Home > Articles

This chapter is from the book

We Can Measure How Secure Our Systems Are

Cybersecurity professionals cringe at the question “how secure are we?”10 The person asking probably thinks there is a nice straightforward answer, such as 90% secure or very secure. Professionals wince because this is not something they can meaningfully quantify. They wish the question could be asked more precisely.

People want a number. They want a value they can point to, saying, “This is how secure I am: I am a 5.” It’s something they can hang on to or publicize to make themselves feel better.

Without a good definition, however, we are left without good metrics. And, to scientists and engineers, metrics are important! As Lord Kelvin wrote, “I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind.”11

“Wait!” some people might say. “What about the traditional C-I-A measures?” In computer science textbooks and classes, we find references to confidentiality, integrity, and availability as the basic components of cybersecurity. But those are also poorly chosen. For instance, what is the dimension of integrity? How do I have two more units of confidentiality? Are three units of confidentiality more important than two units of availability? Furthermore, those are not orthogonal properties: If data is overwritten (poor integrity controls), it is no longer available.

The shortcomings of the C-I-A model are not new realizations. Donn Parker developed his Hexad model12 to add three more properties (Control, Correctness, Utility), and John McCumber developed his Cube model13 to better focus controls on goals and whether data is at rest or in transit. Those are not the only models, either! None of those models fixed the underlying problems, which are rooted in not having a good definition of “secure.”

All of this is complicated by two facts not reflected in most practice (and rarely in commercial advertising): (1) It is impossible to make an arbitrary system secure against all threats and still be usable, functional, and productive, and (2) all security is relative to a security policy. The first fact is relatively easy to illustrate. Think about protecting a PC against a planet-killing asteroid, against the combined cyber attack capabilities of North Atlantic Treaty Organization (NATO) forces, and an invasion of Unidentified Flying Objects (UFOs) carrying telepathic lizard people.14 Those are all potential threats, and there is simply no current, complete defense against all of them—singly or combined. As to usability, consider the quote that opens this chapter.

The second fact is a bit more subtle and not as well understood as it should be. It is about defining what we are protecting and what we are protecting against. Situations and environments are different. If I am a grad student, I might not care about protecting my recipe for chocolate chip cookies on my home computer; if I am an elf running a baked goods empire from my hollow tree, I probably care a great deal about protecting my recipe! In both cases, it might be the same computing hardware, the same underlying Operating System (OS), and perhaps even the same recipe—but the risks and policies are definitely different! As Figure 1.1 illustrates, one size does not fit all.


FIGURE 1.1 Security must fit a myriad of users and situations.

This policy difference also shows up when comparing a home system versus a bank versus the computers used in the Situation Room at the White House. The differences are also interwoven with risk issues (How likely is the system to be attacked, and by whom?) and consequences (Is loss of my cookie recipe equal to compromise of the bank’s balance sheets?) That, in turn, drives decisions about how much time, capital, and effort to spend on controls, countermeasures, and recovery mechanisms. We cannot afford to protect every system at the same level! Protecting a student’s cookie recipe against potential theft by a team of elite cyber criminals is not likely to be a good use of funds and time, but the elves might view the tradeoffs differently.

This, too, has been understood for quite some time. Buried in a lesser-known Request for Comments (RFC) is a gem worth noting here. Internet Engineering Task Force (IETF) RFCs are individually numbered publications that include Internet standards, ideas, and occasional humor. For example, domain names and the Domain Name System (DNS) that underpin our Internet experiences are described in RFC 1034. The Internet Security Glossary is written in RFC 4949.15 It is a fascinating and informative read. It defines terms such as “port scan” and “vulnerability.” Within this document is an entry for Courtney’s Laws, which Robert Courtney defined decades ago:

Courtney’s Laws

Principles for managing system security that were stated by Robert H. Courtney, Jr.

  • Courtney’s first law: You cannot say anything interesting (i.e., significant) about the security of a system except in the context of a particular application and environment.

  • Courtney’s second law: Never spend more money eliminating a security exposure than tolerating it will cost you.

    • First corollary: Perfect security has infinite cost.

    • Second corollary: There is no such thing as zero risk.

  • Courtney’s third law: There are no technical solutions to management problems, but there are management solutions to technical problems.

Bob Courtney was a pioneer in cybersecurity and well understood the limitations of what we can achieve. His three laws enrich the context of cybersecurity, and everybody who deals with cybersecurity should know them.

Trust and Risk

Notice that Bob referred to “risk.” Those who have studied the field in depth generally prefer to talk about either trust, risk, or both rather than security. We try to measure how much we trust a system to address risk. One of the most influential documents in cybersecurity was the Trusted Computer System Evaluation Criteria (TCSEC), also known colloquially as “the Orange Book” because of the color of its cover. It was issued by the U.S. National Computer Security Center in 1983 and described how to build computers with increasing levels of trust.16 We might say they were levels of security, but it was understood early on that it made more sense to speak about what we could do to increase our trust in a system to operate according to policy and minimize risk.

Though related, trust is sometimes confused with security. Consider SSL/TLS17 as an example. We have taught users to look for the lock icon in their browsers when visiting their bank’s website. Somewhere along the line, users also began to use this heuristic for safety to decide if the website was legitimate and not a phishing attempt; that is not necessarily correct, as Figure 1.2 illustrates.18 Uh oh! TLS (usually) provides a secure connection, but this connection could still be with an untrustworthy attacker. This illustrates the difficulty in defining “secure”: The network connection is secure (in a sense), but the actual session is not.


FIGURE 1.2 A lock icon does not necessarily mean there is no risk.

Importantly, as noted in the corollary to Courtney’s Second Law, there is no way to eliminate all the risk. For instance, strong passwords lower the risk of attackers accessing our account, but all passwords can eventually be guessed with enough brute force. The first myth in this chapter acknowledges that it’s difficult to quantify how much risk we face and how much cybersecurity lowers risk. The flip side is that cybersecurity can be so stringent that it degrades the ability to use technology effectively. Risk management is a spectrum from none to complete, where zero risk is unreachable.


When you read some of the definitions of security earlier in the chapter, did you notice that some of them referred to threats? That’s yet another way to define security.

There is a vast difference between threat-based security and risk-based security. “Our firewall prevents network attacks” is vastly different from “our firewall lowers the risk of network attacks.” Bruce Schneier summarized this difference in one of his Crypto-Grams: “Avoiding threats is black and white: either we avoid the threat, or we don’t. Avoiding risk is continuous: there is some amount of risk we can accept, and some we cannot.”19 The end state of risk management is not risk elimination. We can lower our risk of injury or death while driving, but we cannot make it zero.

Despite popular lists of top 10 and best practices security advice, there is no accepted value for precisely how much risk is lowered by patching or firewalls or training. We know they help to an extent, but the remaining risk is still murky in the enormous range between less than 100% and greater than 0%. It would be nice if we could put a number to it, but we cannot.

Security Policy

We define the differences in security posture and risk tolerance in a security policy. The policy helps define assets, authorities, standards, and other issues necessary to build and operate the cybersecurity regime for an organization. Many large organizations have a structured, written policy.20

The relative nature of security policy is one factor that makes cybersecurity difficult. It is further complicated because few organizations have an overall security policy that can be directly applied to developing, procuring, and operating their computing (“cyber”) resources. Most organizations simply have a policy of “no one should do anything to our systems we do not authorize” and depend on off-the-shelf components to enforce it; however, few consumer items are carefully specified and designed to support the full range of possible policies. After all, vendor businesses are focused on making a profit, not on spending unlimited amounts building in support for every possible policy and defense.

This parallels a situation described succinctly by one of our favorite observations from 1985: “A program that has not been specified cannot be incorrect; it can only be surprising.”21 Yes, the majority of security “bugs” and “weaknesses” are not flaws—they are surprises because they occur in systems never fully defined and specified against security policies. If we have no policy, and the software we are running was never explicitly specified and designed, how can there be a security flaw?

It is impossible to avoid yet more surprises given the difficulty vendors have in writing correct programs and the market-driven push to add new features and ever-more layers of complexity to legacy, broken code to drive new sales. Trends involving just-in-time, DevOps, and Agile code development often short-change design, too, leading to surprises. The philosophy is that the coders can quickly fix the problems, but quick fixes are not the same as inherent quality. Consider driving down the highway at 110 kilometers per hour.22 Your car was built to go fast with a shiny exterior, and everything is fine until you need to exit. That’s when you discover the car was built without brakes. Oops! Patching might make things better for the next iteration of the car model, but the situation will not end well for you or anyone else zipping down the road ahead of you. Ease or speed of patching is not the same as good security! Speed of code production is no substitute for considered, comprehensive design, either.

And So. . .

Thus, security is tightly intertwined with risk management, which is related to policy. We need protection because there is a potential danger, harm, or loss of things we value. We have locks on our houses to protect the people and items inside. If we care about the privacy of our cookie recipes and protecting the money in our bank accounts, then we need solid cybersecurity.

So, where does all this leave us? It brings us not to security per se, but rather to the process of building trust and assuring our systems. We want to trust (have confidence) that the mechanisms we have work the way we think they do, that we have backstops and preventive measures in place to add to our confidence in that operation according to policy, and we want to regularly examine everything to ensure there are no gaps or deficiencies. All of that must be within budget, using the lowest-cost software often written by people with minimal (or no) training in sound software engineering and based on architectures and hardware that might have been optimized for running spreadsheets and video games. It should not come as a surprise that the process is fraught with errors and frequent, spectacular failures.

Keep the introduction chapter in mind as you read through the rest of this book. Perhaps the biggest misconceptions in the field are that we know what cybersecurity is, that it is an achievable goal, and that existing technology is sufficient. (And we will not try to explain why safety, correctness, and privacy are also challenging to achieve, although the reasons are roughly similar.)

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020