In my last blog post, I discussed why people often view the web as a uniquely unsuited platform for implementing end-to-end encryption (E2EE). This view is that the web doesn’t offer a long-term trustable notion of what the application is. In that earlier post, I explored the idea of treating the application as untrustworthy and isolating sensitive data from it. In this post, I’m going to pontificate on whether web applications are truly less trustworthy than native applications, especially in an E2EE setting, and if so, how we should bridge the gap. The gap is narrower than it appears at first glance, especially with desktop applications. To close it, though, the devil is in the (UX- and deployment-related) details.
Is the web really worse?
Often, security experts argue that the web isn’t suited for E2EE applications because of the vast attack surface for code injection – abusable by the developer or by an external attacker. For many web applications, a web browser receives and runs code from a zillion servers, retrieved over a zillion TLS connections. Compromising just one of those servers or connections is enough to inject untrustworthy code into the application, likely without detection. This is different from any other mainstream platform, all of which offer stronger statements about the integrity and/or provenance of the application code.
I think this argument is basically factually correct, but less compelling than it appears at first glance.
Web versus mobile
On mobile platforms, the allegedly stronger security model comes from the policies, review processes, and distribution models of app stores, as well as the operating system’s design of security principles and use of sandboxing. Apps are signed with an offline developer key, and usually distributed through a store, with human review against anti-abuse policies and a centralized distribution point operated by the OS vendor. Individually, each of these properties might not provide much in the way of hard guarantees, but together they form a nice defense-in-depth model.
If we compare mobile to web, mobile is convincingly a more secure platform for developing high-security applications. This is especially true if the threat model includes the application developer (e.g., E2EE messaging), because there is another party (the OS vendor) besides the application developer involved in distributing the application.
Web versus desktop
On desktop platforms, the situation is much messier:
- Many desktop applications silently auto-update directly from the application vendor’s servers, which looks a lot more like the web’s code distribution model.
- Desktop applications these days are often built identically to web applications and then packaged up for native distribution, e.g., through Electron.
- Desktop applications might run different code depending on dynamic data (such as experiment configurations) that is loaded and reloaded often, possibly relying on TLS alone to secure these configurations from injection.
- Unlike web apps, desktop applications are often written in vulnerable memory-unsafe languages like C++.
- Desktop applications are often installed at the same privilege level and not isolated from each other as well as mobile apps or even web apps.
Compared to the web, I would say that desktop’s main advantage in application security is the common use of code signing with an offline developer key – and, as a corollary, the notion of a concrete application that can be signed, as opposed to the continuous river of code that forms a web application. Does this delta matter? I’m not sure:
- The security value of code signing boils down to the security of the code signing PKI as well as the UX of verification failures, and I don’t have a lot of confidence in either of those aspects of the system. Further, the legitimate owner of the code signing key is an attacker in the E2EE threat model; thus code signing is only partially relevant to the E2EE threat model.
- The idea of having a discrete application is appealing, but I find it hard to say exactly why. Perhaps there are some advantages in terms of auditing and forensics, or in tightly controlled environments like enterprise deployments, where a knowledgeable administrator might qualify updates manually. It also provides a basis on which to build other security mechanisms, such as comparing the version of the code that one user is running to what everyone else is running – but in practice, this isn’t widely deployed on today’s desktop operating systems.
That said, desktop application security is getting better over time, with new technologies like Mac OS notarization, improvements in code signing UX, more store-like distribution models, and stronger isolation primitives. If there historically hasn’t been a huge gap between desktop application security and web app security, it would be a shame for that gap to widen and leave the web behind. But, today, I don’t believe the gap is very wide, especially if you consider the application developer an attacker, and that is why I looked askance at anyone who claims the web isn’t suited for E2EE while happily distributing desktop versions of their E2EE apps.
Bringing the web closer to parity: a UX problem
If we do care about the delta in security model between the web and other platforms, then we could build some kind of code bundling and signing mechanism for web applications, perhaps with some kind of transparency layer on top to make the code publicly auditable and make it harder to target specific users with malicious code. A bundling/signing/transparency solution for the web could probably be built out of some of a collection of mechanisms that already exist or have at least been explored. Related ideas include Subresource Integrity, Isolated Web Apps, Signed Exchanges and Web Packaging, Meta’s Code Verify extension, and source code and supply chain transparency proposals.
That is, many of the pieces of the solution already exist. It’s not hard to imagine a mechanism for bundling up a web application, signing it with an offline key, and distributing that to the browser to run in isolation, without allowing any dynamic code loading. The daunting challenges that remain are user experience questions and fiddly deployment details:
- Website opt-in/opt-out. Any kind of code
bundling/signing/transparency mechanism has to co-exist with the
open web as it exists today. But then how does the browser know
whether example.com is a normal web application, with all the
normal mechanisms of dynamic code loading, rather than a special
bundled/signed web application? If example.com is normally a
special bundled/signed web application, but the attacker
compromises it to start serving a regular legacy web application,
how does the browser know anything is wrong? Or if the browser
simply punts the information to the user, how should the user
know that anything is wrong? I don’t think there is a good answer
for this yet.
- Separate origin: The browser could use separate web origins for the signed/bundled version of an origin and the regular one. This would isolate secret data like long-term identity keys. However, separating one “physical” origin into different “logical” origins has historically proven to be a complex endeavor, and it opens up UX questions like whether the regular version of the origin could phish the signed/bundled version to break security guarantees.
- Signal in Certificate Transparency: One proposal is to stick a signal into Certificate Transparency logs, but this is somewhat clunky, abusing the web PKI and Certificate Transparency for a purpose for which they weren’t designed. Furthermore, it yields a weak security model. A downgrade could only be detected after the fact, within Certificate Transparency’s somewhat nebulous security guarantees.
- Sticky header: One could imagine using an HSTS-like header, by which bundled/signed web applications instruct the browser to only accept bundled/signed code for the domain. The browser remembers this instruction for some configurable period of time. This would probably be okay, but it’s not ideal. HSTS’s security guarantees only hold if the user visits the site to refresh the browser’s cached directives at some predictable cadence, and it doesn’t protect the first visit from downgrade. Any kind of “sticky” domain-based mechanism becomes problematic when a domain owner transfers the domain, lets it expire, or changes their mind about how they want to configure their website. And HSTS has inspired some unmaintainable hacks for sites that need stronger security. These problems are all somewhat acceptable for HSTS because, in the very long run, we might hope to deprecate HSTS as HTTPS gradually becomes more strongly preferred in browsers. But for code bundling/signing, there isn’t any long-term story in which bundled/signed applications become the default or only supported mechanism, so these drawbacks are perhaps less tolerable.
- Failure UX. Whatever the solution for website opt-in/opt-out, there will have to be some UX in place for when the browser expects a valid bundled/signed web application but doesn’t get one: for example, the signature is invalid, the browser expected a bundled/signed application but got a legacy resource, or there was some failure in the code transparency verification (if such a thing comes to pass). What happens then? Can the user bypass the error? If so, then, as with other security warnings, the warning must be designed just so to allow some users to make reasonable, informed judgment calls without putting less informed users at risk, and without contributing to a broader warning fatigue or habituation effect in E2EE apps. If the browser does not let the user bypass the error, then the entire system must be designed carefully such that false positive errors are virtually nonexistent.
- Development and deployment experience. Finally, whatever the mechanism for app bundling/signing/transparency, it has to be usable by developers. I find that it’s tempting to think of E2EE as a niche feature, thus app bundling/signing as something that only a small number of highly security-sensitive web applications would use. It’s tempting to make the problems more tractable by assuming that the web applications would be small and static, or amenable to custom heavy-handed development workflows, or built by highly sophisticated developers. In reality, some of the largest and most complex web applications in the world – such as facebook.com – need E2EE. The target market includes web applications with vast amounts of legacy code, complex bespoke development workflows, highly performance-sensitive userbases, you name it. Further, if we designed E2EE on the web as a niche feature, it would be a self-fulfilling prophecy. By designing it as a mass-market feature, it increases the chances that E2EE becomes table-stakes for modern applications on any platform. Usable code integrity and provenance mechanisms for the web would also benefit many other types of security-sensitive applications besides E2EE apps.
In the effort to bring E2EE to the web, I hope these challenges in user experience and developer ergonomics don’t get forgotten or underestimated.
Thanks to David Adrian, Richard Barnes, Nick Doty, Wendy Knox Everette, Ryan Hurst, April King, Jon Millican, and Chris Palmer for feedback on this post.