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Building Security Awareness in .NET Assemblies : Part 3 - Learn to break Strong Name .NET Assemblies

, 31 Oct 2004
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Building Security Awareness in .NET Assemblies : Part 3 of 3

Disclaimer

NeCoders shall not be held responsible for any cases of software/files being hacked due to the information provided in this article.

General Overview

Welcome back to part 3 of the Building Security Awareness in .Net Assemblies series. In this article, I will share with you the possibility of breaking Strong Named .Net Assemblies. Make sure you have already read through part 1 and 2 before continuing on.

Questions pertaining to Strong Name

I do believe many others have the same doubts as I do on whether Strong Name keys could really protect the assembly from being tampered. I would like to share my doubts with you in this article.

Questions:

  1. Question : Is Strong Name key secure?
    • Answer : Yes, Strong Name key uses RSA 1024 bit encryption.
  2. Question : Is Strong Name key breakable?
    • Answer : If you have enough computing power, time and knowledge on how to break RSA, the answer is yes.
  3. Question : Can Strong Name key be removed from .NET assemblies?
    • Answer : Yes, it can be removed very easily if you know how.

The Demonstration Test machine specifications :

  • Windows XP Professional Edition SP 1 1

  • Intel Pentium 4 2.6GHz

  • 256MB DDR-RAM

  • Visual Studio .Net 2003 Professional Edition

  • Microsoft .Net Framework 1.1
  1. Download the executable files that come with this article.
  2. Open your Visual Studio .Net 2003 command prompt.
  3. Make sure you are inside the CrackingIL/bin/debug directory.
  4. Type “ildasm CrackingIL.exe /out=CrackingIL.il”.

  1. You must be wondering, why we are repeating what we had done in part 1 and part 2 of the series. If you notice, the way to break Strong Name keys is by manipulating the Intermediate Language. But the problem is in part 2, we did modify the Intermediate Language and at the end when we tried to convert it back to an assembly, we will receive an error. I will explain in detail on which part of the Intermediate Language that you should modified to remove the Strong Name key.
  2. Open up the CrackingIL.il with a text editor. I use notepad.

  1. Take a look at the red boxes. From what I understand, each assemblies like System.Windows.Forms, System and mscorlib contains their own public key token and version number.
  2. Now, does our assembly contain a public key? The answer is yes. Before showing it to you, I will first show you 2 screenshots; one without Strong Name key and one with Strong Name key attached.

Without Strong Name :

With Strong Name :

  1. You will notice that the difference on both sides is that the Strong Name key assembly contains a public key. In order to tamper a Strong Named .Net Assembly, just remove that highlighted section. It will look like this.

  1. Now do some modifications to the existing Intermediate Language. You have to remove the registry checking so it will not prompt you for serial number or license. Look for this code.

  1. Then remove the lines of code from IL_0000 to IL_0075. You should have an output like this.

  1. Now just edit some text to prove that you have hijacked that Strong Named key .Net Assembly. Find the code with the phrase “Welcome to NeCoders” and replace it to “You are being hijacked, Strong Names are useless here”.

Change above to:

  1. Open your Visual Studio .Net 2003 command prompt, and type “ilasm CrackingIL.il”.

  1. Try to run CrackingIL.exe. You will see this.

  1. Congratulations! You had managed to manipulate .Net assemblies with Strong Name key attached to it.

Conclusion

Again, I hope you find this series of the article to be interesting. There will be more articles under this series, in terms of breaking and securing the .Net assemblies. Do check out article 4 when it is available as it will explaining the many theories in .NET security. This in return should provide you with a better understanding in this topic.

References

None

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Chua Wen Ching
Software Developer
Malaysia Malaysia
I am Chua Wen Ching and it is great to be part of CodeProject network Smile | :)

Comments and Discussions

 
GeneralIncredible PinmemberMark Focas1-Nov-04 5:42 
GeneralRe: Incredible Pinmemberchuawenching1-Nov-04 13:49 
GeneralRe: Incredible PinmemberJ.B3-Nov-04 11:27 
Don't forget this is very simplistic implementation of using strong names for security. A developer fluent in assembler could just as easily do something similar with compiled win32 code.
 
Strong names are also used for other reason like assigning privileges on a machine/enterprise based on the strong name or public (see Runtime security policy in .Net configuration, many Microsoft assembly are granted special privileges by their public key). .Restrictions can be applied for access to methods/classes in other systems based on public keys by using codeaccess security. (see StrongNameIdentityPermission).
 
Although as this article demonstrates Strong names could be compromised, if you employ a complete security model these changes can be detected. I would recommend that you sign all code, even if it is only deployed to desktops within your organisation. Security is not so much as preventing tamper completely, but making mechanisms that allow you to detect tamper and take appropriate action to prevent the system being compromised.
 
Rather than take the author at this word here suggest you get a good book on .Net security. The author is technically correct, his article is informative, but he has assumed that you are only using strong names for a single purpose which is only part of the .Net security model.
 
Sorry about the rant but as a consultant I often come into organisations where no security has been applied to .Net code, even on public web servers.
 
.Net code can be made secure, but like any good security system a compromise has to be expected and dealt with appropriately. To assume that you code/System will never be compromised just means you will never know when it is.

:(
GeneralRe: Incredible Pinmemberchuawenching3-Nov-04 13:39 
QuestionIncorrect understanding of strong names? PinmemberS. Senthil Kumar1-Nov-04 4:23 
AnswerRe: Incorrect understanding of strong names? PinsussAnonymous1-Nov-04 4:30 
GeneralEven easier: PinmemberVictor Vogelpoel1-Nov-04 2:18 
GeneralRe: Even easier: Pinmemberchuawenching1-Nov-04 4:18 
GeneralRe: Even easier: PinmemberVictor Vogelpoel1-Nov-04 9:13 
GeneralRe: Even easier: PinmemberFrans Bouma2-Nov-04 1:01 
GeneralRe: Even easier: PinmemberVictor Vogelpoel2-Nov-04 2:45 
GeneralRe: Even easier: PinmemberFrans Bouma2-Nov-04 2:56 

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