I want to make a short philosophical comment about how some approaches to building security are wrong.

Let’s move back in time to the last decade of the XX century, to the 90’s... Back in those days one of the most annoying problems in computer security was viruses, or, more precisely, executable file infectors. Many smart guys were working on both sides to create more stealthy infectors and also better detectors for those infectors…

Russian virus write Z0mbie with his Mistfall engine and Zmist virus went probably closest to the Holy Grail in this arm race – the creation of an undetectable virus. Peter Szor, a Symantec’s chief antivirus researcher, wrote about his work in 2001:

Many of us will not have seen a virus approaching this complexity for a few years. We could easily call Zmist one of the most complex binary viruses ever written.

But nothing is really undetectable if you have a sample of the malware in your lab and can spent XXX hours analyzing it – you will always come up with some tricks to detect it sooner or later. The question is – were any of the A/V scanners back then ready to detect such an infection if it was a 0day in the wild? Will any of the today’s scanners detect a modified/improved Zmist virus, or would they have to count on the virus author being nice enough to send them a sample for an analysis first?

Interestingly, file infectors stopped being a serious problem a few years ago. But this didn’t happen because the A/V industry discovered a miracle cure for viruses, but rather because the users’ habits changed. People do not exchange executables that often as 10 years ago. Today people rather download an executable from the Web (legal or not) rather then copy it from a friend’s computer.

But could the industry have solved the problem of file infectors in an elegant, definite way? The answer is yes and we all know the solution – digital signatures for executable files. Right now, most of the executables (but unfortunately still not all) on the laptop I’m writing this text on are digitally signed. This includes programs from Microsoft, Adobe, Mozilla and even some open source ones like e.g. True Crypt.

With digital signatures we can "detect" any kind of executable modifications, starting form the simplest and ending with those most complex, metamorphic EPO infectors as presented e.g. by Z0mbie. All we need to do (or more precisely the OS needs to do) is to verify the signature of an executable before executing it.

I hear all the counter arguments: that many programs out there are still not digitally signed, that users are too stupid to decide which certificates to trust, that sometimes the bad guys might be able to obtain a legitimate certificate, etc...

But all those minor problems can be solved and probably will eventually be solved in the coming years. Moreover, solving all those problems will probably cost much less then all the research on file infectors cost over the last 20 year. But that also means no money for the A/V vendors.

Does it mean we get a secure OS this way? Of course not! Digital signatures do not protect against malicious code execution, e.g. they can't stop an exploit from executing its shellcode. So why bother? Because certificates allow to verify that what we have is really what we should have (e.g. that nobody infected any of our executable files). It’s the first step in ensuring integrity of an OS.

The case of digital signatures vs. file infectors is a good example of how problems in security should be addressed. But we all know that the A/V industry took a different approach – they invested zillions of dollars into research on polymorphic virus detection, built advanced emulators for analysis of infected files, etc. The outcome – lots of complex heuristics that usually work quite well against known patterns of infection, but are often useless against new 0day engines and also are so complex that nobody really knows how many false positives they can produce and how buggy the code itself is. Tricks! Very complex and maybe even interesting (from a scientific point of view) tricks.

So, do I want to say that all those years of A/V research on detecting file infections was a waste time? I’m afraid that is exactly what I want to say here. This is an example of how the security industry took a wrong path, the path that never could lead to an effective and elegant solution. This is an example of how people decided to employ tricks, instead looking for generic, simple and robust solutions.

Security should not be built on tricks and hacks! Security should be built on simple and robust solutions. Oh, and we should always assume that the users are not stupid – building solutions to protect uneducated users will always fail.