Tested version of twitter oauth authorization header algorithm, at 2014-02-05. Used for only 2 methods: get request token and exchange request token to access token

Twitter oauth header

private string MakeAuthHeader(string state = null, string token = null, string verifier = null)
		{
			//kvp for each possible parameter, participating in header or signing
			var oauthNonce = new Kvp("oauth_nonce", Convert.ToBase64String(Guid.NewGuid().ToByteArray()));
			var oauthCallbackUrl = state != null ? new Kvp("oauth_callback", redirectUrl + "?state=" + state) : null;
			var oauthConsumerKey = new Kvp("oauth_consumer_key", consumerKey);
			var oauthTimestamp = new Kvp("oauth_timestamp", UnixTimestampUTC().ToString(CultureInfo.InvariantCulture));
			var oauthSignatureMethod = new Kvp("oauth_signature_method", "HMAC-SHA1");
			var oauthVersion = new Kvp("oauth_version", "1.0");
			var oauthToken = token != null ? new Kvp("oauth_token", token) : null;
			var oauthVerifier = verifier != null ? new Kvp("oauth_verifier", verifier) : null;

			//sign-only parameters (kvp not needed)
			var requestUrl = apiUrl + (state != null ? "oauth/request_token" : "oauth/access_token");
			const string method = "POST";
			var signKey = Uri.EscapeDataString(consumerSecret) + "&" /*+ no token_secret*/;

			//signing
			var parametersForSigning = string.Join("&", new []
				{
					oauthNonce, oauthCallbackUrl, oauthConsumerKey, oauthTimestamp,
					oauthSignatureMethod, oauthVersion, oauthToken, oauthVerifier
				}
				.Where(kvp => kvp != null)
				.OrderBy(kvp => kvp.Item1)
				.Select(kvp => Uri.EscapeDataString(kvp.Item1) + "=" + Uri.EscapeDataString(kvp.Item2)));
			var signString = string.Concat(method, "&", Uri.EscapeDataString(requestUrl), "&",
				Uri.EscapeDataString(parametersForSigning)); //parameters double percent encoded
			var signature = Sign(signKey, signString);

			var oauthSignature = new Kvp("oauth_signature", signature);

			var headerValue = "OAuth " + string.Join(", ", new[]
				{
					oauthNonce, oauthCallbackUrl, oauthConsumerKey, oauthTimestamp,
					oauthSignatureMethod, oauthSignature, oauthVersion, oauthToken
				}
				.Where(kvp => kvp != null)
				.Select(kvp => Uri.EscapeDataString(kvp.Item1) + "=\"" + Uri.EscapeDataString(kvp.Item2) + "\""));
			return headerValue;
		}

Used stuff

private static string Sign(string key, string signString)
		{
			var keyBytes = Encoding.ASCII.GetBytes(key);
			using (var hmac = new HMACSHA1(keyBytes))
			{
				var hash = hmac.ComputeHash(Encoding.ASCII.GetBytes(signString));
				return Convert.ToBase64String(hash);
			}
		}

		private static int UnixTimestampUTC()
		{
			var utcNow = DateTime.UtcNow;
			var unixEpoch = new DateTime(1970, 1, 1);
			var unixTimestamp = (int)(utcNow.Subtract(unixEpoch)).TotalSeconds;
			return unixTimestamp;
		}
		
		#region Configuration

		private string apiUrl; //https://api.twitter.com/
		private string consumerKey;
		private string consumerSecret;
		private string redirectUrl; //http://localhost:2212/twlogin