A Brief Introduction to Security Engineering


One of the great myths is that security is complicated, hard to understand, and must be opaque to be effective. This is mostly fiction perpetrated by people who would rather you did not question the security theater they are creating in lieu of real security, by security practitioners who don’t really understand what they are doing, or lastly those who are trying to accomplish something in their interests under the false flag of security. This last one is why so much of the government “security” activities are not really about security, but about control – which is not the same. Designing and doing security can be complex, but understanding security is much easier than it is generally portrayed.

Disclaimer – This is not a comprehensive or exhaustive list / analysis. It is a brief introduction that touches on a few of the most practical elements of security engineering.

Security Axioms

Anytime I look at systems security, there are a few axioms I use to set the context, limit the scope and measure the effectiveness. These are:

  1. Perfect security is unachievable, and any practical security is the result of some cost driven tradeoff.
  2. Defining and understanding your threat model is step zero of any security solution. If you don’t know who are are defending against, the solution will not fit.
  3. Defining and understanding success. This means understanding what you trying to protect and what exactly protecting those elements means.
  4. Defending a system is more costly / difficult than attacking that same system. Attacker only need to be successful once, but defenders need to be successful everytime.
  5. Security based on secrecy is weaker than security based on strength. Closed security solutions are more likely to contain flaws that weaken the security versus open security solutions. Yes – this has been validated.

The first of these is a recognition that a security is about a conflict between a system / information defender and an attacker on that system. Somebody is trying to take something of yours and you want to stop them. Each of these two parties can use different approaches and tools to do this, with increasing costs – where costs are monetary, time, resources, or risks of being caught / punished. This first axiom simply states that if an attacker has infinite time, money, resources, and zero risk, your system will be compromised because you are outgunned. For less enabled attackers,  the most cost effective security is that which is just enough to discourage them so they move on to an easier target. This of course leads understanding your attacker, and the next axiom – know your threat.

The second axiom states that any security solution is designed to protect from a certain certain type of threat. Defining and understanding the threats you are defending against is foundational to security design since it will drive every aspect of the system. A security system to keep your siblings, parents, children out of your personal data is completely different than one designed to keep out cyber extortionists out of your Internet accounts.

The third axiom is based on the premise that most of what your system / systems are doing requires minimal protected (depending on the threat model), but some parts of it require significant protection. For example – my Internet browsing history is not that important as compared with my password and account access file. I have strong controls on my passwords and account access (eg KeePass), and my browsing history is behind a system password. Another way to look at this to imagine what the impact could be if a given element were compromised – that should guide the level of protection for that item.

The fourth axiom is based on the premise that the defender must successfully defend every vulnerability in order to be successful, but the attacker only has to be successful on one vulnerability – one time to be successful. This is also why complex systems are more prone to compromise – greater complexity leads to more vulnerabilities (since there are more places for gremlins to hide).

The fifth one is the perhaps the least obvious axiom of this list. Simply put the strength of some security control should not be based on the design being secret. Encryption protocols are probably the best example of how this works. Most encryption protocols over the last few decades are developed, and publicized within the peer community. Invariably, weaknesses are found and corrected, improving the quality of the protocol, and reducing the risk of an inherent vulnerability. These algorithms and protocols are published and well known, enabling interoperability and third party validation reducing the risk of vulnerabilities due to implementation flaws. In application, the security of the encryption is based solely on the key – the keys used by the users. The favorite counter example is from the world of traditional pin tumbler locks , in which locksmith guilds attempted to keep their design / architecture secret for centuries, passed laws making it a crime to possess lock picks or knowing how to pick a lock unless you were a locksmith. Unfortunately, these laws did little to impede criminals and it became an arms race between lock makers, locksmiths and criminals, with the users of locks being kept fairly clueless. Clearly of the lock choices available to a user, some locks were better, some were worse, and some were nearly useless – and this secrecy model of security meant that users did not have the information to make that judgement call (and in general they still don’t). The takeaway – if security requires that the design / architecture of the system be kept secret, it is probably not very good security.

Threat Models

In the world of Internet security and information privacy, there are only a few types of threat models that matter. This is not because there are only a few threats, but because the methods of attack and the methods to defend are common. Generally it is safe to ignore threat distinctions that don’t effect how the system is secured. This list includes:

  1. Immediate family / Friends / Acquaintances – Essentially people who know you well and have some degree of physical access to you or the system your are protecting.
  2. Proximal Threats : Threats you do not know, but are who are physically / geographically close to you and the system you are protecting.
  3. Cyber Extortionists : A broad category of cyber attackers whose intent is to profit by attacking and compromising your information. This group generally targets individuals, but not a specific individual – they look for easy targets.
  4. Service Compromise : Threats who attack large holders of user information – ideally credit card information. This group is looking for bulk information is not targeting individuals directly.
  5. Advanced Persistent Threats (APTs) : Well equipped, well resourced, highly capable and persistent. These attackers are generally supported by governments or large businesses and their targets are usually equally large. This group plans and coordinates their attacks with a specific purpose.
  6. Government (NSA / CIA / FBI / DOJ / DHS / etc): Currently the biggest, baddest threat. They have the most advanced technical resources, the most money, and they use National Security Letters when those are not enough. The collect data in bulk, and they target individuals.

From a personal security perspective we are looking at threats most likely to concern any random user of internet services – you. In that context, we can dismiss a couple of these quickly. Lets do this in reverse order:

Government (NSA et al) – If they are targeting you specifically, and you use Internet services – you are need of more help than I can provide in this article. If your data is part of some massive bulk data collection – there is very little you can do about that either. So in either case,  in the context of personal data security for Joe Internet User, don’t worry about it.

Advanced Persistent Threats (APTs) – Once again, much like the NSA, it is unlikely you would be targeted specifically, and if you are your needs are beyond the scope of this article. So – although you may be concerned about this threat, there is very little you can do to stop this threat.

Service Compromise – I personally pay all of my bills online, and every one of these services wants to store my credit card in their database. Now the question you have to ask is if (for example), the Verizon customer database is compromised and somebody steals all of that credit card information (with 10s of millions of card numbers) and uses them to spend 100s of millions of charges – is Verizon (or any company in that position) going to take full responsibility? Highly unlikely – and that is why I do not store my credit information on their systems. If they are not likely to accept responsibility for any outcome, should you trust them with your credit?

Cyber Extortionists – The most interesting and creative of all these threat classes. I continue to be amazed at every new exploit I hear about. Examples include mobile apps that covertly call money transfer numbers (eg 1-900 numbers in US), or apps that buy other apps covertly. Much like the Salami Slicing attacks (made famous in the movie Office Space), individual attacks represent some very small financial gain, but the hope is that collectively they can represent significant money.

Proximal Threats – If somebody can physically take your laptop, tablet, phone, they have a really good shot at all of the information on that device. Many years ago, I had an iPhone stolen from me on the Washington DC metro, I had not enabled the screen lock, and I had the social security numbers / birthdays of my entire family in my contacts. And yes, there were false attempts to get credit based on this information within hours – unsuccessfully. I now use / recommend everybody use some device access lock, and encrypt very sensitive information in some form of locker. Passwords / accounts and social security numbers in KeePass and sensitive file storage in TruCrypt. These apps are free and provide significant protection for Just In Case. Remember physical control / access to a device is its own special type of attack.

Friends / Family / Acquaintances – In most cases, the level of security to protect from this class of threat is small. More importantly, it is crucial to understand what it is you are trying to protect, why are you protecting it, and what are your recovery options. To repeat – what are your recover options? It is very easy to secure your information, and then forget the password /  passphrase  or corrupt your keyfile. Compromise of private data in this context is orders of magnitude less likely than you locking yourself out of your data – permanently. Yes, I have done this and family photos on a locked TrueCrypt partition cannot be recovered in your lifetime. So when you look at security controls to protect from this threat model, look for built in recovery capabilities and only protect what is necessary to protect.


Fundamentally security engineering is about understanding what you are trying to protect, who / what your threat is, and determining what controls to use to impede the threat while not impeding proper function. Understanding your threat is the first and most important part of that process.

Lastly – I would encourage everybody who finds this the least bit interesting to either read Bruce Schneier’s blog and his books. He provides a very approachable and coherent perspective on IT security / Security Engineering.


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