MD5是一種資訊-摘要演算法，一種單向函式演算法（也就是HASH演算法）。將不同輸入長度的資訊進行雜湊計算，得到固定長度的輸出。它的主要特點是，不可逆
和唯一性。即不能由結果計算出輸入值；且不同的輸入值計算得到的固定長度輸出是唯一的。
目前使用的面向物件程式語言中，基本都有類庫實現好的MD5方法可以使用。為了更好地弄清楚內部執行原理，參考了一些實現演算法，如下，使用C#自己實現MD5演算法。上程式碼看看，

 public class MyMD5
    {
        //static state variables
        private static UInt32 A;
        private static UInt32 B;
        private static UInt32 C;
        private static UInt32 D;

        //number of bits to rotate in tranforming
        private const int S11 = 7;
        private const int S12 = 12;
        private const int S13 = 17;
        private const int S14 = 22;
        private const int S21 = 5;
        private const int S22 = 9;
        private const int S23 = 14;
        private const int S24 = 20;
        private const int S31 = 4;
        private const int S32 = 11;
        private const int S33 = 16;
        private const int S34 = 23;
        private const int S41 = 6;
        private const int S42 = 10;
        private const int S43 = 15;
        private const int S44 = 21;

        /* F, G, H and I are basic MD5 functions.
         * 四個非線性函式:
         * 
         * F(X,Y,Z) =(X&Y)|((~X)&Z)
         * G(X,Y,Z) =(X&Z)|(Y&(~Z))
         * H(X,Y,Z) =X^Y^Z
         * I(X,Y,Z)=Y^(X|(~Z))
         * 
         * （&與，|或，~非，^異或）
         */
        private static UInt32 F(UInt32 x, UInt32 y, UInt32 z)
        {
            return (x & y) | ((~x) & z);
        }
        private static UInt32 G(UInt32 x, UInt32 y, UInt32 z)
        {
            return (x & z) | (y & (~z));
        }
        private static UInt32 H(UInt32 x, UInt32 y, UInt32 z)
        {
            return x ^ y ^ z;
        }
        private static UInt32 I(UInt32 x, UInt32 y, UInt32 z)
        {
            return y ^ (x | (~z));
        }

        /* FF, GG, HH, and II transformations for rounds 1, 2, 3, and 4.
         * Rotation is separate from addition to prevent recomputation.
         */
        private static void FF(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti)
        {
            a = a + F(b, c, d) + mj + ti;
            a = a << s | a >> (32 - s);
            a += b;
        }
        private static void GG(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti)
        {
            a = a + G(b, c, d) + mj + ti;
            a = a << s | a >> (32 - s);
            a += b;
        }
        private static void HH(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti)
        {
            a = a + H(b, c, d) + mj + ti;
            a = a << s | a >> (32 - s);
            a += b;
        }
        private static void II(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti)
        {
            a = a + I(b, c, d) + mj + ti;
            a = a << s | a >> (32 - s);
            a += b;
        }

        private static void MD5_Init()
        {
            A = 0x67452301;　//in memory, this is 0x01234567
            B = 0xefcdab89;　//in memory, this is 0x89abcdef
            C = 0x98badcfe;　//in memory, this is 0xfedcba98
            D = 0x10325476;　//in memory, this is 0x76543210
        }

        private static UInt32[] MD5_Append(byte[] input)
        {
            int zeros = 0;
            int ones = 1;
            int size = 0;
            int n = input.Length;
            int m = n % 64;
            if (m < 56)
            {
                zeros = 55 - m;
                size = n - m + 64;
            }
            else if (m == 56)
            {
                zeros = 0;
                ones = 0;
                size = n + 8;
            }
            else
            {
                zeros = 63 - m + 56;
                size = n + 64 - m + 64;
            }

            ArrayList bs = new ArrayList(input);
            if (ones == 1)
            {
                bs.Add((byte)0x80); // 0x80 = $10000000
            }
            for (int i = 0; i < zeros; i++)
            {
                bs.Add((byte)0);
            }

            UInt64 N = (UInt64)n * 8;
            byte h1 = (byte)(N & 0xFF);
            byte h2 = (byte)((N >> 8) & 0xFF);
            byte h3 = (byte)((N >> 16) & 0xFF);
            byte h4 = (byte)((N >> 24) & 0xFF);
            byte h5 = (byte)((N >> 32) & 0xFF);
            byte h6 = (byte)((N >> 40) & 0xFF);
            byte h7 = (byte)((N >> 48) & 0xFF);
            byte h8 = (byte)(N >> 56);
            bs.Add(h1);
            bs.Add(h2);
            bs.Add(h3);
            bs.Add(h4);
            bs.Add(h5);
            bs.Add(h6);
            bs.Add(h7);
            bs.Add(h8);
            byte[] ts = (byte[])bs.ToArray(typeof(byte));

            /* Decodes input (byte[]) into output (UInt32[]). Assumes len is
             * a multiple of 4.
             */
            UInt32[] output = new UInt32[size / 4];
            for (Int64 i = 0, j = 0; i < size; j++, i += 4)
            {
                output[j] = (UInt32)(ts[i] | ts[i + 1] << 8 | ts[i + 2] << 16 | ts[i + 3] << 24);
            }
            return output;
        }
        private static UInt32[] MD5_Trasform(UInt32[] x)
        {

            UInt32 a, b, c, d;

            for (int k = 0; k < x.Length; k += 16)
            {
                a = A;
                b = B;
                c = C;
                d = D;

                /* Round 1 */
                FF(ref a, b, c, d, x[k + 0], S11, 0xd76aa478); /* 1 */
                FF(ref d, a, b, c, x[k + 1], S12, 0xe8c7b756); /* 2 */
                FF(ref c, d, a, b, x[k + 2], S13, 0x242070db); /* 3 */
                FF(ref b, c, d, a, x[k + 3], S14, 0xc1bdceee); /* 4 */
                FF(ref a, b, c, d, x[k + 4], S11, 0xf57c0faf); /* 5 */
                FF(ref d, a, b, c, x[k + 5], S12, 0x4787c62a); /* 6 */
                FF(ref c, d, a, b, x[k + 6], S13, 0xa8304613); /* 7 */
                FF(ref b, c, d, a, x[k + 7], S14, 0xfd469501); /* 8 */
                FF(ref a, b, c, d, x[k + 8], S11, 0x698098d8); /* 9 */
                FF(ref d, a, b, c, x[k + 9], S12, 0x8b44f7af); /* 10 */
                FF(ref c, d, a, b, x[k + 10], S13, 0xffff5bb1); /* 11 */
                FF(ref b, c, d, a, x[k + 11], S14, 0x895cd7be); /* 12 */
                FF(ref a, b, c, d, x[k + 12], S11, 0x6b901122); /* 13 */
                FF(ref d, a, b, c, x[k + 13], S12, 0xfd987193); /* 14 */
                FF(ref c, d, a, b, x[k + 14], S13, 0xa679438e); /* 15 */
                FF(ref b, c, d, a, x[k + 15], S14, 0x49b40821); /* 16 */

                /* Round 2 */
                GG(ref a, b, c, d, x[k + 1], S21, 0xf61e2562); /* 17 */
                GG(ref d, a, b, c, x[k + 6], S22, 0xc040b340); /* 18 */
                GG(ref c, d, a, b, x[k + 11], S23, 0x265e5a51); /* 19 */
                GG(ref b, c, d, a, x[k + 0], S24, 0xe9b6c7aa); /* 20 */
                GG(ref a, b, c, d, x[k + 5], S21, 0xd62f105d); /* 21 */
                GG(ref d, a, b, c, x[k + 10], S22, 0x2441453); /* 22 */
                GG(ref c, d, a, b, x[k + 15], S23, 0xd8a1e681); /* 23 */
                GG(ref b, c, d, a, x[k + 4], S24, 0xe7d3fbc8); /* 24 */
                GG(ref a, b, c, d, x[k + 9], S21, 0x21e1cde6); /* 25 */
                GG(ref d, a, b, c, x[k + 14], S22, 0xc33707d6); /* 26 */
                GG(ref c, d, a, b, x[k + 3], S23, 0xf4d50d87); /* 27 */
                GG(ref b, c, d, a, x[k + 8], S24, 0x455a14ed); /* 28 */
                GG(ref a, b, c, d, x[k + 13], S21, 0xa9e3e905); /* 29 */
                GG(ref d, a, b, c, x[k + 2], S22, 0xfcefa3f8); /* 30 */
                GG(ref c, d, a, b, x[k + 7], S23, 0x676f02d9); /* 31 */
                GG(ref b, c, d, a, x[k + 12], S24, 0x8d2a4c8a); /* 32 */

                /* Round 3 */
                HH(ref a, b, c, d, x[k + 5], S31, 0xfffa3942); /* 33 */
                HH(ref d, a, b, c, x[k + 8], S32, 0x8771f681); /* 34 */
                HH(ref c, d, a, b, x[k + 11], S33, 0x6d9d6122); /* 35 */
                HH(ref b, c, d, a, x[k + 14], S34, 0xfde5380c); /* 36 */
                HH(ref a, b, c, d, x[k + 1], S31, 0xa4beea44); /* 37 */
                HH(ref d, a, b, c, x[k + 4], S32, 0x4bdecfa9); /* 38 */
                HH(ref c, d, a, b, x[k + 7], S33, 0xf6bb4b60); /* 39 */
                HH(ref b, c, d, a, x[k + 10], S34, 0xbebfbc70); /* 40 */
                HH(ref a, b, c, d, x[k + 13], S31, 0x289b7ec6); /* 41 */
                HH(ref d, a, b, c, x[k + 0], S32, 0xeaa127fa); /* 42 */
                HH(ref c, d, a, b, x[k + 3], S33, 0xd4ef3085); /* 43 */
                HH(ref b, c, d, a, x[k + 6], S34, 0x4881d05); /* 44 */
                HH(ref a, b, c, d, x[k + 9], S31, 0xd9d4d039); /* 45 */
                HH(ref d, a, b, c, x[k + 12], S32, 0xe6db99e5); /* 46 */
                HH(ref c, d, a, b, x[k + 15], S33, 0x1fa27cf8); /* 47 */
                HH(ref b, c, d, a, x[k + 2], S34, 0xc4ac5665); /* 48 */

                /* Round 4 */
                II(ref a, b, c, d, x[k + 0], S41, 0xf4292244); /* 49 */
                II(ref d, a, b, c, x[k + 7], S42, 0x432aff97); /* 50 */
                II(ref c, d, a, b, x[k + 14], S43, 0xab9423a7); /* 51 */
                II(ref b, c, d, a, x[k + 5], S44, 0xfc93a039); /* 52 */
                II(ref a, b, c, d, x[k + 12], S41, 0x655b59c3); /* 53 */
                II(ref d, a, b, c, x[k + 3], S42, 0x8f0ccc92); /* 54 */
                II(ref c, d, a, b, x[k + 10], S43, 0xffeff47d); /* 55 */
                II(ref b, c, d, a, x[k + 1], S44, 0x85845dd1); /* 56 */
                II(ref a, b, c, d, x[k + 8], S41, 0x6fa87e4f); /* 57 */
                II(ref d, a, b, c, x[k + 15], S42, 0xfe2ce6e0); /* 58 */
                II(ref c, d, a, b, x[k + 6], S43, 0xa3014314); /* 59 */
                II(ref b, c, d, a, x[k + 13], S44, 0x4e0811a1); /* 60 */
                II(ref a, b, c, d, x[k + 4], S41, 0xf7537e82); /* 61 */
                II(ref d, a, b, c, x[k + 11], S42, 0xbd3af235); /* 62 */
                II(ref c, d, a, b, x[k + 2], S43, 0x2ad7d2bb); /* 63 */
                II(ref b, c, d, a, x[k + 9], S44, 0xeb86d391); /* 64 */

                A += a;
                B += b;
                C += c;
                D += d;
            }
            return new UInt32[] { A, B, C, D };
        }
        public static byte[] MD5Array(byte[] input)
        {
            MD5_Init();
            UInt32[] block = MD5_Append(input);
            UInt32[] bits = MD5_Trasform(block);

            /* Encodes bits (UInt32[]) into output (byte[]). Assumes len is
             * a multiple of 4.
               */
            byte[] output = new byte[bits.Length * 4];
            for (int i = 0, j = 0; i < bits.Length; i++, j += 4)
            {
                output[j] = (byte)(bits[i] & 0xff);
                output[j + 1] = (byte)((bits[i] >> 8) & 0xff);
                output[j + 2] = (byte)((bits[i] >> 16) & 0xff);
                output[j + 3] = (byte)((bits[i] >> 24) & 0xff);
            }
            return output;
        }

        public static string ArrayToHexString(byte[] array, bool uppercase)
        {
            string hexString = "";
            string format = "x2";
            if (uppercase)
            {
                format = "X2";
            }
            foreach (byte b in array)
            {
                hexString += b.ToString(format);
            }
            return hexString;
        }

        public static string MDString(string message)
        {
            char[] c = message.ToCharArray();
            byte[] b = new byte[c.Length];
            for (int i = 0; i < c.Length; i++)
            {
                b[i] = (byte)c[i];
            }
            byte[] digest = MD5Array(b);
            return ArrayToHexString(digest, false);
        }
        public static string MDFile(string fileName)
        {
            FileStream fs = File.Open(fileName, FileMode.Open, FileAccess.Read);
            byte[] array = new byte[fs.Length];
            fs.Read(array, 0, (int)fs.Length);
            byte[] digest = MD5Array(array);
            fs.Close();
            return ArrayToHexString(digest, false);
        }

        public static string Test(string message)
        {
            return MyMD5.MDString(message);
        }
       
    }
 

呼叫過程如下，

  static void Main(string[] args)
        {
            string name = "yys";

            string myresult = MyMD5.Test(name);

            string result = MD5Hash(name);

            Console.WriteLine(myresult);

            Console.WriteLine(result);

            Console.ReadLine();
        }

       

        public static string MD5Hash(string value)
        {
            using (var md5 = MD5.Create())
            {
                string ss = GetMd5Hash(md5, value);
                var result = md5.ComputeHash(Encoding.UTF8.GetBytes(value));
                var strResult = BitConverter.ToString(result);
                return strResult.Replace("-", "");
            }
        }

        public static string GetMd5Hash(MD5 md5Hash, string input)
        {
            byte[] data = md5Hash.ComputeHash(Encoding.UTF8.GetBytes(input));

            StringBuilder sBuilder = new StringBuilder();


            for (int i = 0; i < data.Length; i++)
            {
                sBuilder.Append(data[i].ToString("x2"));
            }

            return sBuilder.ToString();
        }
 

結果如下，

為了比較，使用了C#預設的MD5方法和自己實現的MD5進行比較，其結果除大小寫外是一致的。
程式碼下載：https://download.csdn.net/download/yysyangyangyangshan/10718208