HTTPS requests with client certificates in Clojure

The vast majority of TLS connections only authenticate the server. When the client opens the connection, the server sends its certificate. The client checks the certificate against the list of certificate authorities that it knows about. The client is typically authenticated, but over the inner HTTP connection, not at a TLS level.

That isn't the only way TLS can work. TLS also supports authenticating clients with certificates, just like it authenticates servers. This is called mutually authenticated TLS, because both peers authenticate each other. At Rackspace Managed Security, we use this for all communication between internal nodes. We also operate our own certificate authority to sign all of those certificates.

One major library, http-kit, makes use of Java's javax.net.ssl, notably SSLContext and SSLEngine. These Java APIs are exhaustive, and very... Java. While it's easy to make fun of these APIs, most other development environments leave you using OpenSSL, whose APIs are patently misanthropic. While some of these APIs do leave something to be desired, aphyr has done a lot of the hard work of making them more palatable with less-awful-ssl. That gives you an SSLContext. Request methods in http-kit have an opts map that you can pass a :sslengine object to. Given an SSLContext, you just need to do (.createSSLEngine ctx) to get the engine object you want.

Another major library, clj-http, uses lower-level APIs. Specifically, it requires [KeyStore][keystore] instances for its :key-store and :trust-store options. That requires diving deep into Java's cryptographic APIs, which, as mentioned before, might be something you want to avoid. While clj-http is probably the most popular library, if you want to do fancy TLS tricks, you probably want to use http-kit instead for now.

My favorite HTTP library is aleph by Zach Tellman. It uses Netty instead of the usual Java IO components. Fortunately, Netty's API is at least marginally friendlier than the one in javax.net.ssl. Unfortunately, there's no less-awful-ssl for Aleph. Plus, since I'm using sente for asynchronous client-server communication, which doesn't have support for aleph yet. So, I'm comfortably stuck with http-kit for now.

In conclusion, API design is UX design. The library that "won" for us was simply the one that was easiest to use.

For a deeper dive in how TLS and its building blocks work, you should watch my talk, Crypto 101, or the matching book. It's free! Oh, and if you're looking for information security positions (that includes entry-level!) in an inclusive and friendly environment that puts a heavy emphasis on teaching and personal development, you should get in touch with me at [email protected].

Call for proposal proposals

I'm excited to announce that I was invited to speak at PyCon PL. Hence, I'm preparing to freshen up my arsenal of talks for the coming year. The organizers have very generously given me a lot of freedom regarding what to talk about.

I'd like to do more security talks as well as shift focus towards a more technical audience, going more in-depth and touching on more advanced topics.

Candidates

Object-capability systems

Capabilities are a better way of thinking about authorization. A capability ("cap") gives you the authority to perform some action, without giving you any other authority. Unlike role-based access control systems, capability based systems nearly always fail-closed; if you don't have the capability, you simply don't have enough information to perform an action. Contrast this with RBAC systems, where authorization constraints are enforced with pinky swears, and therefore often subverted.

I think I can make an interesting case for capability systems to any technical audience with some professional experience. Just talk about secret management, and how it's nearly always terrifying! This gives me an opportunity to talk about icecap (docs) and shimmer (blog, my favorite pastimes.

Putting a backdoor in RDRAND

I've blogged about this before before, but I think I could turn it into a talk. The short version is that Linux's PRNG mixes in entropy from the RDRAND in a way that would allow a malicious implementation to control the output of the PRNG in ways that would be indistinguishable to a (motivated) observer.

As a proof of concept, I'd love to demo the attack, either in software (for example, with QEMU) or even in hardware with an open core. I could also go into the research that's been done regarding hiding stuff on-die. Unfortunately, the naysayers so far have relied on moving the goalposts continuously, so I'm not sure that would convince them this is a real issue.

Retroreflection

An opportunity to get in touch with my languishing inner electrical engineer! It turns out that when you zap radio waves at most hardware, the reflection gets modulated based on what it's doing right now. The concept became known as TEMPEST, an NSA program. So far, there's little public research on how feasible it is for your average motivated hacker. This is essentially van Eck phreaking, with 2015 tools. There's probably some interesting data to pick off of USB HIDs, and undoubtedly a myriad of interesting devices controlled by low-speed RS-232. Perhaps wireless JTAG debugging?

The unfinished draft bin

Underhanded curve selection

Another talk in the underhanded cryptography section I've considered would be about underhanded elliptic curve selection. Unfortunately, bringing the audience up to speed with the math to get something out of it would be impossible in one talk slot. People already familiar with the math are also almost certainly familiar with the argument for rigid curves.

Web app authentication

Some folks asked for a tutorial on how to authenticate to web apps. I'm not sure I can turn that into a great talk. There's a lot of general stuff that's reasonably obvious, and then there's highly framework-specific stuff. I don't really see how I can provide a lot of value for people's time.

Feedback

David Reid and Dwayne Litzenberger made similar, excellent points. They both recommend talking about object-capability systems. Unlike the other two, it will (hopefully) actually help people build secure software. Also, the other two will just make people feel sad. I feel like those points generalize to all attack talks; are they just not that useful?

Everything I've learned about running a financial aid program

For the past two years, I've been running PyCon's financial aid program. Starting this year, the event coordinator has asked all staff members to document what they do for PyCon. Firstly, this helps to objectively recognize the hard work done by our (volunteer) staff, and to help make sure there is continuity when the time comes to pass the torch. Since organizers of other conferences have expressed interest in my opinions for creating their own financial aid programs, I am posting my notes publicly instead.

This is a collection of hard-earned opinions, and is very much work in progress. It's written as if it were a conversation with a hypothetical financial aid organizer; so, whenever I say "you", I mean you, the awesome person running financial aid at a conference somewhere.

Basics

Financial aid programs are one of the most effective ways a software foundation can spend their money. Even if you completely ignore the effect it has on diversity, the number of speakers, sprinters and other contributors that attended PyCon thanks to the financial aid program is staggering.

Naming your program

I inherited the term "Financial Aid". It's a fine term, but some other conferences have come up with different terms that you may want to consider, like "opportunity grants" and "diversity grants".

Taking care of yourself

Running a financial aid program is a lot of work. It scales linearly with the number of applicants; it's your job to create a process that keeps the work per applicant small, so that you can make the number of applicants large. (You'll know you've succeeded when the fixed overhead dominates, and it wouldn't really matter if you added another dozen people.)

It is also an exercise in deferred gratification. Typically, you will need to start preparing about a year before the conference. The gratification part only comes at the conference itself. Since you'll probably be quite busy as an organizer, it may only come after the conference is over. You probably want to make sure that you have an excellent social circle and that you're fairly self-motivated.

As the Financial Aid Chair, I have been on the receiving end of verbal abuse once. Don't put up with it.

Beat the drum

Your financial aid program is useless if people don't know about it.

Some very talented speakers refrain from sending talk proposals because they don't know if they can afford to attend.

Numbers

PyCon's financial aid program is quite large. There are a number of reasons for that:

  • PyCon's financial aid program has been around for many years, so it's quite mature.
  • The program can count on support from PyCon leadership and the Python Software Foundation.
  • Because PyCon US is the largest PyCon in the world, it acts as a nexus for people across the globe; therefore, it's important for the Python community that as many people as possible have a chance to attend.

This is just to give you a ballpark idea of our numbers.

Regardless of what your numbers will be, expect that the primary bottleneck of your financial aid program will simply be lack of funds.

Confusing parts and sad truths

No-shows

A lot of people will not show up, and not notify you (or notify you in the days around the conference). Sometimes, there's good reasons (illness, emergencies...). Sometimes, the reasons are less great. Sometimes, you won't know the reason.

From the perspective of the Financial Aid Chair, no-shows are terrible. It's a dead grant: money that's been allocated that can't easily be translated into an extra attendee. Hence, many of the suggestions I make for running financial aid processes are focused on minimizing no-shows.

Visas and travel

Many recipients have to cancel because they are unable to acquire visas to Canada or the United States. In some cases, the visa process took several months and simply did not complete in time for the conference. In others the visas were declined for various reasons.

Occam's razor tells me that many countries, particularly the United States, to some extent now Canada, but also Schengen zone countries are simply actively hostile to foreigners visiting their country.

Planning

Stand by your Chair

At the end of the day, someone's responsible for making your conference all it can be. That position is typically called the Conference Chair. They get help from teams like the Program Committee and Financial Aid to make that happen. At the end of the day they make the hard calls, and you have to execute within their guidelines. That typically includes the budget, but it also includes how you want to allocate grants. You will almost certainly be resource-constrained, so there are trade-offs to be made:

  • Do you want to help newbies, or advanced programmers?
  • Do you want to help marginalized groups? Which ones? How much?
  • Do you want repeat attendees, or first timers?
  • Do you want a few people from all over the globe, or many locals?
  • Do you want to benefit people who directly contribute to the conference? Which ones (speakers, staff...)? How much?
  • Do we care if people are receiving funds from other places? What if their employer is paying? What if their employer is also a sponsor? Does it make sense for them to give us x sponsorship fees when we're giving them a significant portion of that back in financial aid?

Your budget is, unfortunately, zero sum. Every group you benefit means less for everyone else. Helping everyone is the same as helping no-one. Everyone wants to help everyone, but it's unlikely you'll get to do that. Make everyone understands exactly what you want to accomplish; you don't want to have this argument in the middle of trying to run a financial aid process.

Free or reduced-price tickets

For many conferences, the tickets themselves can be quite expensive. It makes sense to provide them to financial aid recipients at no (or reduced) charge as part of their grant.

PyCon previously provided free registration, but now provides reduced-cost registration. This helps with no-shows, giving applicants a financial incentive to let you know if they can't attend.

Make sure that you document clearly that people will be receiving tickets, at what price they'll be receiving them, and that their spots are reserved. Common concerns from financial aid applicants:

  • "Your conference blog says that you're sold out, and I haven't received financial aid yet. Will I be able to attend?"
  • "I've already registered to reserve my spot; what do I do now that I get financial aid?"
  • "I'm a student. Will I still be able to register at the student rate if I apply for financial aid?"
  • "I applied for a larger amount because I didn't think ticket price would be included." (Unfortunately, most people tell you this far too late.)

As usual, clarity in communication is key here.

Previous years, PyCon optionally provided free registration for people who asked. This optional part was somewhat confusing. Whatever you do, make it part of the default grant application. That also means that you should probably offer the reduced-price ticket to anyone who applies for financial aid, even if you can't otherwise give them a grant. Otherwise, someone who applies for financial aid for whom you simply don't have enough funds will get punished twice: no financial aid, and no access to early bird ticket price.

Housing

Housing, in the context of financial aid, means that you pay for a bunch of hotel rooms for various dates at your conference, and then put financial aid recipients in them. To save costs, you want to pair them up, and you want to utilize the rooms maximally.

Some people think that it's a good idea to organize housing as part of your travel grants. Those people are mistaken. Housing is a terrible idea all round:

  • It doesn't scale, up or down. If you're small, you can't negotiate a worthwhile hotel block contract. If you're big, the system crashes under its /O(N²)/ weight (see below).
  • It's not good for the financial aid recipients. While there are many nice things to be said about conference hotels, they are typically not economical. When we still organized housing, many financial aid recipients opted out: they could get significantly more bang for their buck otherwise.
  • It's not good for the conference. People leave, people join, people change dates, people have preferences (or hard constraints) about who they'll stay with... Doing this for any nontrivial number of people is a logistic nightmare; doing it for trivial number of people isn't worth it.

Having humans solve the allocation problem produces inefficiencies at larger scales (i.e. humans typically come up with fairly suboptimal solutions). It's a pretty tricky problem to solve even with computers (believe me, I've tried extensively), but computers will never solve the logistic issues caused by human factors.

PyCon used to manage housing for financial aid recipients. Getting rid of this was the single best decision I've ever made for the financial aid process.

It worked out quite well for the attendees too. Providing simple tools (i.e. the equivalent of a classifieds section) is more than ample to help people find great groups to room-share with. This increased opportunities for roomsharing, because it made it much less of a hassle to mix-and-match between financial aid recipients and other attendees.

Plenty of FA people stayed in large AirBnBs or the like in groups of 6 or more, and ended up getting fantastic deals that allowed them to stay an extra few days to attend other events like sprints and tutorials.

Before the conference: from applications to allocation

Applications

Keep it simple. Use a form generator (Google Forms or Wufoo or something) for data collection. All of the processing was done with simple Python scripts, most of it in an IPython/Jupyter notebook. This enables you to create well-documented processes, which helps everyone. CSV files are your best friend.

Make as many fields on the application form as possible directly translatable to something in your allocation process. Multiple choice and boolean values are your friend; the review process will make sure the applications are accurate. Have free-form fields for documenting things, but only use them in the review process. For example, you can have a multiple choice field for Python expertise ranging from beginner to expert, and then have a free-form field for applicants' portfolios.

Names are weird. There are lots of falsehoods programmers believe about names (link). You probably want to ask for a legal name; it's quite likely that you need to keep the legal name around for your records. Ask a lawyer and/or an accountant for details. However, you probably want to ask for an (optional) preferred name as well, which you should always use when communicating with them. There's a bunch of reasons those might be different, and people may have excellent reasons for not using their legal names. For example, a legal name might give someone away as being transgender. Sometimes, you want to do that just for your /own/ convenience. People will put all sorts of stuff down as their "name", but full legal names are quite consistent. This can be useful to match up records from different sources, such as your registration database.

Speaking of gender, asking for people's gender is also tricky. Make sure you have at least a cursory understanding of how gender works before you ask. Always have a "decline to answer" button, which is distinct from "other/nonbinary". If all you really want to know is whether someone qualifies for earmarked funds, just ask the specific question you want to know; e.g. "Apply for a PyLadies grant (people who self-identify as women only, please)".

As usual in programming, state is the bane of your existence. Keep it in one place whenever possible. A (Google Drive) spreadsheet works just fine. Your scripts should operate on data extracted from them (again, CSV works fine), but not store any state. This is trickier than it sounds, but the alternative is that you'll probably end up destroying some data.

Review

Reviews are easy to do in parallel, so get help from volunteers if needed. Establish clear guidelines for what your discrete values (e.g. Python experience) mean; not everyone agrees on what "expert" means.

Allocation

I've written about allocation before.

PyCon allocates approximately 15% over budget; i.e. the budget that we allocate is 1.15x the actual grant budget. This does not include aid in the form of e.g. reduced ticket prices; remember to account for those separately!

PyCon's no-show rate is somewhere between 10% and 20%, but it's very hard to predict if the factors that contribute to that will affect your conference equally.

As mentioned in that previous blog post, you don't always want to allocate the full grant asked for. People will still be able to attend with partial grants but typically wouldn't with an empty grant. Therefore it typically makes sense to reduce everyone's grant slightly if that allows you to provide more grants.

We ended up going with a fairly simple "flood fill" algorithm. An applicant's score maps to a fraction of the budget:

$$f_i = \frac{s_i}{\sum_j s_j}$$

Where \(f_i\) is the fraction of the budget you're willing to assign to applicant \(i\), \(s_i\) is an applicant's score.

If \(f_i \ge q \cdot r_i\) (where \(q\) is the fraction of the grant you're willing to allocate and \(r_i\) is what the applicant requested, you grant them \(q \cdot r_i\); otherwise, grant them 0.

Some applicants will be below this fraction, some will be above. They could be below this fraction because they have a very high score (e.g. they are a speaker), or because they're "low hanging fruit" and not asking for very much money.

That means that if you run the algorithm again, the fractions will be bigger; the people that received allocations in the previous round were allocated less than what would've been their "fair share". Rinse, repeat until you're out of money.

Communications

Have a central website where people can see their current status and updates, both specific to the applicant and generic to the entire process. Training people to expect information there will drastically reduce the number of repetitive questions you get to answer by e-mail, which will contribute enormously to your happiness. Therefore, if people ask questions that are answered there; answer, be kind, but point out where they could have gotten that information from.

Most of the things you will have to say will be generic points about the process. Nonetheless, I have spent huge amounts of time answering individual questions about that generic process, which got get quite tedious. Hence, even a static page that doesn't show any information specific to the applicant, but just explaining the process in detail is extremely valuable.

Being up-front about how your process works is also great for prospective applicants who wouldn't feel comfortable asking. This also attracts speakers to submit talk proposals; many speakers would not propose a talk because they know they can't afford to come without financial aid. Therefore, it's important to communicate clearly if you intend to support speakers, both through your financial aid communication and your call for papers.

Once that fails, send e-mail. Once you're over a dozen or so recipients, use Mailgun. Emails particularly automated from your personal email account is a good way to get stuck in spam filters.

Disbursement (giving out money)

First off, talk to your treasurer. Possibly talk to a lawyer, too. It's quite possible, particularly if you're in the United States, that giving out a bunch of money as a non-profit comes with some fairly complex strings attached. For example, you probably have certain standards in terms of what records you have to keep.

As a European, I found it somewhat comical to still see checks in active use, but hey; it works. For many parts of the world (apparently not the United States, though) wire transfers are how you send money to people. PayPal seems to work better for larger, established organizations that use it often. There is less of a problem with the accounts or funds being frozen. It does actively restrict (or, perhaps more accurately, enforces restrictions on) sending funds to certain countries, including Brazil and India.

Cash works, but is hard to scale. With PyCon's budget of well over $100,000.00, managing cash is clearly less than ideal.

At the end of the day, be pragmatic. Do whatever your recipients can accept. Be sure to ask ahead of time, as your financial aid programs may prove to be successful in bringing people in from very different countries, with very different cultures and very different means available to them for accepting your grant. This is the case for PyCon, but I consider that a superb problem to have.

Conclusion

I'd like to thank the following people in no particular order:

  • Ewa Jodlowska, for managing PyCon and being the love of my life
  • Van Lindberg, for continuously inspiring me to serve
  • Diana Clarke, for Charing PyCon US in Montreal