HttpClient Digest authentication VB.Net

I'm writing an .NET application that uses HttpClient to connect to the API and retrieve video.

The Documentation provided for "Cookie-based authentication" details the following to login:

1. Call GET /api/getNonce
2. In response you'll get a JSON object with realm and nonce (nonce is a session key for this user)
3. Calculate authentication hash auth_digest, using realm and nonce (see algorithm below)
4. Call and pass the "auth" parameter in the json request body GET /api/cookieLogin
5. Server will check authentication and set session cookies

Which then expands on "Calculating Authentication Hash" with the following steps:

• Call GET /api/getNonce
• In response you'll get a JSON object with realm and nonce
• Translate user's username to the lower case
• Check the required method ("GET" for HTTP GET requests, "POST" for HTTP POST requests, "PLAY" for RTSP etc)
• digest = md5_hex(user_name + ":" + realm + ":" + password)
• partial_ha2 = md5_hex(method + ":")
• simplified_ha2 = md5_hex(digest + ":" + nonce + ":" + partial_ha2)
• auth_digest = base64(user_name + ":" + nonce + ":" + simplified_ha2)
• Here auth_digest is the required authentication hash

I use this Article written in C# as a reference, converted it to VB.NET, change a little bit of code in it according to the requirement above and then implemented it into my program. But I always get an error like this

"The remote server returned an error: (403) Forbidden."

When I debug the program, the problem appears when reading DigestAuthFixer.vb class in GrabResponse Function on this line response = CType(request2.GetResponse(), HttpWebResponse) . I suspect that "auth_digest" didn't return the correct value so the web service to deny the request with a 403 error. My question is how to properly implement digest authentication using VB.Net ? Are there any standard methods or do I have to do it from scratch? Thanks in advance.

This is the code that I use to connect to the API and retrieve the video:

Download HTTPClient:

Private Shared _host As String = "http://127.0.0.1:7001"
Private Shared _username As String = "user"
Private Shared _password As String = "password"
Dim auto As Boolean = False
Shared ReadOnly client As HttpClient = New HttpClient()

Public Async Function Download_HttpClient(ByVal cameraID As String _
                                          , ByVal videoDT As String _
                                          , ByVal duration As String _
                                          , ByVal resolution As String _
                                          , ByVal path As String) As Task(Of String)

    Dim cookieContainer As New CookieContainer
    Dim httpClientHandler As New HttpClientHandler
    httpClientHandler.AllowAutoRedirect = True
    httpClientHandler.UseCookies = True
    httpClientHandler.CookieContainer = cookieContainer
    Dim httpClient As New HttpClient(httpClientHandler)
    Dim err As Boolean = False
    Dim downloadFileName As String

    'SET HTTPCLIENT HEADERS
    httpClient.DefaultRequestHeaders.Add("User-Agent", "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)")
    'httpClient.DefaultRequestHeaders.Add("Accept", "text/html,application/xhtml+xml,application/xml")
    httpClient.DefaultRequestHeaders.Add("Accept", "text/html,application/xhtml+json,application/json")
    httpClient.DefaultRequestHeaders.Add("Accept-Charset", "ISO-8859-1")

    Dim downloadId As String = Nothing

    Try
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 Or SecurityProtocolType.Tls12 Or SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls
        
        'GET USER RIGHTS
        Dim login_url As String = _host + "/api/getNonce"
        Dim login_parameter As String = "?user_name =" + _username + "&password=" + _password
        Dim login_response As HttpResponseMessage = Await httpClient.GetAsync(login_url + login_parameter)
        Dim login_contents As String = Await login_response.Content.ReadAsStringAsync()

        'CALCULATE AUTHENTICATION HASH
        Dim dir As String = "/dir/index.html"
        Dim url As String = _host + "/api/getNonce"
        Dim digest As NUI.DigestAuthFixer = New NUI.DigestAuthFixer(url, _username.ToLower, _password)
        Dim auth As String = digest.GrabResponse(dir)

        'CALL POST /api/cookieLogin
        Dim client As HttpClient = New HttpClient
        client.DefaultRequestHeaders.Accept.Add(New System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"))
        Dim postData As New List(Of KeyValuePair(Of String, String))
        postData.Add(New KeyValuePair(Of String, String)("auth_digest ", auth))
        Dim content As New FormUrlEncodedContent(postData)
        content.Headers.ContentType = New Headers.MediaTypeHeaderValue("application/x-runtime-guid")
        Dim response As HttpResponseMessage = client.PostAsync(New Uri(_host + "/api/cookieLogin"), content).Result
        If response.IsSuccessStatusCode Then
            MsgBox("POST CookieLogin Successfully")
        Else
            MsgBox(response)
        End If

    Catch ex As Exception
        MessageBox.Show("Invalid response from the server due to connection limitation or firewall blocking your request")
        Return ex.ToString()
    End Try
    
    'CREATE DOWNLOAD URL
    Dim download_url As String = _host + "/media/" + cameraID + ".mkv"
    Dim download_param As String = "?pos=" + videoDT + "&duration=" + duration + "&resolution=" + resolution
    Dim downloadFile As String = download_url + download_param

    'GET REQUEST AND DOWNLOAD
    Dim file_response = Await httpClient.GetAsync(downloadFile)
    Dim file_header = file_response.Content.Headers.GetValues("Content-Disposition")
    Dim temp_string() As String = file_header.ToArray()
    temp_string = temp_string(0).Split("=")
    downloadFileName = temp_string(1).Replace("""", "")

    Try
        Using fs As New FileStream(path & downloadFileName, FileMode.Create)
            Await file_response.Content.CopyToAsync(fs)
        End Using
    Catch ex As Exception
        MessageBox.Show("Directory does not exists, please manually change the folder target")
        Return ex.ToString()
    End Try
    Return "Download successfully"

End Function

DigestAuthFixer.vb class

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Security.Cryptography
Imports System.Text.RegularExpressions
Imports System.Net
Imports System.IO
Imports System.Net.Http

Namespace NUI
Public Class DigestAuthFixer

    Private Shared _host As String
    Private Shared _user As String
    Private Shared _password As String
    Private Shared _realm As String
    Private Shared _nonce As String
    Private Shared _qop As String
    Private Shared _cnonce As String
    Private Shared _cnonceDate As DateTime
    Private Shared _nc As Integer

    Public Sub New(ByVal host As String, ByVal user As String, ByVal password As String)
        _host = host
        _user = user
        _password = password
    End Sub

    Private Function CalculateMd5Hash(ByVal input As String) As String
        Dim inputBytes As Byte() = Encoding.GetEncoding("ISO-8859-1").GetBytes(input)
        Dim hash = MD5.Create().ComputeHash(inputBytes)
        Dim sb As New System.Text.StringBuilder()

        For Each b In hash
            sb.Append(b.ToString("x2"))
        Next b
        Return sb.ToString()
    End Function

    Private Function GrabHeaderVar(ByVal varName As String, ByVal header As String) As String
        Dim regHeader = New Regex(String.Format("{0}=""([^""]*)""", varName))
        Dim matchHeader = regHeader.Match(header)
        If matchHeader.Success Then Return matchHeader.Groups(1).Value
        Throw New ApplicationException(String.Format("Header {0} not found", varName))
    End Function

    Private Function GetDigestHeader(ByVal dir As String) As String
        Dim digest = CalculateMd5Hash(_user.ToLower + ":" + _realm + ":" + _password)
        Dim partial_ha2 = CalculateMd5Hash("GET" + ":")
        Dim simplified_ha2 = CalculateMd5Hash(digest + ":" + _nonce + ":" + partial_ha2)
        Dim auth_digest = (_user.ToLower + ":" + _nonce + ":" + simplified_ha2)
        Dim authKey As Byte() = Encoding.GetEncoding("ISO-8859-1").GetBytes(auth_digest)
        Return Convert.ToBase64String(authKey)
    End Function

    Public Function GrabResponse(ByVal dir As String) As String
        Dim url = _host & dir
        Dim uri = New Uri(url)
        Dim request = CType(WebRequest.Create(uri), HttpWebRequest)

        If Not String.IsNullOrEmpty(_cnonce) AndAlso DateTime.Now.Subtract(_cnonceDate).TotalHours < 1.0 Then
            request.Headers.Add("Authorization", GetDigestHeader(dir))
        End If

        Dim response As HttpWebResponse

        Try
            response = CType(request.GetResponse(), HttpWebResponse)
        Catch ex As WebException

            If ex.Response Is Nothing OrElse (CType(ex.Response, HttpWebResponse)).StatusCode <> HttpStatusCode.Unauthorized Then Throw
            Dim wwwAuthenticateHeader = ex.Response.Headers("WWW-Authenticate")
            _realm = GrabHeaderVar("realm", wwwAuthenticateHeader)
            _nonce = GrabHeaderVar("nonce", wwwAuthenticateHeader)
            _qop = "auth"
            _nc = 0
            _cnonce = New Random().[Next](123400, 9999999).ToString()
            _cnonceDate = DateTime.Now
            Dim request2 = CType(WebRequest.Create(uri), HttpWebRequest)
            request2.Headers.Add("Authorization", GetDigestHeader(dir))
            response = CType(request2.GetResponse(), HttpWebResponse)

        End Try

        Dim reader = New StreamReader(response.GetResponseStream())
        Return reader.ReadToEnd()
    End Function

End Class
End Namespace