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 _@lvh.io.