方法一：
//须添加对System.Web的引用
using System.Web.Security;
...
/// <summary>
/// SHA1加密字符串
/// </summary>
/// <param name="source">源字符串</param>
/// <returns>加密后的字符串</returns>
public string SHA1(string source)
{
return FormsAuthentication.HashPasswordForStoringInConfigFile(source, "SHA1");
}
/// <summary>
/// MD5加密字符串
/// </summary>
/// <param name="source">源字符串</param>
/// <returns>加密后的字符串</returns>
public string MD5(string source)
{
return FormsAuthentication.HashPasswordForStoringInConfigFile(source, "MD5");;
}
方法二(可逆加密解密)：
using System.Security.Cryptography;
...
public string Encode(string data)
{
byte[] byKey = System.Text.ASCIIEncoding.ASCII.GetBytes(KEY_64);
byte[] byIV = System.Text.ASCIIEncoding.ASCII.GetBytes(IV_64);
    DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
int i = cryptoProvider.KeySize;
    MemoryStream ms = new MemoryStream();
    CryptoStream cst = new CryptoStream(ms, cryptoProvider.CreateEncryptor(byKey, byIV), CryptoStreamMode.Write);
    StreamWriter sw = new StreamWriter(cst);
    sw.Write(data);
    sw.Flush();
    cst.FlushFinalBlock();
    sw.Flush();
return Convert.ToBase64String(ms.GetBuffer(), 0, (int)ms.Length);
}
public string Decode(string data)
{
byte[] byKey = System.Text.ASCIIEncoding.ASCII.GetBytes(KEY_64);
byte[] byIV = System.Text.ASCIIEncoding.ASCII.GetBytes(IV_64);
byte[] byEnc;
try
    {
        byEnc = Convert.FromBase64String(data);
    }
catch
    {
return null;
    }
    DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
    MemoryStream ms = new MemoryStream(byEnc);
    CryptoStream cst = new CryptoStream(ms, cryptoProvider.CreateDecryptor(byKey, byIV), CryptoStreamMode.Read);
    StreamReader sr = new StreamReader(cst);
return sr.ReadToEnd();
}
方法三(MD5不可逆)：
using System.Security.Cryptography;
...
//MD5不可逆加密
//32位加密
public string GetMD5_32(string s, string _input_charset)
{
    MD5 md5 = new MD5CryptoServiceProvider();
byte[] t = md5.ComputeHash(Encoding.GetEncoding(_input_charset).GetBytes(s));
    StringBuilder sb = new StringBuilder(32);
for (int i = 0; i < t.Length; i++)
    {
        sb.Append(t[i].ToString("x").PadLeft(2, '0'));
    }
return sb.ToString();
}
//16位加密
public static string GetMd5_16(string ConvertString)
{
    MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
string t2 = BitConverter.ToString(md5.ComputeHash(UTF8Encoding.Default.GetBytes(ConvertString)), 4, 8);
    t2 = t2.Replace("-", "");
return t2;
}
方法四(对称加密)：
using System.IO;
using System.Security.Cryptography;
...
private SymmetricAlgorithm mobjCryptoService;
private string Key;
/// <summary>   
/// 对称加密类的构造函数   
/// </summary>   
public SymmetricMethod()
{
    mobjCryptoService = new RijndaelManaged();
    Key = "Guz(%&hj7x89H$yuBI0456FtmaT5&fvHUFCy76*h%(HilJ$lhj!y6&(*jkP87jH7";
}
/// <summary>   
/// 获得密钥   
/// </summary>   
/// <returns>密钥</returns>   
private byte[] GetLegalKey()
{
string sTemp = Key;
    mobjCryptoService.GenerateKey();
byte[] bytTemp = mobjCryptoService.Key;
int KeyLength = bytTemp.Length;
if (sTemp.Length > KeyLength)
        sTemp = sTemp.Substring(0, KeyLength);
else if (sTemp.Length < KeyLength)
        sTemp = sTemp.PadRight(KeyLength, ' ');
return ASCIIEncoding.ASCII.GetBytes(sTemp);
}
/// <summary>   
/// 获得初始向量IV   
/// </summary>   
/// <returns>初试向量IV</returns>   
private byte[] GetLegalIV()
{
string sTemp = "E4ghj*Ghg7!rNIfb&95GUY86GfghUb#er57HBh(u%g6HJ($jhWk7&!hg4ui%$hjk";
    mobjCryptoService.GenerateIV();
byte[] bytTemp = mobjCryptoService.IV;
int IVLength = bytTemp.Length;
if (sTemp.Length > IVLength)
        sTemp = sTemp.Substring(0, IVLength);
else if (sTemp.Length < IVLength)
        sTemp = sTemp.PadRight(IVLength, ' ');
return ASCIIEncoding.ASCII.GetBytes(sTemp);
}
/// <summary>   
/// 加密方法   
/// </summary>   
/// <param name="Source">待加密的串</param>   
/// <returns>经过加密的串</returns>   
public string Encrypto(string Source)
{
byte[] bytIn = UTF8Encoding.UTF8.GetBytes(Source);
    MemoryStream ms = new MemoryStream();
    mobjCryptoService.Key = GetLegalKey();
    mobjCryptoService.IV = GetLegalIV();
    ICryptoTransform encrypto = mobjCryptoService.CreateEncryptor();
    CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Write);
    cs.Write(bytIn, 0, bytIn.Length);
    cs.FlushFinalBlock();
    ms.Close();
byte[] bytOut = ms.ToArray();
return Convert.ToBase64String(bytOut);
}
/// <summary>   
/// 解密方法   
/// </summary>   
/// <param name="Source">待解密的串</param>   
/// <returns>经过解密的串</returns>   
public string Decrypto(string Source)
{
byte[] bytIn = Convert.FromBase64String(Source);
    MemoryStream ms = new MemoryStream(bytIn, 0, bytIn.Length);
    mobjCryptoService.Key = GetLegalKey();
    mobjCryptoService.IV = GetLegalIV();
    ICryptoTransform encrypto = mobjCryptoService.CreateDecryptor();
    CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Read);
    StreamReader sr = new StreamReader(cs);
return sr.ReadToEnd();
}
方法五：
using System.IO;
using System.Security.Cryptography;
using System.Text;
...
//默认密钥向量
private static byte[] Keys = { 0x12, 0x34, 0x56, 0x78, 0x90, 0xAB, 0xCD, 0xEF };
/**//**//**//// <summary>
/// DES加密字符串
/// </summary>
/// <param name="encryptString">待加密的字符串</param>
/// <param name="encryptKey">加密密钥,要求为8位</param>
/// <returns>加密成功返回加密后的字符串，失败返回源串</returns>
public static string EncryptDES(string encryptString, string encryptKey)
{
try
    {
byte[] rgbKey = Encoding.UTF8.GetBytes(encryptKey.Substring(0, 8));
byte[] rgbIV = Keys;
byte[] inputByteArray = Encoding.UTF8.GetBytes(encryptString);
        DESCryptoServiceProvider dCSP = new DESCryptoServiceProvider();
        MemoryStream mStream = new MemoryStream();
        CryptoStream cStream = new CryptoStream(mStream, dCSP.CreateEncryptor(rgbKey, rgbIV), CryptoStreamMode.Write);
        cStream.Write(inputByteArray, 0, inputByteArray.Length);
        cStream.FlushFinalBlock();
return Convert.ToBase64String(mStream.ToArray());
    }
catch
    {
return encryptString;
    }
}
/**//**//**//// <summary>
/// DES解密字符串
/// </summary>
/// <param name="decryptString">待解密的字符串</param>
/// <param name="decryptKey">解密密钥,要求为8位,和加密密钥相同</param>
/// <returns>解密成功返回解密后的字符串，失败返源串</returns>
public static string DecryptDES(string decryptString, string decryptKey)
{
try
    {
byte[] rgbKey = Encoding.UTF8.GetBytes(decryptKey);
byte[] rgbIV = Keys;
byte[] inputByteArray = Convert.FromBase64String(decryptString);
        DESCryptoServiceProvider DCSP = new DESCryptoServiceProvider();
        MemoryStream mStream = new MemoryStream();
        CryptoStream cStream = new CryptoStream(mStream, DCSP.CreateDecryptor(rgbKey, rgbIV), CryptoStreamMode.Write);
        cStream.Write(inputByteArray, 0, inputByteArray.Length);
        cStream.FlushFinalBlock();
return Encoding.UTF8.GetString(mStream.ToArray());
    }
catch
    {
return decryptString;
    }
} 
方法六(文件加密)：
using System.IO;
using System.Security.Cryptography;
using System.Text;
...
//加密文件
private static void EncryptData(String inName, String outName, byte[] desKey, byte[] desIV)
{
//Create the file streams to handle the input and output files.
    FileStream fin = new FileStream(inName, FileMode.Open, FileAccess.Read);
    FileStream fout = new FileStream(outName, FileMode.OpenOrCreate, FileAccess.Write);
    fout.SetLength(0);
//Create variables to help with read and write.
    byte[] bin = new byte[100]; //This is intermediate storage for the encryption.
    long rdlen = 0;              //This is the total number of bytes written.
    long totlen = fin.Length;    //This is the total length of the input file.
    int len;                     //This is the number of bytes to be written at a time.
    DES des = new DESCryptoServiceProvider();
    CryptoStream encStream = new CryptoStream(fout, des.CreateEncryptor(desKey, desIV), CryptoStreamMode.Write);
//Read from the input file, then encrypt and write to the output file.
    while (rdlen < totlen)
    {
        len = fin.Read(bin, 0, 100);
        encStream.Write(bin, 0, len);
        rdlen = rdlen + len;
    }
    encStream.Close();
    fout.Close();
    fin.Close();
}
//解密文件
private static void DecryptData(String inName, String outName, byte[] desKey, byte[] desIV)
{
//Create the file streams to handle the input and output files.
    FileStream fin = new FileStream(inName, FileMode.Open, FileAccess.Read);
    FileStream fout = new FileStream(outName, FileMode.OpenOrCreate, FileAccess.Write);
    fout.SetLength(0);
//Create variables to help with read and write.
    byte[] bin = new byte[100]; //This is intermediate storage for the encryption.
    long rdlen = 0;              //This is the total number of bytes written.
    long totlen = fin.Length;    //This is the total length of the input file.
    int len;                     //This is the number of bytes to be written at a time.
    DES des = new DESCryptoServiceProvider();
    CryptoStream encStream = new CryptoStream(fout, des.CreateDecryptor(desKey, desIV), CryptoStreamMode.Write);
//Read from the input file, then encrypt and write to the output file.
    while (rdlen < totlen)
    {
        len = fin.Read(bin, 0, 100);
        encStream.Write(bin, 0, len);
        rdlen = rdlen + len;
    }
    encStream.Close();
    fout.Close();
    fin.Close();
}

// 转于： http://www.cnblogs.com/luluping/archive/2009/05/05/1449984.html