Securing an assembly so that it can't be used by a third party

Question

I have written an assembly I don't want other people to be able to use. My assembly is signed with a strong name key file, but how do I secure the code so that only my other assemblies signed with the same key can call the members in this assembly?

Answer 1

There are a few options, none very effective as they merely will make things a tiny bit difficult, but would not prevent a committed user to work around any restriction:

On every one of your entry points, you can call Assembly.GetCallingAssembly() and compare the result with a list of assemblies that are allowed to call into your library, and throw an exception otherwise.

You could use a tool like ilmerge to merge your assemblies into your main application, and flag all of the internals as private. Combine with an obfuscator to make the results slightly better protected.

But securing an assembly is as solid as securing a computer where the attacker has physical access to it: there is very little that you can do to protect the contents once physical access is granted.

Answer 2

You need to add StrongNameIdentityPermissionAttributeattributes to demand security

eg

[assembly:StrongNameIdentityPermissionAttribute(SecurityAction.RequestMinimum, 
 PublicKey="00240000048000009400000006020000002400005253413100040000010001005" +
 "38a4a19382e9429cf516dcf1399facdccca092a06442efaf9ecaca33457be26ee0073c6bde5" +
 "1fe0873666a62459581669b510ae1e84bef6bcb1aff7957237279d8b7e0e25b71ad39df3684" +
 "5b7db60382c8eb73f289823578d33c09e48d0d2f90ed4541e1438008142ef714bfe604c41a4" +
 "957a4f6e6ab36b9715ec57625904c6")]

see this msdn page for more info

or do it in code see this example