using System;

using System.Text;

using System.Security.Cryptography;

using Org.BouncyCastle.Crypto;

using Org.BouncyCastle.Crypto.Parameters;

using Org.BouncyCastle.Security;

​

public class Program

{

    public static void Main()

    {

        //string token = "DMT4QPtYcJnCvt5q6idgwIKo4p+F4yKg4K3cnBiuNwFDxExcR9xgH0wcYrx8YP8vez+i5X7mjRn5CSw=";

        //string actualDecrypted = "wpnwRLk0NOx5EqGDOB17W_qfXGRZIGmX6Xes9c7OFf4";

        string token = "vR0BPcq8Q5brz02hMpMl5XqBNXF5ZZR+ZLdw3IpmExJUmIHFBIU84F7E4f2VYjL9RILbru0Utj3wXOA=";

        string actualDecrypted = "cNWw--Til19eQmwvl6efPdOAWSaUKk7fh9eOAVPs12U";

​

        try 

        {

            string serviceToken = DecryptString(Convert.FromBase64String(token), "password1!");

            Console.WriteLine(string.Format("Decrypted cookie = {0}", serviceToken));

        }

        catch (Exception exp)

        {

            Console.WriteLine(string.Format("Error: {0}", exp.Message));

        }

    }

​

    private static string DecryptString(byte[] encrypted, string password)

    {

        byte[] decryptedBytes;

        // The IV is prepended at the start of the string to be decrpyted, so we remove it and store it separately.

        byte[] ivFromEncrypted = Slice(encrypted, 0, 15);

        byte[] encryptedText = Slice(encrypted, 16);

​

        // The "password" actually gets SHA256 hashed for use.

        byte[] passwordBytes = Encoding.Default.GetBytes(password);

        passwordBytes = SHA256.Create().ComputeHash(passwordBytes);

​

        // Initialize AES CTR (counter) mode cipher from the BouncyCastle cryptography library.

        IBufferedCipher cipher = CipherUtilities.GetCipher("AES/CTR/NoPadding"); 

        cipher.Init(false, new ParametersWithIV(new KeyParameter(passwordBytes), ivFromEncrypted));

        decryptedBytes = cipher.DoFinal(encryptedText);

​

        return Encoding.Default.GetString(decryptedBytes);

    }

​

    /**

     * Generate a "slice" of an array. This could be done using ranges, but ranges is not widely supported in .NET.

     */

    private static byte[] Slice(byte[] bytes, int start, int end)

    {

        byte[] split = new byte[end - start+1];

        int counter = start;

        int zeroCounter = 0;

        while (counter <= end)

        {

            split[zeroCounter] = bytes[counter];

            counter++;

            zeroCounter++;

        }

​

        return split;

    }

​

    /**

     * Generate a slice of an array, starting at start and going until the end of the provided array.

     */

    private static byte[] Slice(byte[] bytes, int start)

    {

        return Slice(bytes, start, bytes.Length-1);

    }

}