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In fact, private methods are implemented in C# that can still be searched with Reflection.

What I am going to do is to write public string Encrypt(string data) and private string Decrypt(string cipher) methods to perform encryption and decryption.

Unfortunately, if someone knows .NET framework, he can use Reflection to find Decrypt methods and it decrypt everything that is encrypted.

It seems that is not that secure. So I want to make Decrypt method to truly private method.

But how to do that?

Updated 09 Jan 2012 10:52PM Sydney Time

bdares provides the technical explanation of this question

Eric Lippert provides the political explanation of this question

Thanks both experts!

Alex Yeung
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5 Answers5

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You can't. If the attacker has access to your code, compiled or source, he can trace your program and find where it's being encrypted or decrypted.

You can add a layer of security by storing the key in a separate location, but generally if the attacker is executing code on your server, you're already screwed.

(You're only worried about this if the attacker is executing code on your server, because otherwise it doesn't matter whether or not the method is private. Also, he can't use reflection to find method names unless he's executing code on your server. In short: you're worrying about the wrong thing here.)

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    Good points. You could also say that `private` is not a concept for security. That's a common misconception I think. – Stefan Steinegger Jan 09 '12 at 06:28
  • I agree your points.You solved my question **technical**. Another question is: does that mean there is **NO** truly secure program we can make if user got the binary of the program? – Alex Yeung Jan 09 '12 at 11:47
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    @AlexYeung: "truly secure" **against what threat**? You can make truly secure programs; you do so when the *cost* to the attacker of them making good on their *threat* is higher than the *benefit* they attain through a successful attack. Describe the threat and the benefit of a successful attack, and only then can you design a security system as a response. – Eric Lippert Jan 09 '12 at 14:38
  • This isn't just a theoretical thing. There is negligible benefit to obsfucating your code if an attacker is executing code on your system. It's like painting your bathroom door the same color as your wall, just in case someone breaks down your front door. It won't fool them, and they're probably not interested in your plumbing anyways. –  Jan 09 '12 at 15:49
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Your fundamental problem is that you've got the trust model wrong. If someone can use reflection then they are the user. You are the software provider. You work for them. Trust flows from them, not from you. They are the person who has to trust you, not you them.

If you don't trust the user then do not sell them your software in the first place. Don't sell weapons to people who you believe plan to attack you.

Eric Lippert
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  • Fair enough but then why are going many companies to great length to try and hide their licensing/encryption/copyprotection mechanisms? Users are not trustworthy - you give them something they will break it and misuse it. Not everybody but quite a few – ChrisWue Jan 09 '12 at 06:56
  • "Don't sell weapons to people who you believe plan to attack you." that happen so many times – Lukasz Madon Jan 09 '12 at 08:36
  • Thank you very much. I also want to mark your reply as my answer but only one answer is possible in stackoverflow. Because your reply definitely helps me to answer questions from my client. – Alex Yeung Jan 09 '12 at 11:51
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    @ChrisWue: And those measures are about as effective as locking your car doors. Cars get stolen all the time that are locked; car door locks keep out honest people who don't want to steal your car in the first place. Having replaced a number of broken windows in my time, I no longer lock my car. It is not an effective security measure against the threat. – Eric Lippert Jan 09 '12 at 14:36
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I believe you are referring to obfuscation, which is an attempt to hide/disguise code from being read by humans when opened in program such as Reflector. Supplied within Visual Studio is a community use license for PreEmptive Solutions dotfuscator which will provide this functionality on small projects, and also for Windows Phone projects (if you download the add-on). There are also commercial platforms available too, from the same vendor and others .

This blog post explains a little more.

svick
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LewisBenge
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  • You may add some information what obfuscation is doing in relation to the question. And what it is not doing. Usually it only changes the name to make the method hard to discover. – Stefan Steinegger Jan 09 '12 at 06:25
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If you're creating your own encryption method, you're doing it wrong. People who know way more about encryption than you or I have already come up with excellent methods for encryption, and MS has implemented most of them already.

For good encryption, it's the keys, not the method, that makes encryption secure. Keep the keys safe and the algorithm can (and should) be published for all to see.

If you're trying to distribute both content and keep it encrypted, aka DRM, you're most probably doomed to failure unless you can keep the keys very well hidden in hardware, and even that will only buy you some time -- maybe months, maybe years.

Allen
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I am not sure about your exact application. But if you are selling a product to a customer who will be doing both the Encryption and Decryption on their own system, then there is no way to keep the encryption secret from them. But you can instead allow them to generate a new Private Key for their own use. In this way each customer's data is 'secure' in regards to other customers; though obviously still not so secure within the same customer's site. In other situations where you control the encrypted content you can also look into creating a private master key to be generated on your side and only allow the customer to have a public key.

Andrew Hanlon
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