The following is an excerpt from the book Designing Secure Software: A Guide for Developers by Loren Kohnfelder, Copyright 2022, No Starch Press

“If you cannot—in the long run—tell everyone what you have been doing, your doing has been worthless.” — Erwin Schrödinger

Join me on a hike through the software security landscape.

My favorite hike begins in a rainforest, near the top of the island of Kaua’i, which is often shrouded in misty rain. The trail climbs moderately at first, then descends along the contour of the sloping terrain, in places steep and treacherously slippery after frequent rains. Further down, passing through valleys choked with invasive ginger or overgrown by thorny lantana bushes, it gets seriously muddy, and the less dedicated turn and head back. A couple of miles out, the trees thin out as the environment gradually warms, becoming arid with the lower elevation. Further on, the first long views of the surrounding Pacific begin to open up, as reminders of the promise the trail offers.

In my experience, many software professionals find security daunting at first: shrouded in mist, even vaguely treacherous. This is not without good reason. If the act of programming corresponded to a physical environment, this would be it.

The last mile of the trail runs through terrain made perilous by the loose volcanic rock that, due to the island’s geologically tender age of five million years, hasn’t had time to turn into soil. Code is as hard and unforgiving as rock, yet so fragile that one small flaw can lead to a disaster, just as one misstep on the trail could here. Fortunately, the hiking trail’s path along the ridge has been well chosen, with natural handholds on the steepest section: sturdy basalt outcroppings, or the exposed, solid roots of ohia trees.

Approaching the end of the trail, you’ll find yourself walking along the rim of a deep gorge, the loose ground underfoot almost like ball bearings. To your right, a precipice drops over 2,000 feet. In places, the trail is shoulder width. I’ve seen acrophobic hikers turn around at this point, unable to summon the confidence to proceed. Yet most people are comfortable here, because the trail is slightly inclined away from the dangerous side. To the left, the risk is minimal; you face the same challenging footing, but on a gentle slope, so at worst you might slide a few feet. I thought about this trail often as I wrote this book and have endeavored to provide just such a path, using stories and analogies like this one to tackle the toughest subjects in a way that I hope will help you get to the good stuff.

Security is challenging for a number of reasons: it’s abstract, the subject is vast, and software today is both fragile and extremely complex. How can one explain the intricacies of security in enough depth to connect with readers, without overwhelming them with too much information? This book confronts those challenges in the spirit of hikers on that trail at the rim of the gorge: by leaning away from the danger of trying to cover everything. In the interest of not losing readers, I err on the side of simplification, leaving out some of the smaller details. By doing so, I hope to avoid readers metaphorically falling into the gorge—that is, getting so confused or frustrated that you give up. The book should instead serve as a springboard, sparking your interest in continued exploration of software security practices.

As you approach the end of the trail, the ridge widens out and becomes flat, easy walking. Rounding the last curve, you’re treated to a stunning 300-degree view of the fabled Na Pali coast. To the right is a verdant hanging valley, steeply carved from the mountain. A waterfall feeds the meandering river visible almost directly below. The intricate coastline extends into the distance, flanked by neighboring islands on the horizon to the west. The rewards of visiting this place never get old. After drinking in the experience, a good workout awaits as you start the climb back up.

Just as I’ll never get to see every inch of this island, I won’t learn everything there is to know about software security, and of course, no book will ever cover this broad topic completely, either. What I do have, as my guide, is my own experience. Each of us charts our own unique path through this topic, and I’ve been fortunate to have been doing this work for a long time. I’ve witnessed firsthand some key developments and followed the evolution of both the technologies and the culture of software development since its early days.

The purpose of this book is to show you the lay of the security land, with some words of warning about some of the hazards of the trail so you can begin confidently exploring further on your own. When it comes to security, cut-and-dried guidance that works in all circumstances is rare. Instead, my aim is to show you some simple examples from the landscape to kick-start your interest and deepen your understanding of the core concepts. For every topic this book covers, there is always much more to say. Solving real-world security challenges always requires more context in order to better assess possible solutions; the best decisions are grounded in a solid understanding of the specifics of the design, implementation details, and more. As you grasp the underlying ideas and begin working with them, with practice it becomes intuitive. And fortunately, even small improvements over time make the effort worthwhile.

When I look back on my work with the security teams at major software companies, a lost opportunity always strikes me. Working at a large and profitable corporation has many benefits: along with on-site massage and sumptuous cafes come on-tap security specialists (like myself) and a design review process. Yet few other software development efforts enjoy the benefits of this level of security expertise and a process that integrates security from the design phase. This book seeks to empower the software community to make this standard practice.

With myriad concerns to balance, designers have their hands full. The good ones are certainly aware of security considerations, but they rarely get a security design review. (And none of my industry acquaintances have even heard of the service being offered by consultants.) Developers also have varying degrees of security knowledge, and unless they pursue it as a specialty, their knowledge is often at best piecemeal. Some companies do care enough about security to hire expert consultants, but this invariably happens late in the process, so they’re working after the fact to shore up security ahead of release. Bolting on security at the end has become the industry’s standard strategy—the opposite of baking in security.

Over the years, I have tried to gently spread the word about security among my colleagues. Invariably, one quickly sees that certain people get it; others, not so much. Why people respond so differently is a mystery, possibly more psychological than technological, but it does raise an interesting question. What does it mean to “get” security, and how do you teach it? I don’t mean world-class knowledge, or even mastery, but a sufficient grasp of the basics to be aware of the challenges and how to make incremental improvements. From that point, software professionals can continue their research to fill in any gaps. That’s the objective that this book endeavors to deliver.

Throughout the process of writing this book, my understanding of the challenge this work entailed has grown considerably. At first, I was surprised that a book like this didn’t already exist; now I think I know why. Security concepts are frequently counterintuitive; attacks are often devious and nonobvious, and software design itself is already highly abstract. Software today is so rich and diverse that securing it represents a daunting challenge. Software security remains an unsolved problem, but we do understand large parts of it, and we’re getting better at it—if only it weren’t such a fast-moving target! I certainly don’t have perfect answers for everything. All of the easy answers to security challenges are already built into our software platforms, so it’s the hard problems that remain. This book strategically emphasizes concepts and the development of a security mindset. It invites more people to contribute to security, to bring a greater diversity of fresh perspectives and more consistent security focus.

I hope you will join me on this personal tour of my favorite paths through the security landscape, in which I share with you the most interesting insights and effective methodologies that I have to offer. If this book convinces you of the value of baking security into software from the design phase, of considering security throughout the process, and of going beyond what I can offer here, then it will have succeeded.