Mar 13, 2020

Strong Name Signing in .NET

Using a strong name protects your assembly against manipulation.

For example your software using a class named Class1 in ClassLibrary1 namespace.

If you compile a new dll contains ClassLibrary1.Class1 then you can use this dll instead of original one.

This is very basic example but it can be applicable in real life.

To be sure that your software is using your assembly version then you can sign your project.

Microsoft description is:
When a strong-named assembly is created, it contains the simple text name of the assembly, the version number, optional culture information, a digital signature, and the public key that corresponds to the private key used for signing.
Example here shows signing process in Visual Studio. You can use also command line tool (sn.exe).


Our solution has 2 projects, executable and a class library. Main program is using ClassLibrary1.Class1

using ClassLibrary1;
using System;

namespace strongname
    class Program
        static void Main(string[] args)
            Console.WriteLine("strongname loaded");

            Class1 c = new Class1();


using System;

namespace ClassLibrary1
    public class Class1
        public Class1()
            Console.WriteLine("Hello from Class1");


Right click to project in VS and select Properties. In Signing tab check "Sign the assembly" and select "New" in listbox. Enter password for your private key.

This will create a pfx (Personal Information Exchange Format) file in your project folder.


strongname loaded
Hello from Class1


If you create a new project with the same namespace, class and functions then you can use the new dll with current executable.

For test change the code and recompile dll in outside of project folder.

csc.exe /t:library /out:ClassLibrary1.dll Class1.cs
Then replace the old ClassLibrary1.dll with the new file.


When you try to run your executable an error will be shown because the new dll has not signed with correct key.


Unhandled Exception: System.IO.FileLoadException: Could not load file or assembly 'ClassLibrary1, Version=, Culture=neutral, PublicKeyToken=77d705841860f240' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
at strongname.Program.Main(String[] args)

If you completely remove signing from your projects then your manipulated dll will work.


Anonymous said...

Where does one find this mythical signing tab? I see project settings, SQLCLR--which has a signing button but doesn't have an effect on the signature of the assembly as SQL Server keeps complaining--, SQLCLR Build, build, sqlcmd variables, build events, debug, reference paths, code analysis. I don't see any signing tab in VS 2019. I saw someone asking about it on microsoft's support site, but the bot shot it down as 'depressing', I guess because it was closed without answer even with the requested info.

Oktay said...

If you are using "SQL Server Database Project" you can see Signing button in SQLCLR tab. Enter a name and password then build. It should work.
.Net framework and .Net core projects still have Signing tab.
Your problem is not clear.