using System;

public class Program

{

    public static void Main()

    {

        var correlationId = "IbtdvxxH40G8uojzDWnAiQ";

        var shortGuid = correlationId.Replace('-', '/') + "==";

        var decodedGuid = new Guid(Convert.FromBase64String(shortGuid));

        Console.WriteLine(decodedGuid.ToString("N"));

    }

}

​