using System;

using System.Text;  

using System.Web;

​

public class Program

{

    public static void Main()

    {

        InitializeDecodingTable();

        // Example: id 2051788 is encoded to _xJfEhMUFRY

        long testDecodedId = 2051788;

        string encoded = EncodeId(testDecodedId);

        string testEncodedId = "_xJfEhMUFRY";

        long decoded = DecodeId(testEncodedId);

​

    }

    public static long DecodeId(string encodedId)

    {

        if (string.IsNullOrEmpty(encodedId))

        {

            throw new Exception("Unable to decode empty id");

        }

​

        encodedId += Version; // Add the trailing Version (which indicate to UrlTokenDecode that the base 64 contains 1 = at the end)

​

        try

        {

            // get bytes

            byte[] keyBytes = HttpServerUtility.UrlTokenDecode(encodedId);

​

            // ReSharper disable once AssignNullToNotNullAttribute

            return ClarifyNumber(BitConverter.ToInt64(keyBytes, 0));

        }

        catch (Exception)

        {

            Console.WriteLine("Unable to decode unique id. Make sure it is using the proper format.");

            throw;

        }

    }

    private static long ClarifyNumber(long val)

    {

        // Get the 64-bit value as bytes

        byte[] bytes = BitConverter.GetBytes(val);

​

        byte xorValue = 0;

​

        // Replace each byte with the corresponding value from the decoding table, after XOR.

        for (int i = 0; i < bytes.Length; ++i)

        {

            byte b = bytes[i];

​

            if (i == 0)

            {

                xorValue = b;

            }

            else

            {

                b ^= xorValue;

            }

​

            b = DecodingTable[b];

            bytes[i] = b;

            ++xorValue;

        }

​

        var decoded = BitConverter.ToInt64(bytes, 0);

        Console.WriteLine("Decoding: " + val + " resulted: " + decoded);

        return decoded;

    }

    private const char Version = '1';

    // Random-generated encoding table. // DO NOT CHANGE!

    private static readonly byte[] EncodingTable = new byte[]

    {

        0x10, 0xA9, 0x6D, 0x39, 0x17, 0x3B, 0x69, 0x1F,

        0xA4, 0xD0, 0xB8, 0xC5, 0x5C, 0x61, 0x75, 0x1A,

        0x21, 0x8D, 0x19, 0x62, 0x2F, 0x5F, 0x53, 0x40,

        0xC4, 0x49, 0x3F, 0xD9, 0xD4, 0x0F, 0x50, 0x5E,

        0x4D, 0xB3, 0xDA, 0x6F, 0xE2, 0xEA, 0x37, 0x71,

        0x0C, 0x6C, 0x4F, 0x4E, 0x79, 0xDE, 0xF3, 0x1B,

        0x20, 0x28, 0x5D, 0x92, 0xEF, 0xCB, 0xE6, 0x97,

        0x23, 0x25, 0x6A, 0x8F, 0xE7, 0x90, 0xF7, 0xA8,

        0xBF, 0x4B, 0x76, 0xAA, 0x9D, 0xEE, 0x6E, 0x3C,

        0x38, 0x34, 0x4A, 0x94, 0x65, 0xD6, 0x12, 0x43,

        0x8C, 0x16, 0x1E, 0x52, 0x99, 0xB7, 0xED, 0x6B,

        0xA2, 0x0D, 0x08, 0x57, 0x07, 0xF0, 0x42, 0xB0,

        0x02, 0x14, 0xCA, 0xBB, 0x00, 0x51, 0x31, 0xDC,

        0x1D, 0xE3, 0x7F, 0x0E, 0x78, 0xA5, 0xAD, 0x3A,

        0x85, 0xC3, 0xA0, 0x68, 0x88, 0xA1, 0x91, 0x87,

        0x45, 0x5B, 0xAE, 0x09, 0xD5, 0x63, 0x7E, 0x4C,

        0x05, 0x13, 0x82, 0xF9, 0xC2, 0xBE, 0xFD, 0x96,

        0x9C, 0x9B, 0x26, 0x98, 0x55, 0x95, 0xB2, 0x2A,

        0xFC, 0x81, 0x7C, 0xBC, 0x59, 0x60, 0xB4, 0xF5,

        0x66, 0x29, 0x7D, 0x35, 0xDF, 0x80, 0xFB, 0xE0,

        0x36, 0xF2, 0x56, 0xEB, 0x7B, 0xC9, 0xBD, 0x9A,

        0xC6, 0x2E, 0x3E, 0xD2, 0xD8, 0x22, 0xAC, 0xB1,

        0x47, 0xAF, 0x8A, 0x73, 0xD1, 0xC1, 0xF4, 0xA7,

        0xF8, 0xC0, 0x64, 0x5A, 0x2D, 0x48, 0x44, 0x06,

        0x8E, 0xE9, 0x27, 0x24, 0xCD, 0xD3, 0x58, 0xFA,

        0xBA, 0xE4, 0x11, 0xEC, 0xFF, 0x0B, 0x3D, 0xA3,

        0x72, 0xB9, 0x93, 0xC8, 0x86, 0x84, 0xCE, 0xD7,

        0xA6, 0x54, 0x8B, 0x77, 0x46, 0x70, 0xB5, 0x32,

        0x18, 0xE8, 0x67, 0xF1, 0x2B, 0x03, 0xF6, 0xFE,

        0x7A, 0x01, 0x74, 0x0A, 0xAB, 0xC7, 0xDB, 0x1C,

        0xCF, 0xCC, 0x30, 0x2C, 0xE1, 0x04, 0x41, 0x83,

        0x9F, 0xB6, 0x15, 0xE5, 0x89, 0x33, 0x9E, 0xDD

    };

​

    public static byte[] DecodingTable;

​

    public static void InitializeDecodingTable()

    {

        // initialize the decoding table

        DecodingTable = new byte[256];

        for (int i = 0; i < 256; ++i)

        {

            byte b = EncodingTable[i];

            DecodingTable[b] = (byte)i;

        }

    }

    public static string EncodeId(long uniqueId)

    {

        // get bytes

        byte[] keyBytes = BitConverter.GetBytes(ObfuscateNumber(uniqueId));

​

        // get urlbase64 value

        string urlEncoded = HttpServerUtility.UrlTokenEncode(keyBytes);

        var encoded = urlEncoded.TrimEnd(Version);// Remove trailing Version

        Console.WriteLine("Enconding: " + uniqueId + " resulted: " + encoded);

        return encoded;

    }

    private static long ObfuscateNumber(long val)

    {

        // Get the 64-bit value as bytes

        byte[] bytes = BitConverter.GetBytes(val);

​

        byte xorValue = 0;

​

        // Replace each byte with the corresponding value from the encoding table, after XOR.

        for (int i = 0; i < bytes.Length; ++i)

        {

            byte b = EncodingTable[bytes[i]];

            if (i == 0)

            {

                xorValue = b;

            }

            else

            {

                b ^= xorValue;

            }

​

            bytes[i] = b;

            ++xorValue;

        }

        return BitConverter.ToInt64(bytes, 0);

    }

}