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       How s that usually solved  How can I generate the key and how can it be validated by the application  How can I also avoid having a key getting published on the Internet and used by others that haven t paid 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

User · Answer

You can use a free third party solution to handle this for you such as Quantum-Key.Net It's free and handles payments via paypal through a web sales page it creates for you, key issuing via email and locks key use to a specific computer to prevent piracy.

Your should also take care to obfuscate/encrypt your code or it can easily be reverse engineered using software such as De4dot and .NetReflector. A good free code obfuscator is ConfuserEx wich is fast and simple to use and more effective than expensive alternatives.

You should run your finished software through De4Dot and .NetReflector to reverse-engineer it and see what a cracker would see if they did the same thing and to make sure you have not left any important code exposed or undisguised.

Your software will still be crackable but for the casual cracker it may well be enough to put them off and these simple steps will also prevent your code being extracted and re-used.

https://quantum-key.net

How to use ConfuserEx?

https://github.com/0xd4d/de4dot

https://www.red-gate.com/dynamic/products/dotnet-development/reflector/download

User · 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

User · Answer

I strongly believe  that only public key cryptography based licensing system is the right approach here  because you don t have to include essential information required for license generation into your sourcecode   In the past  I ve used Treek s Licensing Library many times  because it fullfills this requirements and offers really good price  It uses the same license protection for end users and itself and noone cracked that until now  You can also find good tips on the website to avoid piracy and cracking

User · Answer

I m one of the developers behind the Cryptolens software licensing platform and have been working on licensing systems since the age of 14  In this answer  I have included some tips based on experience acquired over the years   The best way of solving this is by setting up a license key server that each instance of the application will call in order to verify a license key   Benefits of a license key server  The advantages with a license key server is that    you can always update or block a license key with immediate effect  each license key can be locked to certain number of machines  this helps to prevent users from publishing the license key online for others to use     Considerations  Although verifying licenses online gives you more control over each instance of the application  internet connection is not always present  especially if you target larger enterprises   so we need another way of performing the license key verification   The solution is to always sign the license key response from the server using a public-key cryptosystem such as RSA or ECC  possibly better if you plan to run on embedded systems   Your application should only have the public key to verify the license key response   So in case there s no internet connection  you can use the previous license key response instead  Make sure to store both the date and the machine identifier in the response and check that it s not too old  eg  you allow users to be offline at most 30 days  etc  and that the license key response belongs to the correct device      Note you should always check the certificate of license key response  even if you are connected to the internet   in order to ensure that it has not been changed since it left the server  this still has to be done even if your API to the license key server uses https    Protecting secret algorithms  Most  NET applications can be reverse engineered quite easily  there is both a diassembler provided by Microsoft to get the IL code and some commercial products can even retrieve the source code in eg  C    Of course  you can always obfuscate the code  but it s never 100  secure   I most cases  the purpose of any software licensing solution is to help honest people being honest  i e  that honest users who are willing to pay don t forget to pay after a trial expires  etc    However  you may still have some code that you by no means want to leak out to the public  eg  an algorithm to predict stock prices  etc   In this case  the only way to go is to create an API endpoint that your application will call each time the method should be executed  It requires internet connection but it ensures that your secret code is never executed by the client machine    Implementation  If you don t want to implement everything yourself  I would recommend to take a look at this tutorial  part of Cryptolens

User · Answer

The only way to do everything you asked for is to require an internet access and verification with a server  The application needs to sign in to the server with the key  and then you need to store the session details  like the IP address  This will prevent the key from being used on several different machines  This is usually not very popular with the users of the application  and unless this is a very expensive and complicated application it s not worth it   You could just have a license key for the application  and then check client side if the key is good  but it is easy to distribute this key to other users  and with a decompiler new keys can be generated

User · Answer

It is not possible to prevent software piracy completely  You can prevent casual piracy and that s what all licensing solutions out their do    Node  machine  locked licensing is best if you want to prevent reuse of license keys  I have been using Cryptlex for about a year now for my software  It has a free plan also  so if you don t expect too many customers you can use it for free

User · Answer

I ve used Crypkey in the past  It s one of many available   You can only protect software up to a point with any licensing scheme

User · Answer

Simple answer - No matter what scheme you use it can be cracked   Don t punish honest customers with a system meant to prevent hackers  as hackers will crack it regardless   A simple hashed code tied to their email or similar is probably good enough   Hardware based IDs always become an issue when people need to reinstall or update hardware   Good thread on the issue  http   discuss joelonsoftware com default asp biz 5 82298 34

User · Answer

I don t know how elaborate you want to get  but i believe that  net can access the hard drive serial number   you could have the program send you that and something eles   like user name and mac address of the nic   you compute a code based off that and email them back the key   they will keep them from switching machines after they have the key

User · Answer

Besides what has already been stated      Any use of  NET applications are inherently breakable because of the intermediate language issues   A simple disassembly of the  NET code will open your product to anyone   They can easily bypass your licensing code at that point    You can t even use hardware values to create a key anymore   Virtual machines now allow someone to create an image of a  licensed  machine and run it on any platform they choose   If it s expensive software there are other solutions   If it s not  just make it difficult enough for the casual hacker   And accept the fact that there will be unlicensed copies out there eventually   If your product is complicated  the inherent support issues will be create some protection for you

User · Answer

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    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  A license key should be useable on only one computer  or at least you should be able to control this very tightly  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     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      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  Well  just eliminate redundant characters like  1    l    0    o  from your keys  Split the license key string into groups of characters

User · Answer

The C     NET engine we use for licence key generation is now maintained as open source   https   github com appsoftware  NET-Licence-Key-Generator   It s based on a  Partial Key Verification  system which means only a subset of the key that you use to generate the key has to be compiled into your distributable  You create the keys your self  so the licence implementation is unique to your software   As stated above  if your code can be decompiled  it s relatively easy to circumvent most licencing systems

User · Answer

I solved it by interfacing my program with a discord server  where it checks in a specific chat if the product key entered by the user exists and is still valid  In this way to receive a product key the user would be forced to hack discord and it is very difficult

User · Answer

When generating the key  don t forget to concatenate the version and build number to the string you calculate the hash on  That way there won t be a single key that unlocks all everything you ever released     After you find some keys or patches floating in astalavista box sk you ll know that you succeeded in making something popular enough that somebody bothered to crack  Rejoice

User · Answer

I ve implemented internet-based one-time activation on my company s software  C   net  that requires a license key that refers to a license stored in the server s database   The software hits the server with the key and is given license information that is then encrypted locally using an RSA key generated from some variables  a combination of CPUID and other stuff that won t change often  on the client computer and then stores it in the registry     It requires some server-side coding  but it has worked really well for us and I was able to use the same system when we expanded to browser-based software   It also gives your sales people great info about who  where and when the software is being used   Any licensing system that is only handled locally is fully vulnerable to exploitation  especially with reflection in  NET   But  like everyone else has said  no system is wholly secure     In my opinion  if you aren t using web-based licensing  there s no real point to protecting the software at all   With the headache that DRM can cause  it s not fair to the users who have actually paid for it to suffer

[c#] How to generate and validate a software license key?

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