how to generate a unique token which expires after 24 hours?

Question

I have a WCF Webservice which checks if the user is valid.

If the user is valid I want to generate a token which expires after 24 hours.

public bool authenticateUserManual(string userName, string password,string language,string token)
{
    if (Membership.ValidateUser(userName,password))
    {
        //////////
        string token = ???? 
        //////////

        return true;
    }
    else 
    {
        return false;
    }
}   

Answer 1

There are two possible approaches; either you create a unique value and store somewhere along with the creation time, for example in a database, or you put the creation time inside the token so that you can decode it later and see when it was created.

To create a unique token:

string token = Convert.ToBase64String(Guid.NewGuid().ToByteArray());

Basic example of creating a unique token containing a time stamp:

byte[] time = BitConverter.GetBytes(DateTime.UtcNow.ToBinary());
byte[] key = Guid.NewGuid().ToByteArray();
string token = Convert.ToBase64String(time.Concat(key).ToArray());

To decode the token to get the creation time:

byte[] data = Convert.FromBase64String(token);
DateTime when = DateTime.FromBinary(BitConverter.ToInt64(data, 0));
if (when < DateTime.UtcNow.AddHours(-24)) {
  // too old
}

Note: If you need the token with the time stamp to be secure, you need to encrypt it. Otherwise a user could figure out what it contains and create a false token.