本文转自：http://blog.csdn.net/veryhappy/archive/2006/01/09/574152.aspx

下面的类实现了文件的加密和解密操作，试验了几种文件类型均没有问题，现在和大家共享一下。

using System;
using System.Collections.Generic;
using System.Text;
using System.Security.Cryptography;
using System.IO;

namespace MyCryptoHelp
{
 /// <summary>
 /// 异常处理类
 /// </summary>
 public class CryptoHelpException : ApplicationException
 {
  public CryptoHelpException(string msg):base(msg){}
 }

 /// <summary>
 /// CryptHelp
 /// </summary>
 public class CryptoHelp
 {
  private const ulong FC_TAG = 0xFC010203040506CF;

  private const int BUFFER_SIZE = 128*1024;
  
  /// <summary>
  /// 检验两个Byte数组是否相同
  /// </summary>
  /// <param name="b1">Byte数组</param>
  /// <param name="b2">Byte数组</param>
  /// <returns>true－相等</returns>
  private static bool CheckByteArrays(byte[] b1, byte[] b2)
  {
   if(b1.Length == b2.Length)
   {
    for(int i = 0; i < b1.Length; ++i)
    {
     if(b1[i] != b2[i])
      return false;
    }
    return true;
   }
   return false;
  }

  /// <summary>
  /// 创建Rijndael SymmetricAlgorithm
  /// </summary>
  /// <param name="password">密码</param>
  /// <param name="salt"></param>
  /// <returns>加密对象</returns>
  private static SymmetricAlgorithm CreateRijndael(string password, byte[] salt)
  {
   PasswordDeriveBytes pdb = new PasswordDeriveBytes(password,salt,"SHA256",1000);
   
   SymmetricAlgorithm sma = Rijndael.Create();
   sma.KeySize = 256;
   sma.Key = pdb.GetBytes(32);
   sma.Padding = PaddingMode.PKCS7;
   return sma;
  }

  /// <summary>
  /// 加密文件随机数生成
  /// </summary>
  private static RandomNumberGenerator rand = new RNGCryptoServiceProvider();

  /// <summary>
  /// 生成指定长度的随机Byte数组
  /// </summary>
  /// <param name="count">Byte数组长度</param>
  /// <returns>随机Byte数组</returns>
  private static byte[] GenerateRandomBytes(int count)
  {
   byte[] bytes = new byte[count];
   rand.GetBytes(bytes);
   return bytes;
  }

  /// <summary>
  /// 加密文件
  /// </summary>
  /// <param name="inFile">待加密文件</param>
  /// <param name="outFile">加密后输入文件</param>
  /// <param name="password">加密密码</param>
  public static void EncryptFile(string inFile, string outFile, string password)
  {
   using(FileStream fin = File.OpenRead(inFile),
       fout = File.OpenWrite(outFile))
   {
    long lSize = fin.Length; // 输入文件长度
    int size = (int)lSize;
    byte[] bytes = new byte[BUFFER_SIZE]; // 缓存
    int read = -1; // 输入文件读取数量
    int value = 0;
    
    // 获取IV和salt
    byte[] IV = GenerateRandomBytes(16);
    byte[] salt = GenerateRandomBytes(16);
    
    // 创建加密对象
    SymmetricAlgorithm sma = CryptoHelp.CreateRijndael(password, salt);
    sma.IV = IV;   
    
    // 在输出文件开始部分写入IV和salt
    fout.Write(IV,0,IV.Length);
    fout.Write(salt,0,salt.Length);
    
    // 创建散列加密
    HashAlgorithm hasher = SHA256.Create();
    using(CryptoStream cout = new CryptoStream(fout,sma.CreateEncryptor(),CryptoStreamMode.Write),
        chash = new CryptoStream(Stream.Null,hasher,CryptoStreamMode.Write))
    {
     BinaryWriter bw = new BinaryWriter(cout);
     bw.Write(lSize);
     
     bw.Write(FC_TAG);

     // 读写字节块到加密流缓冲区
     while( (read = fin.Read(bytes,0,bytes.Length)) != 0 )
     {
      cout.Write(bytes,0,read);
      chash.Write(bytes,0,read); 
      value += read;
     }
     // 关闭加密流
     chash.Flush();
     chash.Close();

     // 读取散列
     byte[] hash = hasher.Hash;
     
     // 输入文件写入散列
     cout.Write(hash,0,hash.Length);

     // 关闭文件流
     cout.Flush();
     cout.Close();
    }
   }
  }

  /// <summary>
  /// 解密文件
  /// </summary>
  /// <param name="inFile">待解密文件</param>
  /// <param name="outFile">解密后输出文件</param>
  /// <param name="password">解密密码</param>
  public static void DecryptFile(string inFile, string outFile, string password)
  {
   // 创建打开文件流
   using(FileStream fin = File.OpenRead(inFile),
       fout = File.OpenWrite(outFile))
   {
    int size = (int)fin.Length;
    byte[] bytes = new byte[BUFFER_SIZE];
    int read = -1;
    int value = 0;
    int outValue = 0;

    byte[] IV = new byte[16];
    fin.Read(IV,0,16);
    byte[] salt = new byte[16];
    fin.Read(salt,0,16);
    
    SymmetricAlgorithm sma = CryptoHelp.CreateRijndael(password,salt);
    sma.IV = IV;

    value = 32;
    long lSize = -1;
    
    // 创建散列对象, 校验文件
    HashAlgorithm hasher = SHA256.Create();

    using(CryptoStream cin = new CryptoStream(fin,sma.CreateDecryptor(),CryptoStreamMode.Read),
        chash = new CryptoStream(Stream.Null,hasher,CryptoStreamMode.Write))
    {
     // 读取文件长度
     BinaryReader br = new BinaryReader(cin);
     lSize = br.ReadInt64();
     ulong tag = br.ReadUInt64();
     
     if(FC_TAG != tag)
      throw new CryptoHelpException("文件被破坏");
     
     long numReads = lSize / BUFFER_SIZE;

     long slack = (long)lSize % BUFFER_SIZE;
     
     for(int i = 0; i < numReads; ++i)
     {
      read = cin.Read(bytes,0,bytes.Length);
      fout.Write(bytes,0,read);
      chash.Write(bytes,0,read);
      value += read;
      outValue += read;
     }

     if(slack > 0)
     {
      read = cin.Read(bytes,0,(int)slack);
      fout.Write(bytes,0,read);
      chash.Write(bytes,0,read);
      value += read;
      outValue += read;
     }

     chash.Flush();
     chash.Close();

     fout.Flush();
     fout.Close();

     byte[] curHash = hasher.Hash;

     // 获取比较和旧的散列对象
     byte[] oldHash = new byte[hasher.HashSize / 8];
     read = cin.Read(oldHash,0,oldHash.Length);
     if((oldHash.Length != read) || (!CheckByteArrays(oldHash,curHash)))
      throw new CryptoHelpException("文件被破坏");
    }
    
    if(outValue != lSize)
     throw new CryptoHelpException("文件大小不匹配");
   }
  }
 }
}

// 调用

public class TestClass
{
    string myPassword = "TEST_PASSWORD_~!@#";
    string myPlainFile = "test.txt";
    string myEncryptedFile = "test.encrypted";
    string myDecryptedFile = "test.decrypted";

    [STAThread]
    static void Main()
    {
        CryptoHelp.EncryptFile(myPlainFile, myEncryptedFile, myPassword);
        CryptoHelp.DecryptFile(myEncryptedFile,myDecryptedFile, myPassword);
    }
}

// 微软的加密解密文本文件的方法

http://support.microsoft.com/kb/307010/zh-cn