header_size = (long)BitConverter.ToInt32(_header, 0);
      byte[] _header_buffer=new byte[_header_size];
      fsread.Seek(0,SeekOrigin.Begin);
      fsread.Read(_header_buffer,0,_header_buffer.Length);
      //头部写入新文件
      fswrite.Write(_header_buffer,0,_header_buffer.Length);
      //定位到头部，准备读取需要加密的部分
      fsread.Seek(_header_size,SeekOrigin.Begin);
      /*-----------------定位加密部分完成-------------------*/
                    int length;
                    //while ((length = fsread.ReadByte()) != -1)
                        //cs.WriteByte((byte)length);
                    while ((length = fsread.Read(buffer, 0, 4096)) > 0){
                        cs.Write(buffer, 0, (int)length);
      }

                    fsread.Close();
                    cs.Close();
                    fswrite.Close();
                    Console.WriteLine("Encrypt Success");
            }
            catch (Exception e)
            {
                Console.WriteLine("Encrypt Faile"+e.ToString());
            }
        }
        //用于解密的函数
        public static void decryption(RijndaelManaged rij, string readfile, string writefile)
        {
            try
            {
                //byte[] key = rij.Key;
     byte[] key=Encoding.UTF8.GetBytes (strKey);
                byte[] iv = rij.IV;
                byte[] buffer=new byte[4096];
                    Rijndael crypt = Rijndael.Create();
                    ICryptoTransform transform = crypt.CreateDecryptor(key, iv);
                    //读取加密后的文件 
                    FileStream fsopen = new FileStream(readfile, FileMode.Open);
                    CryptoStream cs = new CryptoStream(fsopen, transform, CryptoStreamMode.Read);
                    //把解密后的结果写进文件
                    FileStream fswrite = new FileStream(writefile, FileMode.OpenOrCreate);
      /*------------------定位要解密的部分-----------------*/
      long _file_size=fsopen.Length;
      byte[] _header = new byte[8];
      //定位GUID
      fsopen.Seek(16, SeekOrigin.Begin);
      //读取header size
      fsopen.Read(_header, 0, _header.Length);
      //头部长度
      long _header_size = (long)BitConverter.ToInt32(_header, 0);
      byte[] _header_buffer=new byte[_header_size];
      fsopen.Seek(0,SeekOrigin.Begin);
      fsopen.Read(_header_buffer,0,_header_buffer.Length);
      //头部写入新文件
      fswrite.Write(_header_buffer,0,_header_buffer.Length);
      //定位到头部，准备读取需要加密的部分
      fsopen.Seek(_header_size,SeekOrigin.Begin);
      /*-----------------定位要解密的部分完成-------------------*/

                    int length;
                    //while ((length = cs.ReadByte()) != -1)
                        //fswrite.WriteByte((byte)length);
                    while ((length = cs.Read(buffer, 0, 4096)) > 0){
                        fswrite.Write(buffer, 0, (int)length);
      }
                    fswrite.Close();
                    cs.Close();
                    fsopen.Close();
                    Console.WriteLine("Decrypt Success");
            }
            catch (Exception e)
            {
                Console.WriteLine("Decrypt Failed"+e.ToString());
            }
        }
    }