Now when you compile your project, Visual Studio will automatically sign your assembly with the new strong name key you have just created.
Or if you prefer to use the command-line, you can create a key pair file with the strong name utility sn.exe in the .NET SDK, for example:
sn -k MyKey.snk
Then you reference that key file to when compiling your code with the C# compiler csc.exe:
csc /keyfile:MyKey.snk MyCodeFile.cs
What does it mean to sign an assembly?
.NET uses digital signatures to verify the integrity of an assembly. The signatures are generated and verified using public key cryptography, specifically the RSA public key algorithm and SHA-1 hash algorithm. The developer uses a pair of cryptographic keys: a public key, which everyone can see, and a private key, which the developer must keep secret.To create a strong-named assembly, the developer signs the assembly with his private key when building the assembly. When the system later loads the assembly, it verifies the assembly with the corresponding public key.
How do I sign an assembly?
When you compile your assembly with a strong name key file, the compiler digitally signs the assembly:
- The compiler calculates the cryptographic digest (a hash) of your assembly contents. This is known as the compile-time digest. Modifying just a single byte of your assembly will change this hash value.
- The compiler encrypts the digest using the 1024-bit private key from your public-private key pair file.
- The compiler then stores the encrypted digest and public key into the assembly.
How does the system verify a signed assembly?
Sometime later, when an application attempts to load your signed assembly:
- The .NET assembly loader calculates the cryptographic digest of the current assembly contents. This is known as the run-time digest.
- The loader extracts the stored compile-time digest and public key from the assembly.
- The loader uses the public key to decrypt the compile-time digest.
- The loader then compares the run-time digest with the decrypted compile-time digest to ensure they match. If not, then the assembly has been modified since you compiled it, and the assembly load fails.
This process is different when loading shared assemblies from the GAC. Because assemblies are verified when they are first installed into the GAC–and they cannot be modified while in the GAC–the .NET assembly loader does not verify an assembly when loading it from the GAC. This can improve the startup speed of your application if you load many shared assemblies.
What is delay signing?
Delay signing is signing an assembly with its strong name public key, which is freely distributable, instead of using the private key as usual. This allows developers to use and test a strong-named assembly without access to the private key. Then at a later stage (typically just before shipping the assembly), a manager or trusted keyholder must sign the assembly with the corresponding private key.
How do I protect my private keys?
Private keys must remain secret. A hacker with your private key could spoof your signed assemblies by replacing or injecting them with a virus or other malicious code. There are a few strategies you can use to protect your private keys:
- Password Protection. As shown above, Visual Studio will allow you to protect your strong name key file with a password.
- Delay Signing. As mentioned above, delay signing enables your development team to build and test your assembly without access to the private key.
- Cryptographic Container. One of the most secure ways to protect your strong name key is to store it in a secure cryptographic .
How many private keys should I have?
There are three main strategies for how many private keys a developer should use:
- One private key for all your applications and assemblies
- One private key for each application (an application may have multiple assemblies)
- One private key for each assembly
Which option to use depends on your security situation and risk tolerance. With option 1, it's easier to keep a single key secure, but if your one private key is compromised, then all of your assemblies are compromised. With option 3, there are more keys to manage and hence lose, but if one key is compromised, then only one of your many assemblies is compromised. I recommend option 2 or 3 to reduce your overall exposure.
Are there problems with using strong names?
Strong names are not perfect. There are some issues to consider when using strong names:
- Requires Exact Match. If you use strong names, your application or library must load the assembly with the exact strong name that you specify, including version and culture. Note that you can bypass this requirement with a publisher policy.
- Cannot Lose Private Key. If your private key is lost or stolen, the security of your assembly is compromised. You will be forced to re-issue a new assembly signed with a new public-private key pair.
- Cannot Stop Full Replacement. Strong names cannot prevent a hacker from removing the strong name signature, maliciously modifying your assembly, re-signing it with his own key, and then passing off his assembly as yours. The user must have some way to ensure the public key they have from your assembly is valid and truly came from you. Note that you can use more sophisticated signing to help with this issue.
Where are shared assemblies stored?
A shared assembly is used by multiple applications. You can store shared assemblies pretty much anywhere. However, the challenge is to ensure that all dependent applications can find the shared assembly. The recommended way to ensure this is to store shared assemblies in the Global Assembly Cache.
What is the Global Assembly Cache (GAC)?
The Global Assembly
Cache is a system folder (typically C:\Windows\assembly) that contains .NET shared assemblies. Companies that wish to share assemblies with others or even just among their own applications typically store these shared assemblies in the GAC. All of the .NET framework libraries are stored in the GAC.
Why should I store my shared assemblies in the GAC?
You should install assemblies in the GAC only when necessary. As a general guideline, assemblies should be kept private and stored in the application's folder unless you explicitly need to share them. There are some benefits to storing shared assemblies in the GAC:
Global Location
The GAC is the known standard location for .NET shared assemblies. When an application attempts to load an assembly, the GAC is one of the first places it looks. If there's any chance that an application outside your control may someday require access to your shared assembly, you should install your assembly in the GAC so the application is sure to find it.
Security
The GAC is a system folder typically protected by administrator rights. Once an assembly is installed in the GAC, it cannot be easily modified. Also, assemblies stored in the GAC must be signed with a cryptographic key. These protections make it difficult to spoof your assembly, in other words, replace or inject your assembly with a virus or malicious code.
Version Management
.NET allows multiple versions of the same assembly to reside in the GAC so that each application can find and use the version of your assembly to which it was compiled. This helps avoid DLL Hell, where applications that may be compiled to different versions of your assembly could potentially break because they are all forced to use a single version of your assembly.
Faster Loading
The system verifies assemblies when they are first installed in the GAC, eliminating the need to verify an assembly each time it is loaded from the GAC. This can improve the startup speed of your application if you load many shared assemblies.
Why would I avoid the GAC?
The GAC should contain "global" shared assemblies only, so there are many instances when you would NOT install an assembly in the GAC:
- The assembly is private to your application and not to be shared with other applications.
- You want to use XCOPY or FTP copy to install a .NET application to a single folder. This eliminates the need to access the Registry and GAC and does not require administrator rights.
- The assembly is not strong-named or you do not want tight version control.
- COM interop and unmanaged code do not require the GAC.
How do I see assemblies installed in the GAC?
The .NET Framework includes an Assembly Cache Viewer. Open Windows Explorer, enter %windir%\assembly in the address bar, and all global assemblies will appear in a special view that shows the assembly name, version, culture, public key token, and processor architecture.
Can I install multiple versions of the same assembly in the GAC?
Yes. Normally you would not be able to have two files with the same name in a Windows folder, but the GAC is a special folder that stores its contents by strong name. Hence, two assemblies with the same name but different versions or cultures may coexist in the GAC.
How do I add/remove assemblies from the GAC?
Assemblies added to the GAC must be signed with a strong name. There are multiple ways to add/remove assemblies from the GAC:
Windows Installer
The preferred way to add/remove assemblies from the GAC is with Microsoft Windows Installer 2.0. Visual Studio includes a limited version of Windows Installer, and most major setup programs such as InstallShield also use Windows Installer. There are benefits to using Windows Installer:
- Windows Installer provides a simple interface for developers to add/remove shared assemblies in the GAC and can handle private assemblies as well.
- Installer provides a familiar interface and setup experience for the user.
- Installer can also install application shortcuts and supporting files such as ReadMe and license agreements and can run other installation programs and scripts.
- Installer registers and tracks references to assemblies installed in the GAC to determine which assemblies are still required.
- Installer can repair and patch assemblies and rollback unsuccessful installations.
- Installer can install assemblies on-demand as they are needed by applications.
GAC Utility
The .NET developer's kit includes a command line utility GACutil.exe to interact with the GAC. This utility is intended for use in a development environment only and should not be used to install assemblies on a client PC because:
- The GACutil license agreement states that it is not freely distributable.
- GACutil is part of the .NET SDK, which may not be installed on many target PCs.
- GACutil lacks many important features found in Windows Installer such as assembly repair and rollback.
Assembly Cache Viewer
Using the Assembly Cache Viewer shown above, you can drag & drop assemblies from any folder into the GAC and also delete assemblies installed in the GAC.
.NET Framework Configuration Administrative Tool
To access the .NET Framework Configuration Tool:
- Click Start > Control Panel.
- Double-click on Administrative Tools.
- Double-click on Microsoft .NET Framework 2.0 Configuration.
- Ensure My Computer is selected in the tree.
- In the Tasks group, click the Manage the Assembly Cache link.
- Two links appear, enabling you to view and add assemblies in the GAC.
How do I access the GAC programmatically?
You can access the GAC from code with the target="_blank">fusion.dll library. Here is an excellent C# wrapper for the GAC.
You are strongly advised NOT to access the GAC from code unless you are creating an administrative or setup tool. The Fusion APIs expose your application to the inner workings of assembly binding and may cause your application to fail on future .NET versions.
How do I add my shared assembly to the Visual Studio "Add Reference" dialog?
If you add an assembly to the GAC, it will NOT automatically appear in the Visual Studio "Add Reference" dialog; instead you must add your assembly manually.
Happy programming!!!
