Network Programming in .NET

Recently I had to integrate with Spotler’s REST API from a .NET application. Spotler provides a powerful marketing automation platform, and their API uses OAuth 1.0 HMAC-SHA1 signatures for authentication.

They provided a working PHP client, but I needed to port this to C#. Here’s what I learned (and how you can avoid some common pitfalls).

🚀 The Goal

We started with a PHP class that:

✅ Initializes with:

• consumerKey
• consumerSecret
• optional SSL certificate verification

✅ Creates properly signed OAuth 1.0 requests

✅ Makes HTTP requests with cURL and parses the JSON responses.

I needed to replicate this in C# so we could use it inside a modern .NET microservice.

🛠 The Port to C#

🔑 The tricky part: OAuth 1.0 signatures

Spotler’s API requires a specific signature format. It’s critical to:

1. Build the signature base string by concatenating:
   • The uppercase HTTP method (e.g., GET),
   • The URL-encoded endpoint,
   • And the URL-encoded, sorted OAuth parameters.
2. Sign it using HMAC-SHA1 with the consumerSecret followed by &.
3. Base64 encode the HMAC hash.

This looks simple on paper, but tiny differences in escaping or parameter order will cause 401 Unauthorized.

💻 The final C# solution

We used HttpClient for HTTP requests, and HMACSHA1 from System.Security.Cryptography for signatures. Here’s what our C# SpotlerClient does:

✅ Generates the OAuth parameters (consumer_key, nonce, timestamp, etc).
✅ Creates the exact signature base string, matching the PHP implementation character-for-character.
✅ Computes the HMAC-SHA1 signature and Base64 encodes it.
✅ Builds the Authorization header.
✅ Sends the HTTP request, with JSON bodies if needed.

We also added better exception handling: if the API returns an error (like 401), we throw an exception that includes the full response body. This made debugging much faster.

🐛 Debugging tips for OAuth 1.0

1. Print the signature base string.
   It needs to match exactly what Spotler expects. Any stray spaces or wrong escaping will fail.
2. Double-check timestamp and nonce generation.
   OAuth requires these to prevent replay attacks.
3. Compare with the PHP implementation.
   We literally copied the signature generation line-by-line from PHP into C#, carefully mapping rawurlencode to Uri.EscapeDataString.
4. Turn off SSL validation carefully.
   During development, you might disable certificate checks (ServerCertificateCustomValidationCallback), but never do this in production.

using System.Security.Cryptography;
using System.Text;

namespace SpotlerClient
{
 
    public class SpotlerClient
    {
        private readonly string _consumerKey;
        private readonly string _consumerSecret;
        private readonly string _baseUrl = "https://restapi.mailplus.nl";
        private readonly HttpClient _httpClient;

        public SpotlerClient(string consumerKey, string consumerSecret, bool verifyCertificate = true)
        {
            _consumerKey = consumerKey;
            _consumerSecret = consumerSecret;

            var handler = new HttpClientHandler();
            if (!verifyCertificate)
            {
                handler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) => true;
            }

            _httpClient = new HttpClient(handler);
        }

        public async Task<string> ExecuteAsync(string endpoint, HttpMethod method, string jsonData = null)
        {
            var request = new HttpRequestMessage(method, $"{_baseUrl}/{endpoint}");
            var authHeader = CreateAuthorizationHeader(method.Method, endpoint);
            request.Headers.Add("Accept", "application/json");
            request.Headers.Add("Authorization", authHeader);

            if (jsonData != null)
            {
                request.Content = new StringContent(jsonData, Encoding.UTF8, "application/json");
            }

            var response = await _httpClient.SendAsync(request);

            if (!response.IsSuccessStatusCode)
            {
                var body = await response.Content.ReadAsStringAsync();
                return body;
            }

            return await response.Content.ReadAsStringAsync();
        }

        private string CreateAuthorizationHeader(string httpMethod, string endpoint)
        {
            var timestamp = DateTimeOffset.UtcNow.ToUnixTimeSeconds().ToString();
            var nonce = Guid.NewGuid().ToString("N");

            var paramString = "oauth_consumer_key=" + Uri.EscapeDataString(_consumerKey) +
                              "&oauth_nonce=" + Uri.EscapeDataString(nonce) +
                              "&oauth_signature_method=" + Uri.EscapeDataString("HMAC-SHA1") +
                              "&oauth_timestamp=" + Uri.EscapeDataString(timestamp) +
                              "&oauth_version=" + Uri.EscapeDataString("1.0");

            var sigBase = httpMethod.ToUpper() + "&" +
                          Uri.EscapeDataString(_baseUrl + "/" + endpoint) + "&" +
                          Uri.EscapeDataString(paramString);

            var sigKey = _consumerSecret + "&";

            var signature = ComputeHmacSha1Signature(sigBase, sigKey);

            var authHeader = $"OAuth oauth_consumer_key=\"{_consumerKey}\", " +
                             $"oauth_nonce=\"{nonce}\", " +
                             $"oauth_signature_method=\"HMAC-SHA1\", " +
                             $"oauth_timestamp=\"{timestamp}\", " +
                             $"oauth_version=\"1.0\", " +
                             $"oauth_signature=\"{Uri.EscapeDataString(signature)}\"";

            return authHeader;
        }

        private string ComputeHmacSha1Signature(string data, string key)
        {
            using var hmac = new HMACSHA1(Encoding.UTF8.GetBytes(key));
            var hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(data));
            return Convert.ToBase64String(hash);
        }
    }
}

✅ The payoff

Once the signature was constructed precisely, authentication errors disappeared. We could now use the Spotler REST API seamlessly from C#, including:

• importing contact lists,
• starting campaigns,
• and fetching campaign metrics.

📚 Sample usage

var client = new SpotlerClient(_consumerKey, _consumerSecret, false);
var endpoint = "integrationservice/contact/email@gmail.com";
var json = client.ExecuteAsync(endpoint, HttpMethod.Get).GetAwaiter().GetResult();

🎉 Conclusion

Porting from PHP to C# isn’t always as direct as it looks — especially when it comes to cryptographic signatures. But with careful attention to detail and lots of testing, we managed to build a robust, reusable client.

If you’re facing a similar integration, feel free to reach out or clone this approach. Happy coding!