using System;

using System.Text;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        string method = "GET";

        string url = "https://news-api.apple.com/channels/cdb737aa-FFFF-FFFF-FFFF-FFFFFFFFFFFF";

        string dateString = "2018-06-12T18:15:45Z";

        string apiKeySecret = "HgyfMPjFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF=";

​

        var MyResult = GetSignature(method, url, dateString, apiKeySecret);

        var DocumentedResult = "f3cOzwH7HGYPg481noBFwKgVOGAhH3jy7LQ75jVignA=";

        Console.WriteLine(MyResult);

        Console.WriteLine(MyResult == DocumentedResult);

    }

    public static string GetSignature(string method, string url, string dt, string APISecret)

    {

        var hmac = new HMACSHA256(Convert.FromBase64String(APISecret));

        var hashed = hmac.ComputeHash(Encoding.ASCII.GetBytes(method + url + dt));

        return Convert.ToBase64String(hashed);

    }

}