Trustworthy Virus

The concept of a "trustworthy virus" sounds like a joke, but let's pretend to take ot seriously just a little while. When the virus narcs on the suspect, it really ought to digitally sign the squealing, otherwise the accusation could have been made by any virus out there, not necessarily the reliable rat. But is it possible in principle to keep its key secret? I would think that, since the code has to exist on the target machine, anyone with a disassembler and enough persistance can find the key, and once it's out, it's out. The formerly trusetd virus could be impersonated by any comon virus, or even by someone that isn't really a virus at all! Yeah, I know, the whole thing is silly. But is the reasoning correct? George