A basic digitally signed token in C#. More info at: http://www.benlesh.com/2012/08/creating-digitally-signed-security.html

Program.cs

class Program
{
	static void Main(string[] args)
	{
		TokenTesting();
	}

	private static void TokenTesting()
	{
		// create a private key to use in our tests.
		var privateKey = GetPrivateKey();

		// create a token and token string to test with.
		var token = new Token("testdata", DateTime.Now.AddMinutes(1));
		var tokenString = token.GetTokenString(privateKey);

		//try getting the token from the token string.
		var token2 = Token.FromTokenString(tokenString, privateKey);
		if (token2 != null)
		{
			Console.WriteLine("Valid token!");
			Console.WriteLine("Token Value: {0}", token2.Value);
		}

		//try tampering with the token string (this is a corny way to do it, but whatever)
		tokenString = "blah" + tokenString; 
		var token3 = Token.FromTokenString(tokenString, privateKey);
		if (token3 == null)
		{
			Console.WriteLine("Invalid token string!");
		}

		//try an expired token
		var token4 = new Token("this is expired", DateTime.Now.AddDays(-1));
		var expiredTokenString = token4.GetTokenString(privateKey);
		var token5 = Token.FromTokenString(expiredTokenString, privateKey);
		if (token5 == null)
		{
			Console.WriteLine("This token is expired, silly.");
		}

		Console.ReadKey();
	}

	private static string GetPrivateKey()
	{
		using (var rsa = new RSACryptoServiceProvider())
		{
			return rsa.ToXmlString(true);
		}
	}
}

Token.cs

class Token
{
	public readonly string Value;
	public readonly DateTime Expires;
	public readonly byte[] Data;
	public byte[] Signature { private set; get; }

	public Token(string value, DateTime expires)
	{
		Value = value;
		Expires = expires;
		using (var ms = new MemoryStream())
		using (var writer = new BinaryWriter(ms))
		{
			writer.Write(Expires.Ticks);
			writer.Write(Value);
			Data = ms.ToArray();
		}
	}

	private void Sign(string privateKey)
	{
		using (var rsa = new RSACryptoServiceProvider())
		using (var sha1 = new SHA1CryptoServiceProvider())
		{
			rsa.FromXmlString(privateKey);
			Signature = rsa.SignData(Data, sha1);
		}
	}

	public string GetTokenString(string privateKey)
	{
		if (Signature == null)
		{
			Sign(privateKey);
		}
		return Convert.ToBase64String(Data.Concat(Signature).ToArray());
	}

	public static Token FromTokenString(string tokenString, string key)
	{
		var buffer = Convert.FromBase64String(tokenString);
		var data = buffer.Take(buffer.Length - 128).ToArray();
		var sig = buffer.Skip(data.Length).Take(128).ToArray();
		using (var rsa = new RSACryptoServiceProvider())
		using (var sha1 = new SHA1CryptoServiceProvider())
		{
			rsa.FromXmlString(key);
			if (rsa.VerifyData(data, sha1, sig))
			{
				using (var ms = new MemoryStream(data))
				using (var reader = new BinaryReader(ms))
				{
					var ticks = reader.ReadInt64();
					var value = reader.ReadString();
					var expires = new DateTime(ticks);
					if (expires > DateTime.Now)
					{
						return new Token(value, expires);
					}
				}
			}
		}
		return null;
	}
}