How to generate and validate a software license key?


Question

I'm currently involved in developing a product (developed in C#) that'll be available for downloading and installing for free but in a very limited version. To get access to all the features the user has to pay a license fee and receive a key. That key will then be entered into the application to "unlock" the full version.

As using a license key like that is kind of usual I'm wondering :

  1. How's that usually solved?
  2. How can I generate the key and how can it be validated by the application?
  3. How can I also avoid having a key getting published on the Internet and used by others that haven't payed the license (a key that basically isn't "theirs").

I guess I should also tie the key to the version of application somehow so it'll be possible to charge for new keys in feature versions.

Anything else I should think about in this scenario?

1
218
5/7/2013 4:23:20 PM

Accepted Answer

Caveat: you can't prevent users from pirating, but only make it easier for honest users to do the right thing.

Assuming you don't want to do a special build for each user, then:

  • Generate yourself a secret key for the product
  • Take the user's name
  • Concatentate the users name and the secret key and hash with (for example) SHA1
  • Unpack the SHA1 hash as an alphanumeric string. This is the individual user's "Product Key"
  • Within the program, do the same hash, and compare with the product key. If equal, OK.

But, I repeat: this won't prevent piracy


I have recently read that this approach is not cryptographically very sound. But this solution is already weak (as the software itself has to include the secret key somewhere), so I don't think this discovery invalidates the solution as far as it goes.

Just thought I really ought to mention this, though; if you're planning to derive something else from this, beware.

121
12/10/2010 3:08:27 PM

There are many ways to generate license keys, but very few of those ways are truly secure. And it's a pity, because for companies, license keys have almost the same value as real cash.

Ideally, you would want your license keys to have the following properties:

  1. Only your company should be able to generate license keys for your products, even if someone completely reverse engineers your products (which WILL happen, I speak from experience). Obfuscating the algorithm or hiding an encryption key within your software is really out of the question if you are serious about controlling licensing. If your product is successful, someone will make a key generator in a matter of days from release.

  2. A license key should be useable on only one computer (or at least you should be able to control this very tightly)

  3. A license key should be short and easy to type or dictate over the phone. You don't want every customer calling the technical support because they don't understand if the key contains a "l" or a "1". Your support department would thank you for this, and you will have lower costs in this area.

So how do you solve these challenges ?

  1. The answer is simple but technically challenging: digital signatures using public key cryptography. Your license keys should be in fact signed "documents", containing some useful data, signed with your company's private key. The signatures should be part of the license key. The product should validate the license keys with the corresponding public key. This way, even if someone has full access to your product's logic, they cannot generate license keys because they don't have the private key. A license key would look like this: BASE32(CONCAT(DATA, PRIVATE_KEY_ENCRYPTED(HASH(DATA)))) The biggest challenge here is that the classical public key algorithms have large signature sizes. RSA512 has an 1024-bit signature. You don't want your license keys to have hundreds of characters. One of the most powerful approaches is to use elliptic curve cryptography (with careful implementations to avoid the existing patents). ECC keys are like 6 times shorter than RSA keys, for the same strength. You can further reduce the signature sizes using algorithms like the Schnorr digital signature algorithm (patent expired in 2008 - good :) )

  2. This is achievable by product activation (Windows is a good example). Basically, for a customer with a valid license key, you need to generate some "activation data" which is a signed message embedding the computer's hardware id as the signed data. This is usually done over the internet, but only ONCE: the product sends the license key and the computer hardware id to an activation server, and the activation server sends back the signed message (which can also be made short and easy to dictate over the phone). From that moment on, the product does not check the license key at startup, but the activation data, which needs the computer to be the same in order to validate (otherwise, the DATA would be different and the digital signature would not validate). Note that the activation data checking do not require verification over the Internet: it is sufficient to verify the digital signature of the activation data with the public key already embedded in the product.

  3. Well, just eliminate redundant characters like "1", "l", "0", "o" from your keys. Split the license key string into groups of characters.


Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow
Icon