If I use DateTime.Now as part of software licensing authentication to enforce the number of days software can be used, can this be circumvented by simply changing the system clock?
If so is there a better way to get real UTC time?
If I use DateTime.Now as part of software licensing authentication to
enforce the number of days software can be used, can this be
circumvented by simply changing the system clock?
DateTime.Now uses the system clock, so yes it could be circumvented.
If so is there a better way to get accurate system time?
Yes, you could query a time server over the www. But you know, even this could be circumvented because the user can modify his hosts file and replace the address you are using with his own proxy and return whatever time he likes. So it will all depend on how far do you want to get into protecting your software. If you really care about the intellectual property of your software you are better of purchasing a commercial licensing software.
Everytime i have used this method for testing it has affected this, so the answer is yes it does affect it
Yes of course, the DateTime.Now obtains its value from the system clock.
A better way is to use a NTP server on Internet.
In this article on codeproject you could find code that allows a client to connect to a NTP server
Related
I'm creating in-memory local cache of a network filesystem.
I want every directory/file to have LastDownloadedTime attribute, and then compare it with current time to decide whether the directory/file is too old to be used from cache.
But I'm uncertain whether DateTime.Now() is what I need. What happens if the user changes system time? How can simestamps suitable for my task be obtained in .NET?
Take a look at System.DateTime.UTCNow(). It'll return the UTC time. Just make sure you are consistently calling on both ends (when you store the cache as well as when you test it for expiration).
More info: https://msdn.microsoft.com/en-us/library/system.datetime.utcnow(v=vs.110).aspx
You should use DateTimeOffset. According to the docs:
Uniquely and unambiguously identify a single point in time. The DateTimeOffset type can be used to unambiguously define the meaning of "now", to log transaction times, to log the times of system or application events, and to record file creation and modification times.
If your application is running on a user's machine, though, they could change their system time (as you mentioned). The only way to prevent that is to have your application ping a time server to get an authoritative timestamp.
I think I'm going to use Stopwatch for timestamps, because my filesystem cache is in-memory, it does not survive system reboot. Timestamps are going to be relative to the start of the Stopwatch.
Though, Stopwatch is known to be buggy.
But I consider system clock-dependent solutions to be even more dangerous, as the user may set system clock to some date in the past, messing everything up.
If you are staying within bounds of local server then DateTime.UtcNow is the most convenient option for the task. Yes, if user changes local time, then that will invalidate cache and trigger unnecessary update.
You will need to use NTP (time) servers to avoid that problem. So, instead of using local server UTC time, you will obtain UTC time from NTP server (there are plenty of them available around the world).
Please refer to NTP wikipedia article
I have developed an application in C# which uses the time of the system. My problem is that if someone changes the time of the system, it affects the application also. The application would then show data of that changed time and not the current time.
How do i avoid this? Is there any other way to get a single universal time?
You can use NTP to ask to an authoritative time server what time it is, for example. If you google for NTP client C# you'll find plenty of examples.
If time is so critical, keep anything that is dependent on the time isolated on the server. Move the actual processing to the server, and just display the result on the client.
This question may be easy and the answer obvious, but I can't seem to be able to find a solution right now.
I built an application which has a big flaw in it. In a property of my User class, I check to see if the user subscription is expired. To do this, I compare the ending date of the subscription with DateTime.Now:
return (DateTime.Compare(DateTime.Now, subEndDate) > 0);
It doesn't take a genius user to realize that all it is needed is to change the Windows date to an earlier one, and then the application won't expire at all. So I think that comparing to DateTime.Now should not be done, is there a better method that I could use in order to validate a subscription date?
Thanks.
Regards,
Call a webservice or check a database to determine if the subscription is still active
Does this actually matter? If your product is purely web based the only time you have to worry about is your server time. If the server time is able to be altered without your consent you probably have larger problems to worry about.
If your product is desktop based, then how much protection do you want build in? If you just want to protect against your casual user the solution you have is probably enough. If someone is determined to pirate your software then they will probably be successful. If you want to make it harder for these users one solution would be to keep a log of all the times the application has been run. This way you can get an idea of they are playing with the clock.
Maybe you could extract the subscription expiration logic out of your client program and put it into an external service, then your client app could connect to a different server and retrieve expiration details based on a user parameter passed in?
There are several NTP servers out there which you can use for free... they return the exact time and your casual user won't have a hand in manipulating those... to access them you have several options - though none built-in:
http://dotnet-snippets.com/dns/simple-network-time-ntp-protocol-client-SID571.aspx
http://www.codeproject.com/KB/IP/ntpclient.aspx
http://www.codeproject.com/KB/datetime/SNTPClient.aspx
http://www.rebex.net/time.net/ (commercial)
I'm developing an application and I need to make a demo version of it that will expire in lets say 30 days.
My idea for now is to store the current time when the application is first started in a simple txt file stored in the projects resource (so it doesen't have to be written on the hard disk or the registry), and every time the program is started check if 30 days have passed.
But, what if the user resets the time to an earlyer state?
Then my app would still start becouse now the "current date" can be 1 day after the "first start"..
Is there any way I can get some info along with the first date (or, insted of) that would assure that specific time that the application is first started?
Thank you for your time.
One of the option is to check the date time from some external server and not from local system. But this is also possible to crack as the user can put a proxy in between which will act as the external server to your app and this proxy can send any time the user want. In case you want the solution for novice users, external server approach is fine, but any smart user can still crack the application. Remember, any code that runs on a client machine can be cracked, you just need to flip the correct bit in the application code :)
I have a simple check in the code of one of my programs when in beta. It pretty much does what you say. In the initial code is a hard coded date. The logic then just tests to see if the current date is earlier than the hard coded date. Pretty primitive and it relies on the users system date. However once it is past the program will not run unless the system date is changed. As I mention I use this for betas not for my production programs that have an evaluation period. For that I use Infralution License Tracker. This is payware but it does allow the setting of evaluation periods and also the actual licensing of the code.
Interesting question - how about encoding the initial datetime somehow - base64 or even encryption of somekind), then also storing the datetime each time your application is run.
That way you can detect inconsistencies by checking each datetime and if there's funny business going on (eg. todays date is older than the last date) you can shut the user out.
I want to provide a trial version of my software. This version should only be evaluated within a specific period of time. Let's say only during January 2011.
As this software massively uses the system clock in processing, it would be quite annoying to set the clock to an earlier time to be able to use it over and over. So because of this, I wound't think of a more complicated protection mechanism.
So I have thought about exiting after a test like:
if (DateTime.Now.Year != 2011 && DateTime.Now.Month != 1)
{
MessageBox.Show("expired!");
Application.Exit();
}
How easy will this be cracked :-) ?
Is there a "safe" way to do this ?
Basicly you can say it is impossible to secure trial software against cracking. Everything you do in Software can be bypassed (Yes! Even Ring-0 Drivers).
Even if you have an external dongle from which you recieve the authentication to start can be spoofed through software. Although it isn't easy :-)
You can only make it hard not impossible :-)
It's not exactly related to cracking it, but it's worth noting that if this is an app that can be used internationally, it'll show as being 'expired' for many users before they've even had a chance to try it at all. The values returned by DateTime reflect the local users culture, so DateTime.Now.Year returns 1431 for Arabic cultures and 2553 for the Thai culture for instance. Months can similarly differ, so you shouldn't hardcode values for them without checking the culture first.
You can get round this by using the InvariantCulture each time ,e.g. DateTime.Now.Year.ToString(System.Globalization.CultureInfo.InvariantCulture);
This method could be cracked if the user set his computer Date to 2009: your software will be used for other two years.
If you want to use this method I think the best way is to check the actual date on the internet.
However some users couldn't have a connection; in that case you can use something like a countdown that makes the software executable for n-days.
You ship your software with an encrypted file that contains the date of installation;
The next time you will check for that file:
If exists and the day is different increment a counter;
If exists but is corrupted or if it doesn't exists update the counter to "max-day";
If exists but the counter is equal to "max-day" exit the application;
Update the old file with the new values;
Obviously the counter will be another encrypted file and, as for the other file, you have to check his state (corruption or deletion).
Here's my opinion.
The weak point of any time-limit or dongle-protection is that in the end everything must be checked with an 'if' statement. In the old 'x86 days there is JNE, JE, JNZ family of instructions. Such 'if' statement must exist in hundreds if not thousands or more in the application. So any cracker must find out where to start looking, for instance, dongle checkers almost always use DeviceIoControl API, which could be pinpointed quickly. After the calls to DeviceIoControl API found, the cracker just reverse engineered the routine around the API call, and try change JNE instructions around that to JE or the other way around.
In your case, the usage of DateTime is the tell-tale (but of course, there is a lot of place where DateTime being used for other things, that makes it harder for the cracker). To make things difficult for the cracker, copy the current date to some object's values, and try to make, like, 20 places or something that stores the DateTime. Or even better, retrieve current date from the net and also from the current computer. Don't check the DateTime directly, but use the value that you store in some objects before, to make it harder for the cracker. Use consistency check mechanism to ensure the dates are within tolerance, and kill the app if you find out that 2 of the datetime is different to the other stored datetime (give 2 days tolerance or so).
Also check whether the clock is not turned back by the user, if you found out that CurrentDateTime < StoredDateTimeInRegistry then you should store a kill flag somewhere in the registry. Or you might also want to use a file in addition to the registry.
For every kind checks you do, try to do it in many places and in different ways.
At the end, I must say that what Bigbohne said is true (nothing is impossible to crack) - it is just that, by making it difficult for the cracker, we changed his/her effort-to-result ratio, and hopefully discouraging him from continuing the cracking process.
Checking trial period expiration in C# code is easy to crack, even if you will obfuscate code due to it is compiled into CLR. It is better carry out this check into code that is compiled to byte code. Also you can read topic Protect .NET code from reverse engineering? about protecting .NET code from reverse engineering
Software licensing is a complete subject on its own, and it looks like that you are looking for a simplest solution to be implemented for your trial software.
What simply you can do, on startup of your application log the current date/time in registry and use it is as a reference point for validation. So even if the system time would be changed it wouldn't effect your application validation logic.
If possible, write the registry access library in C++, which wouldn't be possible to crack. Good luck.