private static readonly String strAesKey = "iwww.maoblog.comi123.maoblog.com";//加密所需32位密匙    
    /// <summary>  
    /// AES加密  
    /// </summary>  
    /// <param name="str">要加密字符串</param>  
    /// <returns>返回加密后字符串</returns>  
    public static String Encrypt_AES(String str)
    {
        Byte[] keyArray = System.Text.UTF8Encoding.UTF8.GetBytes(strAesKey);
        Byte[] toEncryptArray = System.Text.UTF8Encoding.UTF8.GetBytes(str); 
        System.Security.Cryptography.RijndaelManaged rDel = new System.Security.Cryptography.RijndaelManaged();
        rDel.Key = keyArray;
        rDel.Mode = System.Security.Cryptography.CipherMode.ECB;
        rDel.Padding = System.Security.Cryptography.PaddingMode.PKCS7; System.Security.Cryptography.ICryptoTransform cTransform = rDel.CreateEncryptor();
        Byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);
        return Convert.ToBase64String(resultArray, 0, resultArray.Length);
    }

    /// <summary>  
    /// AES解密  
    /// </summary>  
    /// <param name="str">要解密字符串</param>  
    /// <returns>返回解密后字符串</returns>  
    public static String Decrypt_AES(String str)
    {
        Byte[] keyArray = System.Text.UTF8Encoding.UTF8.GetBytes(strAesKey);
        Byte[] toEncryptArray = Convert.FromBase64String(str); System.Security.Cryptography.RijndaelManaged rDel = new System.Security.Cryptography.RijndaelManaged();
        rDel.Key = keyArray;
        rDel.Mode = System.Security.Cryptography.CipherMode.ECB;
        rDel.Padding = System.Security.Cryptography.PaddingMode.PKCS7; System.Security.Cryptography.ICryptoTransform cTransform = rDel.CreateDecryptor();
        Byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);
        return System.Text.UTF8Encoding.UTF8.GetString(resultArray);
    }

byte[] keyArray = null;
using (System.Security.Cryptography.MD5CryptoServiceProvider m = new System.Security.Cryptography.MD5CryptoServiceProvider()) {
    keyArray  = m.ComputeHash(Texts.MyEncoder.GetBytes(strAesKey));
}

------解决方案--------------------

参考：http://www.google.com.hk/search?q=Rfc2898DeriveBytes&hl=zh-CN&newwindow=1&safe=strict&rls=com.microsoft:en-US:IE-Address&prmd=imvns&source=lnt&tbs=lr:lang_1zh-CN%7Clang_1zh-TW&lr=lang_zh-CN%7Clang_zh-TW&sa=X&ei=ryUVUNzLDaWPiAfH9IDoCw&ved=0CFEQpwUoAQ&biw=1440&bih=815