MD5 Algorithm

MD5 is a widely used cryptographic hash function with a 128-bit hash value. Usually passwords are not stored directly in file or database, the MD5 hash code is stored. Then compute hash code of inputted password and compare it to previously stored value. You use the following code to compute MD5 hash code:

    static string GetMd5Hash(string input)
    {
        byte[] data = Encoding.UTF8.GetBytes(input);

        System.Security.Cryptography.MD5 md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
        byte[] hash = md5.ComputeHash(data);

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

If you want to know how the algorithm works, you can look the code below:

    /// <summary>
    /// MD5 Alogrithm
    /// </summary>
    public class MD5
    {
        //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 nonlinear MD5 functions.
         * 
         * 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))
         * 
         *  & and, | or, ~ not, ^ xor
         */
        /// <summary>
        /// Fs the specified x.
        /// </summary>
        /// <param name="x">The x.</param>
        /// <param name="y">The y.</param>
        /// <param name="z">The z.</param>
        /// <returns></returns>
        private static UInt32 F(UInt32 x, UInt32 y, UInt32 z)
        {
            return (x & y) | ((~x) & z);
        }
        /// <summary>
        /// Gs the specified x.
        /// </summary>
        /// <param name="x">The x.</param>
        /// <param name="y">The y.</param>
        /// <param name="z">The z.</param>
        /// <returns></returns>
        private static UInt32 G(UInt32 x, UInt32 y, UInt32 z)
        {
            return (x & z) | (y & (~z));
        }
        /// <summary>
        /// Hs the specified x.
        /// </summary>
        /// <param name="x">The x.</param>
        /// <param name="y">The y.</param>
        /// <param name="z">The z.</param>
        /// <returns></returns>
        private static UInt32 H(UInt32 x, UInt32 y, UInt32 z)
        {
            return x ^ y ^ z;
        }
        /// <summary>
        /// Is the specified x.
        /// </summary>
        /// <param name="x">The x.</param>
        /// <param name="y">The y.</param>
        /// <param name="z">The z.</param>
        /// <returns></returns>
        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 input length 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 };
        }
        /// <summary>
        /// Ms the d5 array.
        /// </summary>
        /// <param name="input">The input.</param>
        /// <returns></returns>
        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;
        }

        /// <summary>
        /// Arrays to hex string.
        /// </summary>
        /// <param name="array">The array.</param>
        /// <param name="uppercase">if set to <c>true</c> [uppercase].</param>
        /// <returns></returns>
        public static string ArrayToHexString(byte[] array, bool uppercase)
        {
            string hexString = "";
            string format = uppercase ? "X2" : "x2";
            foreach (byte b in array)
            {
                hexString += b.ToString(format);
            }
            return hexString;
        }

        /// <summary>
        /// MDs the string.
        /// </summary>
        /// <param name="message">The message.</param>
        /// <returns></returns>
        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);
        }
        /// <summary>
        /// MDs the file.
        /// </summary>
        /// <param name="fileName">Name of the file.</param>
        /// <returns></returns>
        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);
        }

        /// <summary>
        /// Tests the specified message.
        /// </summary>
        /// <param name="message">The message.</param>
        /// <returns></returns>
        public static string Test(string message)
        {
            return "\r\nMD5 (\"" + message + "\") = " + MD5.MDString(message);
        }
        /// <summary>
        /// Tests the suite.
        /// </summary>
        /// <returns></returns>
        public static string TestSuite()
        {
            string s = "";
            s += Test("");
            s += Test("a");
            s += Test("abc");
            s += Test("message digest");
            s += Test("abcdefghijklmnopqrstuvwxyz");
            s += Test("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789");
            s += Test("12345678901234567890123456789012345678901234567890123456789012345678901234567890");
            return s;
        }
    }