Posting articles to apple news api

Question

I am trying to post articles to Apple News API but I am getting the following error:

Unauthorized Error (Wrong signature).

Apple instructs to do the following:

Create a canonical version of the request as a byte-wise concatenation of the following:

The HTTP method (for example, GET or POST, in all caps)

The full URL of the request

The current date in ISO 8601 format If the request is a POST request and it includes an entity, include the following:

The value of the Content-Type header

The full content of the entity

Decode the API key’s secret from Base64 to raw bytes. Create the hash using HMAC SHA-256 over the canonical request with the decoded API key secret. Encode the hash with Base64. Set the Authorization header as:

Authorization: HHMAC; key=; signature=; date= where is the date string from step 1. Send the request.

Below is my C# code:

string path = "https://news-api.apple.com/channels/channelid/articles";

var httpWebRequest = (HttpWebRequest)WebRequest.Create(path);
httpWebRequest.ContentType = "multipart/form-data";
httpWebRequest.Method = "POST";
httpWebRequest.Accept = "application / json";
httpWebRequest.Host = "news-api.apple.com";
httpWebRequest.UseDefaultCredentials = true;
httpWebRequest.PreAuthenticate = true;

httpWebRequest.ProtocolVersion = HttpVersion.Version11;
httpWebRequest.KeepAlive = true;
string appleDate= String.Format("{0}Z", DateTime.UtcNow.ToString("s"));
string credentials = String.Format("{0}:{1}", "Content-Disposition", "form-data; ");
credentials += String.Format("{0}:{1}", "filename", "article.json; ");
credentials += String.Format("{0}:{1}", "name", "article.json; ");

credentials += String.Format("{0}:{1}", "Authorization", "HHMAC; ");
credentials += String.Format("{0}:{1}", "key", "api key; ");

string decodedSecret = base64Decode("secret_key");
string canonical_request = "POST" + path + appleDate;
string hash = Class1.HmacSha256Digest(canonical_request, decodedSecret);
string Encodedhash = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(hash));

credentials += String.Format("{0}:{1}", "signature", Encodedhash + "; ");
credentials += String.Format("{0}:{1}", "date", appleDate + "; ");    

httpWebRequest.Headers.Add("Authorization", "Basic " +credentials);

httpWebRequest.Credentials = new NetworkCredential("xxx.com", "xxxx");

using (StreamReader r = new StreamReader(HttpContext.Current.Server.MapPath("~/article.json")))
{
    string json = r.ReadToEnd();
    dynamic jsonObj = JsonConvert.DeserializeObject(json);

    ASCIIEncoding encoding = new ASCIIEncoding();
    Byte[] bytes = encoding.GetBytes(json);
    Stream newStream = httpWebRequest.GetRequestStream();
    newStream.Write(bytes, 0, bytes.Length);
    newStream.Close();
}

var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();

using (var streamReader = newStreamReader(httpResponse.GetResponseStream()))
{
    var result = streamReader.ReadToEnd();
}

Decode method:

public static string base64Decode(string data)
{
    var base64EncodedBytes = System.Convert.FromBase64String(data);
    return System.Text.Encoding.UTF8.GetString(base64EncodedBytes);
}

HmacSha256 extension method:

public static class Class1
{
    public static string HmacSha256Digest(this string message, string secret)
    {
        ASCIIEncoding encoding = new ASCIIEncoding();
        byte[] keyBytes = encoding.GetBytes(secret);
        byte[] messageBytes = encoding.GetBytes(message);
        System.Security.Cryptography.HMACSHA256 cryptographer = new System.Security.Cryptography.HMACSHA256(keyBytes);

        byte[] bytes = cryptographer.ComputeHash(messageBytes);

        return BitConverter.ToString(bytes).Replace("-", "").ToLower();
    }
}

I've got an

Unauthorized Error (Wrong signature).