Question

I have a requirement to have a restriction in an assembly such that only assemblies that are signed with a given key can use it. I am inexperienced, but what I understand is signing is done to help identify who created the assembly. Hence, just signing this assembly should not be enough to ensure that all calling assemblies are signed. Probably the reverse is true, i.e. if an assembly is signed, all the assemblies it depends on should be signed(by the same key perhaps). What would be the way to meet the requirement?

Answer 1

You can use PublisherIdentityPermissionAttribute.

If you apply PublisherIdentityPermissionAttribute to MyClass class then only classes in assemblies signed by the mycert.cer certificate can use your class. You need to put SecurityAction.Demand

All callers higher in the call stack are required to have been granted
the permission specified by the current permission object

Use like

[PublisherIdentityPermission(SecurityAction.Demand, CertFile = "mycert.cer")]
public class MyClass 

You can also use it on assembly level to protect entire assembly (however assembly level security will not work since .Net 4.0 unless you set <NetFx40_LegacySecurityPolicy enabled="true"/> in configuration).