Tweets uses HTTPWebRequest to authenticate and post to Twitter.

Tweet.cs

/* Submit to Twitter. */
public void Twitter(Post post) {
  // Get the OAuth params.
	string status = post.title + " http://blog.smockle.com/" + post.id;
	string postBody = "status=" + Uri.EscapeDataString(status);

	string oauth_consumer_key = Properties.Settings.Default.Twitter_Consumer_Key;
	string oauth_nonce = Convert.ToBase64String(new ASCIIEncoding().GetBytes(DateTime.Now.Ticks.ToString()));
	string oauth_signature_method = "HMAC-SHA1";
	string oauth_token = Properties.Settings.Default.Twitter_Token;
	TimeSpan ts = DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
	string oauth_timestamp = Convert.ToInt64(ts.TotalSeconds).ToString();
	string oauth_version = "1.0";

	// When building the signature string the params must be in alphabetical order.
	SortedDictionary<string, string> sd = new SortedDictionary<string, string>();

	sd.Add("status", Uri.EscapeDataString(status));
	sd.Add("oauth_version", oauth_version);
	sd.Add("oauth_consumer_key", oauth_consumer_key);
	sd.Add("oauth_nonce", oauth_nonce);
	sd.Add("oauth_signature_method", oauth_signature_method);
	sd.Add("oauth_timestamp", oauth_timestamp);
	sd.Add("oauth_token", oauth_token);

	// Build the signature string.
	string baseString = String.Empty;
	baseString += "POST" + "&";
	baseString += Uri.EscapeDataString("https://api.twitter.com/1.1/statuses/update.json") + "&";

	foreach (KeyValuePair<string, string> entry in sd) {
		baseString += Uri.EscapeDataString(entry.Key + "=" + entry.Value + "&");
	}

	// Remove the trailing ampersand char, remember it's been urlEncoded so you have to remove the last 3 chars - %26.
	baseString = baseString.Substring(0, baseString.Length - 3);

	// Build the signing key.
	string consumerSecret = Properties.Settings.Default.Twitter_Consumer_Secret;
	string oauth_token_secret = Properties.Settings.Default.Twitter_Token_Secret;
	string signingKey = Uri.EscapeDataString(consumerSecret) + "&" + Uri.EscapeDataString(oauth_token_secret);

	// Sign the request.
	HMACSHA1 hasher = new HMACSHA1(new ASCIIEncoding().GetBytes(signingKey));
	string signatureString = Convert.ToBase64String(hasher.ComputeHash(new ASCIIEncoding().GetBytes(baseString)));

	// Tell Twitter we don't do the 100 continue thing.
	ServicePointManager.Expect100Continue = false;

	// Instantiate a web request and populate the authorization header.
	HttpWebRequest hwr = (HttpWebRequest)WebRequest.Create(@"https://api.twitter.com/1.1/statuses/update.json");
	hwr.KeepAlive = false;

	string authorizationHeaderParams = String.Empty;
	authorizationHeaderParams += "OAuth ";
	authorizationHeaderParams += "oauth_consumer_key=" + "\"" + Uri.EscapeDataString(oauth_consumer_key) + "\",";
	authorizationHeaderParams += "oauth_nonce=" + "\"" + Uri.EscapeDataString(oauth_nonce) + "\",";
	authorizationHeaderParams += "oauth_signature=" + "\"" + Uri.EscapeDataString(signatureString) + "\",";
	authorizationHeaderParams += "oauth_signature_method=" + "\"" + Uri.EscapeDataString(oauth_signature_method) + "\",";
	authorizationHeaderParams += "oauth_timestamp=" + "\"" + Uri.EscapeDataString(oauth_timestamp) + "\",";
	authorizationHeaderParams += "oauth_token=" + "\"" + Uri.EscapeDataString(oauth_token) + "\",";
	authorizationHeaderParams += "oauth_version=" + "\"" + Uri.EscapeDataString(oauth_version) + "\"";
	hwr.Headers.Add("Authorization", authorizationHeaderParams);

	// POST off the request.
	hwr.Method = "POST";
	hwr.ContentType = "application/x-www-form-urlencoded";
	Stream stream = hwr.GetRequestStream();
	byte[] bodyBytes = new ASCIIEncoding().GetBytes(postBody);

	stream.Write(bodyBytes, 0, bodyBytes.Length);
	stream.Flush();
	stream.Close();

	// Allow us a reasonable timeout in case Twitter is busy.
	hwr.Timeout = 3 * 60 * 1000;

	try {
		HttpWebResponse rsp = hwr.GetResponse() as HttpWebResponse;
	}
	catch (WebException e) {
		// Do some clever error handling here.
	}
}