using System;

using System.IO;

using System.IO.Compression;

using System.Text;

public class Program

{

    public static void Main()

    {

        var mystr = @"i45WKkktLlHSUXLOzynNzTNUitWBC5Ul5pSmggXiNbRilGKUgGI6MTF5CjFAoKAUGwsA";

        byte[] data = Convert.FromBase64String(mystr);

        Console.WriteLine(Unzip(data));

    }

    public static string Unzip(byte[] bytes) {

        using (var msi = new MemoryStream(bytes))

        using (var mso = new MemoryStream()) {

            using (var decompressor = new DeflateStream(msi, CompressionMode.Decompress)) {

                CopyTo(decompressor, mso);

            }

​

            return Encoding.UTF8.GetString(mso.ToArray());

        }

    }

    public static void CopyTo(Stream src, Stream dest) {

        byte[] bytes = new byte[4096];

​

        int cnt;

​

        while ((cnt = src.Read(bytes, 0, bytes.Length)) != 0) {

            dest.Write(bytes, 0, cnt);

        }

    }

}