Generating and validating license keys is a common requirement for commercial desktop applications. This article shows a state of the art implementation in 2021. It is simple and cryptographically secure.
Scope
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When you browse StackOverflow for licensing implementations, you frequently read the following warning:
No license scheme is 100% secure.
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It is true. So, given that our task is ultimately impossible, we don't want to think about it for too long. At the same time, we want something that is reasonably safe.
This article is about registration codes that work offline. No phoning home to a license server. Even if you use a server, you likely don't want your app to stop working just because your user doesn't have internet for a brief while. To achieve this, you will need an offline way of validating licenses.
Cracks vs. keygens
There are several ways in which people can work around the copy protection in your software. The most common are cracks. These usually patch your application's executable, to trick it into believing that there is a valid license. Every desktop application can be fooled in this way. Fortunately, cracks usually only work for specific versions of an app (eg. 5.1.2 but not 5.1.3).
The worst case for software vendors are key generators. They can be used to create arbitrarily many valid serial numbers. If a keygen exists for your app, then your licensing algorithm is compromised beyond repair.
Partial key verification
To prevent keygens from working for all versions of your software, a commonly used technique is partial key verification. Under this scheme, you only use some bits to check the validity of a license key. For example, the first version of your app might only check the first character in each group of a product key:
If someone publishes a keygen for your app, then you can release a new version that checks the second character (say) for a different requirement:
This limits the potential damage of a single key generator. But it doesn't prevent other keygens from appearing for your new app version.
Key length
Historically, license keys had to be entered manually. For instance, when you bought Windows XP, you received a CD-ROM and a printed product key that you had to type in upon installation:
To make this workable, license keys had to be short and consist of simple characters such as A - Z and 0 - 9.
Nowadays, hardly anyone types in license keys by hand. When a user purchases your software, you send them an email. They either download the license key, or copy/paste it into your application. Because of this, the length of license keys has little practical relevance today.
Older articles about license verification spend a lot of brainpower on 1) encoding information in the limited-length license key, such as a maximum app version, and 2) on partial key verification. If we drop the requirement that license keys be easy to type, we can get a simpler and more secure solution.
A modern approach
At the end of the day, a license check boils down to code like the following:
Note that this even applies to more exotic solutions. For example, say your app's binary is encrypted and only valid license keys can 'decrypt' it somehow. Then license_key_is_valid() amounts to asking 'can this key be used to decrypt the binary?'.
We thus need to choose an implementation for license_key_is_valid(). Fortunately, modern cryptography gives us just the right tool for this: We can use RSA signature verification to sign the licensing data with a private key, then verify the signature with an associated public key.
Below is an example in Python that uses the rsa library. Because RSA is so ubiquitous, you should be able to easily port this to another language if required.
First, create an RSA key pair on your development machine. We use 512 bits here because it leads to shorter signatures. In practice, you probably want 2048 bits or more.
When a user purchases, generate a license key:
This prints the following:
Send this to your user. Then, in your application, check the validity of the license key as follows:
Once execution reaches the last line, you can trust that data was not tampered with. This lets you include information relevant to licensing in the data, such as a maximum app version to which your user is entitled.
The above code works as-is when you type it into one interactive Python interpreter session. In practice, you will have to ship the public key with your app and decide where the user will put the license key. These are just details however. The important parts of the implementation are all here.
Caveats & Summary
Assuming you use a large enough bit size, the above implementation should be safe from key generators. It is not immune to cracking however – as mentioned above, no desktop app is. If you want to make your app even more secure, you could look at obfuscation. This makes reverse-engineering and thus circumventing your copy protection more difficult.
Michael is the creator of fman, a cross-platform file manager. Frustrated with how difficult it was to create this desktop application, Michael open sourced fman's build system (fbs). It saves you months when creating desktop apps with Python and Qt. A few days of these months come from using fbs's well-integrated licensing implementation.
Mac OS X 10.2 was released by Apple in 2002. Ten years later, in the summer of 2012, Apple discontinued Software Update for Mac OS X 10.3 and earlier. (Panther users.
The Good Greatly improves Mac, Windows, and Internet networking; refines Web and hard drive search tool; upgrades performance and responsiveness; includes Bluetooth support; new discount for five-user licenses. The Bad No upgrade pricing for copies of OS X purchased before July 17; bundled iChat 1.0 isn't as versatile as AIM; still no Google search in Sherlock. The Bottom Line Take a good, hard look at Mac OS X 10.2 if you're adding a Mac to a Windows network-Jaguar's new tools can't be beat. But home users, beware the $129 upgrade price if you're not looking for networking options. Visit for details. Jaguar, Apple's new version of Mac OS X, packs a much bigger upgrade wallop than its 10.2 version number suggests. Part of the punch is the price: Jaguar costs $129 (no upgrade discount if you bought before July 17) for one user or $199 for a five-user license.
But its slightly improved interface, powerful new networking tools, three new applications, and better performance and stability finally bring to fruition Mac OS X's potential. If you bought recently and aren't looking for networking support, $129 is too much to pay. But if you're a business user or have a cross-platform home network, take a serious look at Jaguar. Jaguar, Apple's new version of Mac OS X, packs a much bigger upgrade wallop than its 10.2 version number suggests.
Part of the punch is the price: Jaguar costs $129 (no upgrade discount if you bought before July 17) for one user or $199 for a five-user license. But its slightly improved interface, powerful new networking tools, three new applications, and better performance and stability finally bring to fruition Mac OS X's potential.
If you bought recently and aren't looking for networking support, $129 is too much to pay. But if you're a business user or have a cross-platform home network, take a serious look at Jaguar.
Installation and interface OS X's installation hasn't changed much in Jaguar; it's as easy as ever, despite involving two discs. You're forced to enter quite a bit of personal information at setup, which gets sent to Apple for registration and marketing purposes.
Read the before you give out your phone number or e-mail address. Jaguar uses some of the registration information for your benefit; for example, Sherlock's new Yellow Pages search feature automatically provides driving directions from the location that you specify as your home address.
Once you start up in Jaguar, you'll notice subtle, refined changes to the Aqua interface. The bulbous, primary-colored buttons now look flatter and are more subtly tinted, while other indicators, including drag-and-drop plus signs, sport a 3D design. Dialog boxes now display more information, too. For example, the streamlined Show Info box displays all data at once, instead of dividing it into tabs.
Plus, you can now open multiple Show Info dialogs simultaneously. Still longing for the handy spring-loaded folders feature from OS 8 and 9? Jaguar makes a concession just for you: folder after folder will open automatically as you drag a file over them, and all but the last one will close when you release the mouse button. Find anything, anywhere Jaguar's most compelling interface changes lie in its revamped Find commands. Apple has moved hard disk search functions out of Sherlock, OS X's search tool, and into the Finder. Now, when you open a Finder window, its toolbar offers a handy field that lets you search a particular folder, drive, or network volume. The Find command in the Finder's File Menu (command-F) brings up a streamlined advanced file-search dialog.
We love the separated search, as the previous Sherlock often presented a confusing list of online and offline results. In contrast, Sherlock 3.0 now searches the Internet only, neatly aggregating common Web search results into a single desktop window-no need for a browser. Sherlock even organizes Web search choices into channels, which it displays as toolbar buttons. For example, the channel lets you search and track items by name and price range, then returns photos and auction data right in your Sherlock window. The Yellow Pages channel displays maps and directions, and the AppleCare channel presents entire Apple Knowledge Base articles, all within Sherlock.
Our only complaint is that the general Internet channel includes only five search engines-and the preeminent is still not among them. Worse, both the five search engines and the Yellow Pages are fixed, so you can't switch, for example, to your favorite map site. Networking wonders Simple productivity changes aside, Jaguar adds some seriously impressive networking tools.
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Any of these improvements would be a major breakthrough for the Mac OS; together, they more than justify Jaguar's price tag for anyone who runs a Mac on a network. Rendezvous, a dynamic discovery and self-configuration technology based on the, makes connecting to IP devices as easy as it was with the old AppleTalk. Plug in two Jaguar Macs to an Ethernet cable, for instance, and the systems configure their own addresses and locate each other in seconds. Jaguar also immediately recognizes any available wireless 802.11b network, presents it by name, and lets you connect with a simple click (and a password, of course). But Jaguar's new Windows networking features take the cake. Adding a Mac to a Windows network is as easy as breathing. You no longer need to type in a server IP address or URL (as with 10.1).
Windows servers now show up by name in the Connect To Server command of the Go menu. Simply double-click a name to mount the server on your Jaguar desktop and start browsing it. Windows browsing from a Mac On top of that, Windows users can now log on to Mac OS X and access Mac files. To enable the sharing, click System Preferences Sharing, and check the box next to Windows File Sharing.
Jaguar generates a URL that you can give out to approved Windows users so that they can access public files using the Windows protocol-something that was impossible in Mac OS X 10.1. And, thank goodness, Jaguar now includes built-in virtual private networking. In the Internet Connect utility in the Utilities folder under Applications, click File 'New VPN connection window,' and Jaguar lets you create a connection to a Windows VPN server so that you can connect remotely over high-speed access.
Performance Jaguar is marketed as a 'features' release mostly because there are so many new features. New features are also easier to sell than incremental performance increases. Mac OS X performance was in the 10.1 review, and for the most part those observations continue to hold for 10.2. But while 10.2 does not provide as big a leap as 10.1 did, performance has improved in many areas. Boot time is substantially faster in Jaguar, thanks largely to its ability to start services in parallel, provided there are no dependencies between them (e.g.
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Network time synchronization can't start until the network is up.) This is especially helpful when one of the startup items takes a long time to complete (e.g. Requesting an IP address from a busy DHCP server.) Rather than blocking the entire boot sequence while waiting for this long-running task to complete, Jaguar will continue to launch other independent services. On the G3/400, the boot sequence was dominated by POST and the new 'gray apple' startup screen. Once the startup progress bar appeared, it zipped to the end very quickly, shaving a full 20 seconds off the 10.1 boot time. The G4/800 got a bit faster as well, but it has a static IP address and was already very fast (no waiting for an address from a DHCP server.) On the front, Jaguar now enables the window buffer compression feature of the window server.
In Jaguar, 'inactive' window buffers are compressed in order to save memory. Since many windows contain large areas of 'empty space' (i.e. Long lines of the same color) they tend to compress very nicely even using a simple algorithm like RLE (run length encoding, e.g. 'white pixel x 200' rather than 'white pixel, white pixel, white pixel.' ) Window compression is independent of Quartz Extreme, so everyone who can run OS X will benefit from it. Application launch speed has improved in Jaguar, but the improvement is not comparable to the that 10.1 made over 10.0. I actually repeated the entire from the 10.1 review, but the results were somewhat inconclusive because almost all the applications tested have been revamped since 10.1.
In general, the launch times were a 1-3 seconds faster on Jaguar. Several bundled applications also stopped bouncing sooner than they did in 10.1. The speed-up is likely due to in Jaguar's Mach kernel. Here's the explanation from Apple's: In order to reduce application launch times, the kernel now maintains information about the working set of an application between launches (in '/var/vm/appprofile'). These so-called 'pre-heat files' are a variation on a well-known optimization for decreasing application launch time, but it's somewhat disappointing the the benefit is not more substantial. Nevertheless, it's good to see that Apple is working hard to improve this part of the user experience.
(Yes, I'd definitely call modifying the kernel to improve application launch times 'working hard':-) The dreading 'spinning rainbow disc' has an all new look in Jaguar, but it appears just about as often as it did in 10.1. Since the rainbow disc is merely an indication of when an application has not responded to events for a few seconds, it's really an application problem. But if the application is blocking on a system call that's taking much too long, it may also be an OS problem. Either way, it's annoying. Jaguar minimizes the annoyance by changing the cursor back to normal when it is not over a window owned by the unresponsive application.
This makes it clear to the user that other applications are still okay and will respond correctly if clicked on. While this was also true in earlier versions of OS X, the cursor did not change, causing many users to assume they had to wait.
Although scrolling is still often much slower than it should be, the performance has improved significantly in Jaguar. Previous versions of Mac OS X would redraw the entire contents of the window during scrolling. Jaguar simply shifts the existing content upward or downward as necessary (something that can be done in hardware on the video card, even without Quartz Extreme), and then redraws only the newly revealed portion of the window. (This effect is easy to see using the Quartz Debug application included on the developer tools disk. Just turn on the 'flash screen updates' option and scroll a few windows.) Window resizing remains very slow, especially in 'brushed metal' applications like iTunes and iPhoto. Window resizing sometimes actually seems worse overall in Jaguar-an impression created by the proliferation of the brushed metal interface.
Whatever is making window resizing so slow needs to be found and fixed. Quartz Extreme doesn't help in any measurably way, so obviously window composition overhead is not the culprit. To explore the actual performance benefits of Quartz Extreme on the G4/800, I returned to the old standby: the transparent terminal window. First, I downloaded the largest version of the Star Wars trailer. To set a baseline, I played the movie as-is. As expected, the G4/800 had no problem playing it at its full framerate of 24 fps. The torture began with the placement of a single transparent terminal window (80x24, 0.25 transparent, where 1 is totally opaque) on top of the movie.
I noted the maximum and minimum sustained (for more then 1 second) framerate during playback. Then I repeated the test after positiong two transparent terminal windows on top of the movie, then three, then four, and so on. The results are shown in the table below: Framerate (Min/Max) # Win Mac OS X 10.1 10.2 w/ QE 0 24fps 24fps 1 8.6fps - 11fps 24fps 2 5fps - 10fps 24fps 3 1fps - 6fps 24fps 4.5fps - 5fps.
24fps 5 0fps - 3fps. 24fps. Sound drop-outs during playback I actually ran tests with many more than 5 windows on top of the movie.
In Mac OS X 10.1, the movie playback was pretty much shot after 5 windows were added. The movie actually stopped entirely (0fps) for a second or more several times, and the sound (which began dropping out with 4 windows) was completely broken up. In Jaguar with Quartz Extreme enabled, the GPU really flexes its muscles. I actually kept adding transparent windows until I hit 25 and got bored. The framerate never dropped at all.
It was time to pull out all the stops and unleash the dreaded shaking transparent terminal window. The test is the same as described above, except that the top-most terminal window is shaken as fast as humanly possible over the top of the rest of the pile. This is the test that caused iTunes to skip in Mac OS X 10.0 and could bring movie playback to its knees in 10.1. Here are the results for 10.2 with Quartz Extreme, listing the minimum sustained (for more than 1 second) framerate. (Results from 10.1 are shown just for completeness:-) Minimum Framerate # Win Mac OS X 10.1 10.2 w/ QE 1 0fps 24fps 1 0fps 24fps 2 0fps 24fps 3 0fps 24fps 4 0fps 21fps 5 0fps 17fps Things are grim in Mac OS X 10.1. I was able to stop the movie dead by shaking just one transparent terminal window vigorously. Not surprisingly, things didn't get any better as the number of (stationary) transparent terminal windows below it increased.
Jaguar with QE was a champ right up until the 4 window mark. After that, it began to degrade fairly linearly. Although this performance is worlds better than 10.1 without QE, I wanted to know exactly what it was about shaking the top window that cause the performance loss. After all, the amount of compositing calculations seemed equal, and could potentially be even less, since the top window occasionally moved away from the playing movie completely during shaking. My next thought was that it might be a bandwidth problem. Perhaps shaking the window somehow caused the AGP bus to be swamped?
But that didn't make much sense, especially with 64MB of VRAM and a set of windows that weren't changing at all (except for the movie). The unchanging window buffers for all the terminal windows could fit comfortably in the card's video memory.
So I tried to look at it more logically. QuickTime needs CPU cycles to decode the audio and video in the movie.
A reduction in framerate means that QuickTime is not getting enough CPU cycles. Therefore shaking the transparent terminal window must be taking up CPU cycles.
Running top verified it: the window server was taking roughly 70% of the G4/800's CPU cycles when I was shaking the window on top of four other windows. It took progressively less CPU time as the number of stationary transparent terminal windows decreased.
That explains why the movie's framerate started to drop, but what explains the window server's increased need for CPU cycles? The test with the stationary windows showed how well compositing calculations are handled by the GPU.
With 25 stationary windows, there was no drop in framerate. Why does a shaking window require so much more CPU involvement? And, even more puzzling, why does the amount of CPU involvement increase in proportion to the number of stationary windows sitting below the shaking window? Unfortunately, I didn't have time to explore the boundaries of Quartz Extreme much further, as there are many other interesting parts of Jaguar that required my attention. Furthermore, the example above is not likely to come up in daily use.
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Stationary windows are much more common than shaking ones, and QE handles them without breaking a sweat. Still, it would be nice if window movement was just as smooth. While QE doesn't help window resizing and isn't responsible for the scrolling speed increase, you can most certainly make a Mac OS X system 'feel' faster by playing to QE's big strength: compositing.