How to Protect Dlls

How to protect dlls?

The short answer is that beyond the obvious things, there is not much you can do.

The obvious things that you might want to consider (roughly in order of increasing difficulty and decreasing plausibility) include:

Static link so there is no DLL to attack.
Strip all symbols.
Use a .DEF file and an import library to have only anonymous exports known only by their export ids.
Keep the DLL in a resource and expose it in the file system (under a suitably obscure name, perhaps even generated at run time) only when running.
Hide all real functions behind a factory method that exchanges a secret (better, proof of knowledge of a secret) for a table of function pointers to the real methods.
Use anti-debugging techniques borrowed from the malware world to prevent reverse engineering. (Note that this will likely get you false positives from AV tools.)

Regardless, a sufficiently determined user can still figure out ways to use it. A decent disassembler will quickly provide all the information needed.

Note that if your DLL is really a COM object, or worse yet a CLR Assembly, then there is a huge amount of runtime type information that you can't strip off without breaking its intended use.

EDIT: Since you've retagged to imply that C# and .NET are the environment rather than a pure Win32 DLL written in C, then I really should revise the above to "You Can't, But..."

There has been a market for obfuscation tools for a long time to deal with environments where delivery of compilable source is mandatory, but you don't want to deliver useful source. There are C# products that play in that market, and it looks like at least one has chimed in.

Because loading an Assembly requires so much effort from the framework, it is likely that there are permission bits that exert some control for honest providers and consumers of Assemblies. I have not seen any discussion of the real security provided by these methods and simply don't know how effective they are against a determined attack.

A lot is going to depend on your use case. If you merely want to prevent casual use, you can probably find a solution that works for you. If you want to protect valuable trade secrets from reverse engineering and reuse, you may not be so happy.

How to protect against DLL Hijacking?

DLL hijacking can be achieved for apps that request an DLL without using an absolute path. This triggers a search process and by placing the compromised DLL higher in the search patch that the real version, it is possible to have execute malicious code. However, your installation directory should be first in the search path, so it does not seem to be applicable in your case. Nevertheless you should be careful when passing sensitive data to your DLL, for example passwords and usernames.

Prevent others from using my dlls

How I can prevent others from using my compiled libraries?

I would like to know how I can prevent someone from using my toothbrush. I have found a solution. I don't give them access to my toothbrush.

If you don't want people to use your software then do not give them your software.

If you want someone to be able to use the functionality of your code without knowing the implementation details then write a web service, put the software behind a web server, and people can use your software via the service you've provided. They only see the web server, not your implementation details.

Is it authenticode or strong names or something else?

No. Your scenario is that you wish to protect yourself, a software provider, from your users. That is exactly backwards. The .NET security system was designed to protect your users from bad software providers.

Both authenticode and strong names are systems whereby a user of software can obtain evidence that the software really was provided by the person they believe it to be provided by, rather than by evil hackers pretending to be you.

For example, suppose I become evil and send you a new version of System.DLL that I say is a software upgrade from Microsoft, but actually watches you type in passwords and emails them to me. I can send you a DLL called "System.DLL", but I can't send you one that has the Microsoft strong name because I can't make a Microsoft strong name. Only Microsoft can do that, because the Microsoft signing key is buried deep within Building 11 and guarded by sharks with laser beams or something. Strong names protect you from me; they don't protect Microsoft.

Similarly, strong names protect your customers from attackers pretending to be you. They don't protect you from your customers. You are not the one who is under attack; they are!

the libraries are part of a commercial desktop software

The commercial desktop software model presupposes that customers are trusted by software providers to use the software in accordance with their license. If you don't trust your customers then you need an entirely different model, like keeping the software on your web server and only allowing access via a web interface.

Protect C# DLL from third party

This is simply not possible.

The whole code access security system is based upon the notion that security decisions come from the user running the code, not from the author of the code. You, the code author, do not get to tell your users what their security decisions are; you are the user's servant, not the user's master.

Now, you can make it difficult for third-party code to use your code. You can use various security attributes to document that your intention is for third-party code to not use your code. Those are good steps, but they do not actually solve your problem in any world where your users are hostile to you. In the CAS model, users always win.

For example: you can have methods in your code do security demands that trigger stack walks to check to see whether everyone on the call stack has certain evidence associated with them. For example, the evidence "this DLL was signed with Guillaume's strong name private key, which is locked in a drawer in Guillaume's office" would be good evidence to check for. That would almost guarantee that everyone calling your code is also your code.

But that is not the purpose of strong name signing; the purpose of strong name signing is to help the user know that the code they think they are running actually came from you. Using a security tool for a purpose other than what it was intended for is dangerous, as we'll see. It gives you a completely false sense of security.

Suppose your user wants to make an application that isn't yours that uses your DLL. Users can write fully trusted code, and fully trusted code has the right to forge evidence. That's what "fully trusted" means. So the user makes an application that was not signed by you, but because the user can fully trust that application, the fully-trusted application is allowed to forge the evidence to say that the code comes from you.

For that matter, there is nothing stopping the user from simply taking your code, removing your signature, and replacing it with their signature. You can say that your EULA forbids that, and you can sue them if you find out, but you cannot do anything to stop them.

And heck, the user can disable the ENTIRE security system if they want, in which case anything can run.

You should consider whether you want to ship your DLL to customers at all. If it contains secrets that you do not want to get out, then don't share those secrets with thousands of customers, some of whom might be hostile to you. If you keep your DLL on your own servers and provide your service via the web then you don't ever ship your DLL to customers and therefore they cannot use it on their own machines for purposes other than what you intend.

Protecting source code of a DLL

If you are really worried about the intellectual property of your algorithm, then you have some options.

1) Patent the algorithm. I can hear the howls, but that is how you protect intellectual property in a legal manner.

2) Do not give the DLL with the algorithm to the users. Host the algorithm in the cloud and only allow customers to access it that way.

Some of the rest i don't understand. You say you can write C++, but don't know how to write an interface to C#? Use COM. C# can use COM libraries. Or, export your function in C++ and write a .NET interop library that calls the C++ DLL using PInvoke.

Protect .NET DLL from CLI modification

How to prevent spoofing of DLLs in .NET

Also, SmartAssembly is able to provide a feature which is able to bypass modification to your ilcode.

"Creates a proxy for calls to methods outside of your assembly to make it hard to find external calls."

How to prevent an applications DLL to be decompiled?

Before I get into anything else, I will state that it is impossible to protect your application entirely.

That being said, you can still make things more difficult. There are many obfuscators out there that will help you make it more difficult for someone to decompile your application and understand it.

http://en.wikipedia.org/wiki/List_of_obfuscators_for_.NET
.NET obfuscation tools/strategy

That's truly the best you can hope for.

Personally, I really wouldn't bother going too deep, if at all. You'll find that you are either spending too much money or time (or both) trying to protect your application from no-gooders. These are the same people who, no matter what barriers you throw up at them, will continue to try and given the nature of managed languages, they will most likely succeed. In fact, most obfuscators can be deobfuscated with simple tools... In the meantime, you've let other important features and bug fixes slip by because you spent more time and effort on security measures.

How to Protect Dlls