Planet GNOME

When you log into a service you’re given an authentication token. Each further request to the site includes that token, allowing the server to figure out who you are and ensuring that you have access to your data. Depending on site policy, this token may either be stored in memory (and so vanish if you restart your browser) or disk. The token is the proof of your identity. As far as the site is concerned, anyone with your token is you. These tokens may be traditional browser cookies, but they may also be stored in either site local storage or (if you’re not using a browser) in some other storage location.

In recent years we’ve seen infostealer malware (like LummaC2) gain the ability to exfiltrate user tokens, allowing attackers to gain access to the user’s data without needing to retain access to the user’s machine. This attack is viable even if the site has strong MFA requirements, so passkeys don’t help. Encrypting the tokens on disk doesn’t prevent the malware from scraping them out of the browser’s RAM or obtaining whatever key is used to encrypt them. This feels like a pretty hard problem to solve.

But that hasn’t stopped people from trying! Dirk Balfanz wrote an IETF draft describing a mechanism for using self-signed certificates for TLS authentication. This uses the mutual authentication feature of the TLS protocol that requires both sides prove their identity to each other. In regular TLS, the remote site presents a signed certificate that tells you who it is. When performing mutual authentication, you then present a certificate to the remote site telling it who you are. These client certificates are largely unused outside enterprise environments because they’re a huge pain to deploy. It’s not so much that this has sharp edges, it’s that it’s entirely made of sharp edges. Managing certificate deployment to your devices is hard. Browsers get confused if the certificates change under them. You have one certificate and it lives forever, so sites you present it to can track your identity. Users are prompted to choose a certificate to authenticate with, and if they pick the wrong one everything breaks and is hard to recover. I’ve deployed this and I did not have a good time.

But Balfanz’s idea was simple. Rather than require certificates to be deployed, browsers would simply generate a certificate on the fly. The goal wasn’t to prove the device or user’s identity in any global way - but it would associate a TLS session with a specific certificate. You could then, for example, include a hash of the certificate in the cookie, and if someone tried to use that cookie without presenting that certificate then the cookie could be rejected. If the browser used a hardware-backed private key for the certificate then it would be impossible for an attacker to steal it. Sure, you could still steal cookies, but you wouldn’t be able to use them.

This was written almost 15 years ago, and seems simple, elegant, and functional. It didn’t happen. Part of the reason for that is that, well, it wasn’t quite so simple. One problem was privacy related. Cookies are only sent after the TLS session is established, so anyone monitoring the network doesn’t know anything about the user identity. A naive implementation of this approach would have meant the client certificate being sent before session establishment, and now user identity can be tracked (no longer an issue if this was implemented on top of TLS 1.3, but this was a log time ago). This was avoided by reordering the client handshake, but that meant having to modify the TLS specification and implementations would have to be updated to support this. Another was that figuring out the granularity of the certificates was difficult. You’d want to use different certificates for every site to avoid them effectively becoming tracking cookies, but you need to provide the certificate before cookies are set, and you don’t know what origin the site is going to set in its cookies. If you generate a certificate for a.example.com and a different one for b.example.com, and a.example.com sets a cookie for *.example.com and includes the certificate you used for a.example.com, that cookie isn’t going to work on b.example.com and things are broken. This meant supporting it wasn’t as straightforward as it seemed - you’d need to ensure that your cookie scope was compatible with the certificate scope. You could probably make this work well enough by aligning it with the Public Suffix List, but there was still some risk of expectations not being aligned.

And, perhaps most importantly, TLS session resumption (replaced by pre-shared keys in TLS 1.3) somewhat defeats the purpose of the exercise - clients store state that allows them to re-establish a TLS connection without performing certificate exchange (this reduces overhead if a connection gets interrupted or you switch to a new network or anything along those lines), and anyone in a position to steal cookies could steal that state as well.

The followup attempt was channel IDs. This simplified the implementation somewhat - rather than certificates, a raw public key would be sent, along with proof of possession of the private key in the form of a signature over a portion of the TLS handshake. This was required even in the event of session resumption, which avoided having to worry about theft of session secrets. The timing of the exchange was after the encrypted session had been established, so user identity couldn’t be leaked that way either. Cookies could then be bound to this identifier. Unfortunately it didn’t really deal with the problem of scoping keys in a way that would match cookie requirements, and the spec suggests that the right way of handling this is to scope keys to TLDs, which would enable user tracking across sites (Chrome’s implementation apparently restricted it to eTLD+1, which would match the third party cookie policy and avoid the tracking risk).

Chrome added support for this, but it was removed in early 2018. The discussion of some of the pain points in that message is interesting, explicitly calling out problems with connection coalescing across domains and the incompatibility with zero-RTT TLS1.3. The overall consensus at the time seems to be that trying to solve this entirely at the TLS layer has too many rough edges, and a different approach should be taken.

And so almost 7 years after the initial draft for origin bound certificates, we come to token binding. This ended up being a rather more complex endeavour, covering 3 different RFCs describing how it impacts TLS, how to incorporate it into HTTP, and how to manage all the various parties involved in the process. The short version is that it’s pretty similar to channel ID, except that there’s also a documented mechanism for allowing tokens to be bound to one party and consumed by another, avoiding any need for widely scoped keys. Token binding effectively solved all the issues in the original proposal, but at the cost of somewhat more complexity.

The RFC was finalised in October 2018. Chrome removed its (incomplete, draft) support for token binding in November 2018. Edge carried support until late 2024. Despite getting all the way through the RFC process, it’s functionally dead.

The process up until this point had been largely initiated by Google, with Microsoft contributing significantly to the token binding standards. The work had been focused on identifying a generic solution to the problem rather than tying it to any specific authentication flow. The next step was in a different direction - rather than trying to fix this for the entire internet, how about we try to fix it for OAuth?

RFC 8705 is titled “OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens”. This is basically the 2011 approach, but (a) with an explicit definition of how the certificate should be incorporated into issued auth cookies, and (b) with a proviso that well uh if you’re going to use tokens issued by your IdP to authenticate to someone else then well you’re going to need to use the same cert for both. This is probably fine for the company-owned-laptop case where you’re actually fine with multiple sites being able to tie identities together (that’s kind of the point here!), and also works for “I am using an app and not a browser”, but doesn’t work for more generic scenarios. It also doesn’t seem to take the session resumption case into account at all? Support for RFC8705 seems poor, as far as I can tell of the big players only Auth0 implements it. In theory it works fine with self-signed client certs but in reality that’s going to be almost as difficult to support across multiple platforms as just issuing proper client certs in the first place, so deployment is going to be kind of a pain. But the good news is it doesn’t rely on any TLS extensions or custom browser behaviour, so at the client side it works fine with any browser.

Which brings us on to RFC 9449, “Demonstrating Proof of Possession”. This goes even further than RFC8705 in terms of reducing the burden of deployment - it works fine with existing browsers, and it doesn’t even require any certs. The client generates a keypair and provides the pubkey when requesting the cookie. The cookie contains the pubkey. Every request to the service now provides the cookie with the pubkey and also provides a signature over the URI and HTTP method. If the signature matches the pubkey in the token then clearly the signature came from the machine the token was issued to, and everything is good.

This does come with some downsides, though. The first is that it uses browser interfaces to generate the keys (typically crypto.subtle.generatekey()) and as far as I can tell there are no browsers that guarantee that that key is going to be generated in hardware even if it’s marked non-exportable, so anyone able to steal the cookies can also steal the keys. The second is that the signature only covers the URI and HTTP method, and not the message content or any other headers, so anyone able to exfiltrate a valid signature can replay it against the same URI with different message content. The recommended way to handle this is to reject any signatures that weren’t generated within the last few seconds, which is a wonderful additional way to allow clock skew to give you a Bad Day. And the third is that every single request has to be separately signed, which is not intrinsically a problem because computers are fast and have multiple cores, but if you’re trying to solve the first problem by sticking the key in a TPM then you’re dealing with something that’s slow and single threaded and that’s maybe acceptable if you’re using client certificates (because there’s going to be one signature per session and you can use the same session for multiple requests) but probably not if you’re dealing with a user opening a browser that restores previous tabs and each of those is a webapp that fires off 100 requests in parallel.

In case it wasn’t clear, I don’t like DPoP. It doesn’t feel like it actually solves the underlying problem that we see in the real world (malware running in a context where if it can grab the tokens it can grab the keys), it adds a massive amount of overhead, and it has baked in replay vulnerabilities. I don’t know why it exists and I’m incredibly suspicious of vendors telling me that it fixes my problems, because if they’re telling me that then I’m going to end up assuming that they either don’t understand my problems or they don’t understand their technology, and neither of those is good.

Still. Then we get to the thing that prompted me to write this - Chrome’s announcement that they had launched device-bound session credentials. This is interesting because it’s a Chrome feature that’s explicitly intended to counter on-device malware, which was one of the things that was out of scope in 2018 when token binding was being removed. Since this is entire web level it doesn’t have to be an RFC, and so is instead defined by W3C. I’m going to handwave all the complexity and say that it’s basically a way to register a public key when a cookie is issued, and then prove possession of the private key when it’s time to renew the cookie. By making the cookies shortlived and having support for rotating them in the background, user impact is basically zero and while it’s still possible for an attacker to exfiltrate and use a cookie they’ll only be able to do so for a short window before it needs to be refreshed - something the attacker can’t do, since they don’t have the private key. This avoids the DPoP overhead because you only need to do signing once per cookie per cookie lifetime, and not on every single request. I don’t like this due to the window where exfiltrated tokens can be used, but it feels like a strict improvement over the status quo. An extension called device-bound session credentials for enterprise allows pre-enrollment of device keys, so even though the actual runtime DBCE flow doesn’t involve certificates, certificates can be used for device registration in enterprise environments and you can make sure that auth cookies only go to trusted devices. Unfortunately this is Chrome-only, and so we’re going to need to wait for it to be backported to all the random app frameworks for it to have widespread support on mobile or for almost everyone’s desktop app that’s actually three websites in an Electron wrapper. Mozilla’s current position is that they’re not in favour of it, so I guess we’ll see where Safari lands in terms of broad uptake.

The last thing on my list is another client cert/OAuth binding, this one still in draft state at the time of writing. This one is aimed primarily at the use of agent-driven tooling, where you have something running in the background using a whole bunch of tools that are each acting on your behalf. Authenticating to all of them separately isn’t a fun time, but giving broadly scoped access tokens to a non-deterministic agent and trusting that it’ll never post them somewhere public also isn’t a fun time. The key distinction between it and RFC8705 is that it’s aimed at connections rather than sessions, which avoids the worries about session resumption. This is done with TLS Exporters, which in TLS 1.3 should be unique to the connection even over session resumption (TLS 1.2 may reuse some of the same key material for exporters over session resumption, so it’s recommended to enforce 1.3 for this). By providing a new signature alongside the cookie on every new connection, the client proves that it still has access to the private key. This is a very new spec and I haven’t had much time to work through it yet, but my naive understanding is that unlike RFC8705 this would require some additional client support to be able to regenerate the client signature on every TLS reconnection.

This doesn’t avoid all the problems that RFC8705 has, including how to scope certificates. For the agentic use case that probably doesn’t matter - all these tools are acting on behalf of the same user, it’s fine if all the sites involved know they’re the same user. But it doesn’t solve the general purpose user use case, and right now DBSC seems like the best we have there.

But. Part of me still wonders whether Dirk Balfanz’s approach was the right one. Yes, there’s risk associated with TLS session resumption, but in the worst case you could just switch that off for high risk setups. The cookie scope argument is real, and also in cases where it could violate privacy the site owner could already choose to broaden their cookie scope and violate your privacy, and in cases where it breaks things you could just not make use of it. The other problems are largely fixed by TLS 1.3, and then we’re just left with “Browsers handle client certificates badly” to which my answer is “Yes, and we should fix that anyway”.

Despite having a pretty good answer to this solution over a decade ago, the closest we have to actual deployment is something that offers strictly worse security guarantees. And tokens keep getting stolen, and compromises keep occurring, and for the most part people shrug and get on with things.

This blog post is a sequel to Your _get_type() function is not G_GNUC_CONST.

GNOME developers have long used G_GNUC_CONST, which expands to __attribute__((const)), to annotate GObject _get_type() functions, despite knowing that it is incorrect to do so. const functions by definition have no side effects, but _get_type() functions actually have a side effect the first time the function is called: they initialize the type. Why apply an incorrect annotation to these functions? Because it makes the code faster.

Although this was long known to be incorrect, it worked fine in practice… until now. Regrettably, Sam James has discovered that GCC 16 may optimize away the type initialization, resulting in crashes. This is our fault for providing the compiler with wrong information about our code, so it’s time to audit your use of const attributes to remove them from _get_type() functions. Most GNOME programs use these attributes only for _get_type() functions, but if you use it in more places, then check to make sure those functions are actually const, as defined by the GCC documentation.

Sadly, there is no suitable replacement attribute for _get_type() functions. Two decades ago, Behdad requested a new idempotent attribute for expressing the desired semantics, but nobody has implemented it.

File under: brief half-year catch-ups, me not remembering when I last updated the back-end of this blog

I’m writing this from a terrible AirBnB in Reading, having just wrapped up a four-day visit to my old Typography & Graphic Communication alma mater and happy to have an excuse to put off re-packing. I’ve been fortunate in the first six months of this year to have time to participate in a number of in-person conferences after a few years of not being able to travel, and when I was looking back at the calendar I noticed that these events have been evenly split between FOSS events and type-related events, so I decided it’s worth jotting down a few of the thoughts that presented themselves before they fade away entirely. Here’s a recap:

SCALE: The Southern California Linux Expo (which, for the record, I refuse to bacronymize in a mixed-case form, so don’t at me about the capital A). I’ve been a regular for yikes years at this point, and have helped coordinate the “libre graphics track” for the past several, but this time I actually had to present a workshop as part of that. It was on the subject of learning Scribus if you’re coming from the world of LibreOffice / Microsoft Word / Google Docs / Etc. Turns out that’s an ambitious subject, scope-wise, and we didn’t get through everything I wanted to.

But for me, that highlights the fact that shifting from one Do Real Work application to another is always a monumental effort. Sure, you can fire up GIMP and do some cropping of images in a matter of minutes. But you can’t drop a full production workflow without a lot of spare time, because every individual sub-task now involves a different order of operations (or different operations), a different cycle of tool-switching, and a different set of cross-checks and QA. I don’t think we give this problem enough attention, especially when the well-meaningers on social media (and yeah, that includes Mastodon, as fully as whatever platform you don’t like) entice people to give it a try like it’s no big thing.

It also reiterated for me how important document templates are. Those are fairly forgettable 99% of the time, but what ships in the template choosers of LibreOffice, WhateverOfficeOnline, and in more specialist apps is what people end up using when they need to get things done. The typography in most FOSS templates is pretty awful: the fonts are weak, the hierarchies are non-existent, the alignments are haphazard, and the optical balance is even more non-existent than the hierarchies. I suspect that this can only being improved with a long, slow, many-person grind. I don’t know where to start.

WAVE and ATypI: Both of these are type-related or type-adjacent events, so I don’t have much that feels relevant to say about them here. WAVE is quite specialized, since it focuses on human writing: just as many of the presenters are linguists as anything else, and the type people made a stronger showing this year than they did in the original event a couple of years back but aren’t the majority. It is genuinely a wake-up call to sit down and learn about a written language that does not operate in the way that your own does, and to hear exactly how many people use it every day (as well as to see photos to remind you that all of this happens in the present, when it’s incredibly easy to write off those concerns as belonging to an earlier era in printing and just presume that Computers Fixed All That. Or that OpenType and Unicode did. Or that FOSS did.).

I do think, however, that most of the FOSS projects I’m involved with keep a stout set of blinders on about non-majority language systems and scripts, and that that’s deeply problematic. Indeed, it was not long ago that I mentioned IRL that GNOME could do a valuable bit of good for the global user community by finding and supporting scripts other than Latin/Greek/Cyrillic — in contrast to today, where the attitude is “oh, those users will figure it out for themselves like they always do aren’t they great over there” and/or “Noto Fixed All That.” It’s not hard to say that the next release of your project will also support, say, Arabic, and to at the very least be deliberate about bringing people into the room to find and test the fonts you need to determine if things are working and look good. Yes, you do have to do actual tests. Yes, I mean you.

ATypI, in contrast, is very much a “type production” event. For me, this year the bit that stood out was behind-the-scenes stuff that eeped out around the seams and got more widely discussed. Like, there was evidently initial interest from some on the local organizing side that there would be a verrrry small list of presenters: less than two dozen, total, for 3–4 days (depending on how you count workshops and exhibits). Far less than two dozen. There are two big gotchas there. First, you have to contend with gatekeeping. All of the well-known people are likely to be the ones with The Exciting Announcement to announce and, in theory, they’re a big part of the draw. But that keeps all the new community members out. Second, it crashes the economic viability of attending the event. The locale this time was Stanford, which (despite being perhaps the world’s only Junior University, look it up) is outrageously expensive, even if you already live outrageously in nearby northern California. And any time people can’t justify the cost of travel, yes you’re gatekeeping again (particularly of the hallway track), but you’re also twisting the dial on your conference further from “I Should Go” over to “Going Is Just Paying Money To Be In The Audience Of A YouTube Video That I Can Watch Any Other Time” … which is a hard dial to reverse.

LGM: The Libre Graphics Meeting seems to be back on its feet and in good form after several post-pandemic years of bumping into things. Massive props. I had a lot of side-project stuff I brought along in disorganized form, although I attended every session. I also quasi-roundtabled a session to talk about how the now defunct “Planet LibreGraphics”, may it rest unpeacefully, used to be the clear answer to the perpetual question of “how do we maintain momentum and connection the rest of the year?”

But whereas the old Planet site was a garden variety class-M aggregator of individual blogs’ RSS or Atom feeds, I’m of the unprofessional opinion that a true community aggregator today has to account for different types of inputs and outputs and user modes (sideputs?). E.g., many projects don’t announce new releases on their project blog anymore; they tag a release on GitHub. That’s a different input. A lot of people don’t post long-form content anymore, but do a lot of microblogging. That might suggest having an ActivityPub output … but it would be a bot, and it would incur a TON of overhead and put scores of messages out all day long, which you definitely couldn’t use on the traditional RSS output. So you probably need to handle those differently, maybe batching the Mastodon bits into a once-per-day blog output?

Ultimately, I’m not sure; there are a lot of these details. We’re way out in the high-cosmic-ray environment of the trans-neptunians here, or some other metaphor. Ping me if you find the question interesting, because I want to talk a lot more about it.

LAS: It was my first time going to the Linux Application Summit, as the kids call it these days, so I have more thoughts to get down about that one. For starters, it wasn’t what I expected, because I expected there to be more people there who develop Linux applications. I covered a lot of growth of post–CD-delivered-and-RPM/Apt-updated packaging efforts when I was toiling as a not-so-young FOSS journalist, so I do think I have the right grounding in systems like Snap, Flatpak, FlatHub, AppKit, immutable-image OS approaches and other user-code–confinement technical building blocks. Except Kubernetes; I never cared about that and never will.

Anyway, all of that stuff (hand-waving) is very much where LAS takes place. It’s really good to see that there are people from multiple application ecosystems talking about how they handle the current set of unsolved problems. Selfishly, one of the sessions that stood out most to me was Carlos Garnacho’s talk about the data-search layer that he’s been working on. The gist there is that it’s for searching local data … which you might think you can already do, but you’d be wrong. You can sort of do filesystem-level text searching, but that doesn’t handle complex stuff, and it really, really doesn’t let you handle per-application searching well. I’ve got a keen interest in what people do with their “big data exports” — we FOSS people like to look down our noses at the public and tell them that they should take all of their content out of The Bad Services and walk into the sunlight. But there’s not anything they can do with it when they get here (or there, depending on how you feel), and they’re the ones holding the bag. The TinySparql and LocalSearch stuff, I think, holds the potential to improve on that in a big way.

I was also quite interested to learn more about how the KDE ecosystem does its builds. I don’t do … builds, at least not in the sense that KDE and GNOME do (nor, who else, who else … Enlightenment, maybe?). But I have been forced against my will to get up to speed on some things like GitHub Actions, and the session about KDE’s build architecture did make me want to go back and re-examine some stuff I’ve built. That being the other sense of “build.” I was also very interested in Evangelos Paterakis’s talk about the gritty realities of picking up an abandoned project, reviving it in fork-form, and getting hit with the consequences of that discussion from both upstream and downstream.

For most people, what matters in these sort of pseudoplumbing projects in what’s going to be accessible in end-user applications whenever they land on the next platform releases. That’s why I thought I might encounter app developers at LAS. Instead, much of the session content was about enhancing the the plumbing layer themselves: what’s going on with portals, how sandbox/confinement techniques either fail-open or fail-close, and where the confined-application model is still leaking.

I suggest checking out Sebastien Wick’s talk about the portal situation if nothing else; the slides are available now, even if the video is not. Whenever the videos go up, I also suggest that everyone watches the session about forking a defunct project … not because it’s something everyone should do, although let’s face it, projects appear and fizzle all the time. But mainly because the social aspects of how a revived fork does or doesn’t catch on are things the FOSS community doesn’t say out loud. Not bad or shameful things, just sharp corners.

All that said, I think that what really needs to happen is for application developers to actually go to this event. I know you think you don’t need to. But the platform layer is another one of those things about which you can easily say “Didn’t Somebody Already Handle That?” and be incorrect for a long period of time before it bites you. Moreover, every time I’ve ever asked the developer of a Mac or Windows desktop app what they know about developing Linux apps, they tell me a story about running into giant potholes, missing documentation, and mismatched API expectations. The presence of Linux app developers at a plumbing-layer conference will not instantly fix that, but I do know that a lot of those Linux developers hit the same roadblocks.

LGM made a massive improvement to the ecosystem of creative-arts apps in FOSS specifically because it involved getting users, app developers, and subsystem developers into a single space. That’s not a magic trick, and the general Linux app universe would benefit from repeating the technique.

Grapholinguistics in the 21st century, also known as /gʁafematik/: This is the one I just came from, and I don’t quite have it all simmered from ingredients into stew just yet. Apart from being hosted in an initially un-airconditioned spot at the university in the hottest UK week since 1666, I don’t think I have any complaints. I introduced speakers and acted as moderator for Q&A blocks, and apart from that I visited with most but not all of the rest of the quantitative type-research clique. You know where to find us.

One takeaway, perhaps, that will be generally useful is that I spent much of the inter-session time when I was on moderator duty trying to gauge the level of nervousness of the upcoming speaker and attempting to defuse it. I don’t know how successful that actually was, but I do think there were sessions elsewhere where it might have made a difference. I don’t know; perhaps that’s projecting.

Anyway, before the Internet runs out of bits, I should wrap up. I do need to be sure to express my appreciation to the GNOME Foundation, who chipped in with some travel expense assistance for LAS, as well as to my friend and mentor-in-a-few-very,-very-limited-capacities-I-can’t-emphasize-that-part-enough Sri Ramkrishna for pinging me about it and then reminding me. I’m definitely glad I went.

At the moment, I’m halfway through reconfiguring the blog site here into static format (fighting Unicode support in the old platform), so don’t count on commenting to work. If you want to reply to anything I said here, try me on Mastodon.