This is the sixth post in a series of ten posts. The previous post explored Law 5: Si vis pacem, para bellum. The sixth law is one of the less controversial ones. Security is the result of many elements and principals that interact to build the appropriate defense. As a consequence, security cannot be stronger than its weakest element. Once more, Chinese general Sun Tzu explained it perfectly.

So in war, the way is to avoid what is strong and to strike at what is weak.

A smart attacker analyzes the full system and looks for the weakest points. The attacker focuses on these points. For instance, in 2012, about 80% of the cyber incidents implying data breach were opportunistic. Furthermore, they did not require proficient hacking skills. The targets were not properly protected. Attackers went after these easy targets.

Another example of attacking the weakest link is the use of side-channel attacks. Side-channel attacks are devastating, non-intrusive attacks that reveal secret information. The information leaks through an unintentional channel in a given physical implementation of an algorithm. These channels are the result of physical effects of the actual implementation. They may, for instance, be timing characteristics, power consumption, generated audio noise, or electromagnetic radiation.

As a general rule, the defender has to know its defense mechanisms. When trying to strengthen the defense, the designer must first focus on the weakest elements of its defense. Both for the defender and the attacker, the difficulty is to identify these weakest elements. They may take many forms: humans (see Law 7), design errors, bad implementations, limitations… White box testing is a good way to identify some weak points.

Know the hardware limitations; in this digital world, most of the technical effort is put into developing software. The focus is often on protecting the executed piece of code. Nevertheless, the code executes on hardware. Hardware introduces constraints that are often unknown to contemporary software developers. Ignoring these constraints may lead to interesting attack surfaces that a seasoned attacker will, of course, use. A typical example is the deletion of data in memory. Hardware memories have persistence even when erased or powered off. For instance, some data may be remaining DRAM several minutes after being powered off. Or, memories may have unexpected behavior when used in extreme conditions. The RowHammer attack is a perfect illustration.

Patch, patch, patch; Security is aging. New vulnerabilities are disclosed every week. As a result, manufacturers and publishers regularly issue patches. They are useless if they are not applied. Unfortunately, too many deployed systems are not properly patched. Smart attackers look first for unpatched targets.

Protect always your keys; Keys are probably the most precious security assets of any secure digital system. Their protection should never be the weakest link. Ideally, these protections should represent the strongest link as they defend the ultimate treasure. Keys need protection not only at rest but also while in use. A software implementation of cryptographic algorithms has to be carefully crafted, especially when operating in a hostile environment. In some contexts, the hardware implementation must resist side-channel attacks. Secure implementation of cryptography is expert work.