ING OpenAPI sandbox authentication

IngSandBoxApiAuth.cs

        /// <summary>
        /// Both example certificates and key pairs have been converted to the (PKCS#12 or PFX) standard.
        /// This simplifies the process of accessing the private keys and integrating them with the HttpClient.
        /// This process can be done using the following openssl command: `openssl pkcs12 -export -out certificate.p12 -inkey privateKey.key -in certificate.crt`
        /// </summary>
        public async Task IngApiGetToken()
        {
            const string clientId = "e77d776b-90af-4684-bebc-521e5b2614dd";
            HttpMethod method = HttpMethod.Post;
            const string hostUrl = "https://api.sandbox.ing.com";
            const string path = "/oauth2/token";
            const string payload = "grant_type=client_credentials";

            string payloadDigest;

            using (var shaProvider = new SHA256CryptoServiceProvider())
            {
                payloadDigest = Convert.ToBase64String(shaProvider.ComputeHash(Encoding.UTF8.GetBytes(payload)));
            }

            var digest = $"SHA-256={payloadDigest}";
            var date = $"{DateTime.UtcNow:ddd, dd MMM yyyy HH:mm:ss} GMT";
            var signingString = $"(request-target): {method.ToString().ToLower()} {path}\ndate: {date}\ndigest: {digest}";


            var cert = new X509Certificate2(@"singing_example.p12", "changeit");
            var signature = Convert.ToBase64String(cert.GetRSAPrivateKey().SignData
            (
                Encoding.UTF8.GetBytes(signingString),
                HashAlgorithmName.SHA256,
                RSASignaturePadding.Pkcs1
            ));

            using (var httpClient = new HttpClient(new HttpClientHandler
            {
                ClientCertificates = {new X509Certificate2(@"tls_example.p12")},
                ClientCertificateOptions = ClientCertificateOption.Manual
            }))
            {
                var request = new HttpRequestMessage
                {
                    Method = method,
                    RequestUri = new Uri($"{hostUrl}{path}"),
                    Headers =
                    {
                        {"Accept", "application/json"},
                        {"Digest", digest},
                        {"Date", date},
                        {
                            "authorization", 
                            $"Signature keyId=\"{clientId}\",algorithm=\"rsa-sha256\",headers=\"(request-target) date digest\",signature=\"{signature}\""
                        }
                    },
                    Content = new StringContent(payload, Encoding.UTF8,  "application/x-www-form-urlencoded")
                };

                var response = await httpClient.SendAsync(request);
                var body = await response.Content.ReadAsStringAsync();
            }
        }