Does signing an assembly or an exe with a digital certificate makes it secure against tampering attacks?

Question

I tried creating a temporary certificate using makecert and creating a spc from the certificate using cert2spc. I signed some exe with the generated spc. I then use the binary editor in VS 2008 to flip some bits (tampered it) in the exe. To my surprise I was able to execute the application.

I was expecting that the system will detect the tampering and will complain. Hence the question.

Any guidance would be appreciated.

Answer 1

No, because any code in the binary to check the signature can also be tampered with.

I recommend obtaining a copy of IDA Pro and disassembling one of your binaries. After you have the raw assembler you can edit specific opcodes using a hex editor. In short, this is the tactic that the cracker community uses. I don't believe that there will ever be a way to stop this, the ps3 and xbox360 both use digital signatures to protect their binaries, but this doesn't stop piracy.