转：MD5辅助类

    public class MD5Helper
    {
        private static MD5 md5 = new MD5CryptoServiceProvider();

        private static string MD5ByteToString(byte[] md5Hash)
        {
　　　　　　　md5.Clear();
            StringBuilder result = new StringBuilder();
            for (int i = 0; i < md5Hash.Length; i++)
            {
                result.Append(md5Hash[i].ToString("X2"));
            }
            return result.ToString();
        }

        /// <summary>
        /// 取文件的MD5
        /// </summary>
        /// <param name="fileName"></param>
        /// <returns></returns>
        public static string GetFileMD5(string fileName)
        {
            Stream stream = File.OpenRead(fileName);
            return GetStreamMD5(stream);
        }

        /// <summary>
        /// 取流的MD5
        /// </summary>
        /// <param name="stream"></param>
        /// <returns></returns>
        public static string GetStreamMD5(System.IO.Stream stream)
        {
            byte[] md5Hash = md5.ComputeHash(stream);
            stream.Close();
            return MD5ByteToString(md5Hash);
        }

        /// <summary>
        /// 取字符串的MD5
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static string GetStringMD5(string str)
        {
            byte[] source = System.Text.Encoding.Default.GetBytes(str);
            byte[] md5Hash = md5.ComputeHash(source);
　　　　　　　// return BitConverter.ToString(md5Hash);
            return MD5ByteToString(md5Hash);
        }

        /// <summary>
        /// 取byte的MD5
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static string GetBytesMD5(byte[] bytes)
        {
            byte[] md5Hash = md5.ComputeHash(bytes);
            return MD5ByteToString(md5Hash);
        }


        /// <summary>
        /// 对给定文件路径的文件加上标签
        /// </summary>
        /// <param name="path">要加密的文件的路径</param>
        /// <returns>标签的值</returns>
        public static bool AddMD5(string path)
        {
            bool flag = true;
            if (MD5Helper.CheckMD5(path))
            {
                flag = false;
            }
            try
            {
                FileStream fileStream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read);
                byte[] array = new byte[fileStream.Length];
                fileStream.Read(array, 0, (int)fileStream.Length);
                fileStream.Close();
                if (flag)
                {
                    string s = MD5Helper.MD5Buffer(array, 0, array.Length);
                    byte[] bytes = Encoding.ASCII.GetBytes(s);
                    FileStream fileStream2 = new FileStream(path, FileMode.Open, FileAccess.ReadWrite);
                    fileStream2.Write(array, 0, array.Length);
                    fileStream2.Write(bytes, 0, bytes.Length);
                    fileStream2.Close();
                }
                else
                {
                    FileStream fileStream3 = new FileStream(path, FileMode.Open, FileAccess.ReadWrite);
                    fileStream3.Write(array, 0, array.Length);
                    fileStream3.Close();
                }
            }
            catch
            {
                return false;
            }
            return true;
        }

        /// <summary>
        /// 添加MD5的前缀，便于检查有无篡改
        /// </summary>
        /// <param name="input">输入字符串</param>
        /// <returns></returns>
        public static string AddMD5Profix(string input)
        {
            return MD5Helper.GetMD54(input) + input;
        }

        /// <summary>
        /// 对给定路径的文件进行验证
        /// </summary>
        /// <param name="path">要加密的文件的路径</param>
        /// <returns>是否加了标签或是否标签值与内容值一致</returns>
        public static bool CheckMD5(string path)
        {
            bool result;
            try
            {
                FileStream fileStream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read);
                byte[] array = new byte[fileStream.Length];
                fileStream.Read(array, 0, (int)fileStream.Length);
                fileStream.Close();
                string a = MD5Helper.MD5Buffer(array, 0, array.Length - 32);
                string @string = Encoding.ASCII.GetString(array, array.Length - 32, 32);
                result = (a == @string);
            }
            catch
            {
                result = false;
            }
            return result;
        }

        /// <summary>
        /// 获得16位的MD5加密
        /// </summary>
        /// <param name="input">输入字符串</param>
        /// <returns></returns>
        public static string GetMD516(string input)
        {
            return MD5Helper.GetMD532(input).Substring(8, 16);
        }

        /// <summary>
        /// 获得32位的MD5加密
        /// </summary>
        /// <param name="input">输入字符串</param>
        /// <returns></returns>
        public static string GetMD532(string input)
        {
            MD5 mD = MD5.Create();
            byte[] array = mD.ComputeHash(Encoding.Default.GetBytes(input));
            StringBuilder stringBuilder = new StringBuilder();
            for (int i = 0; i < array.Length; i++)
            {
                stringBuilder.Append(array[i].ToString("x2"));
            }
            return stringBuilder.ToString();
        }

        /// <summary>
        /// 获得4位的MD5加密
        /// </summary>
        /// <param name="input">输入字符串</param>
        /// <returns></returns>
        public static string GetMD54(string input)
        {
            return MD5Helper.GetMD532(input).Substring(8, 4);
        }

        /// <summary>
        /// 获得8位的MD5加密
        /// </summary>
        /// <param name="input">输入字符串</param>
        /// <returns></returns>
        public static string GetMD58(string input)
        {
            return MD5Helper.GetMD532(input).Substring(8, 8);
        }

        /// <summary>
        /// 移除MD5的前缀
        /// </summary>
        /// <param name="input">输入字符串</param>
        /// <returns></returns>
        public static string RemoveMD5Profix(string input)
        {
            return input.Substring(4);
        }

        /// <summary>
        /// 验证MD5前缀处理的字符串有无被篡改
        /// </summary>
        /// <param name="input">输入字符串</param>
        /// <returns></returns>
        public static bool ValidateValue(string input)
        {
            bool result = false;
            if (input.Length >= 4)
            {
                string input2 = input.Substring(4);
                if (input.Substring(0, 4) == MD5Helper.GetMD54(input2))
                {
                    result = true;
                }
            }
            return result;
        }

        private static string MD5Buffer(byte[] md5File, int index, int count)
        {
            MD5CryptoServiceProvider mD5CryptoServiceProvider = new MD5CryptoServiceProvider();
            byte[] value = mD5CryptoServiceProvider.ComputeHash(md5File, index, count);
            string text = BitConverter.ToString(value);
            return text.Replace("-", "");
        }
    }