alex gaynor's blago-blog

Why Crypto

Posted February 12th, 2014.

People who follow me on twitter or github have probably noticed over the past six months or so: I've been talking about, and working on, cryptography a lot. Before this I had basically zero crypto experience. Not a lot of programmers know about cryptography, and many of us (myself included) are frankly a bit scared of it. So how did this happen?

At first it was simple: PyCrypto (probably the most used cryptographic library for Python) didn't work on PyPy, and I needed to perform some simple cryptographic operations on PyPy. Someone else had already started work on a cffi based cryptography library, so I started trying to help out. Unfortunately the maintainer had to stop working on it. At about the same time several other people (some with much more cryptography experience than I) expressed interest in the idea of a new cryptography library for Python, so we got started on it.

It's worth noting that at the same time this was happening, Edward Snowden's disclosures about the NSA's activities were also coming out. While this never directly motivated me to work on cryptography, I also don't think it's a coincidence.

Since then I've been in something of a frenzy, reading and learning everything I can about cryptography. And while originally my motivation was "a thing that works on PyPy", I've now grown considerably more bold:

Programmers are used to being able to pick up domain knowledge as we go. When I worked on a golf website, I learned about how people organized golf outings, when I worked at rdio I learned about music licensing, etc. Programmers will apply their trade to many different domains, so we're used to learning about these different domains with a combination of Google, asking folks for help, and looking at the result of our code and seeing if it looks right.

Unfortunately, this methodology leads us astray: Google for many cryptographic problems leaves you with a pile of wrong answers, very few of us have friends who are cryptography experts to ask for help, and one can't just look at the result of a cryptographic operation and see if it's secure. Security is a property much more subtle than we usually have to deal with:

>>> encrypt(b"a secret message")
b'n frperg zrffntr'

Is the encrypt operation secure? Who knows!

Correctness in this case is dictated by analyzing the algorithms at play, not by looking at the result. And most of us aren't trained by this. In fact we've been actively encouraged not to know how. Programmers are regularly told "don't do your own crypto" and "if you want to do any crypto, talk to a real cryptographer". This culture of ignorance about cryptography hasn't resulted in us all contacting cryptographers, it's resulted in us doing bad crypto:

Usually when we design APIs, our goal is to make it easy to do something. Cryptographic APIs seem to have been designed on the same principle. Unfortunately that something is almost never secure. In fact, with many libraries, the path of least resistance leads you to doing something that is extremely wrong.

So we set out to design a better library, with the following principles:

  • It should never be easier to do the wrong thing than it is to do the right thing.
  • You shouldn't need to be a cryptography expert to use it, our documentation should equip you to make the right decisions.
  • Things which are dangerous should be obviously dangerous, not subtly dangerous.
  • Put our users' safety and security above all else.

I'm very proud of our work so far. You can find our documentation online. We're not done. We have many more types of cryptographic operations left to expose, and more recipes left to write. But the work we've done so far has stayed true to our principles. Please let us know if our documentation ever fails to make something accessible to you.

This entry was tagged with python, open-source.
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