Friday, January 9, 2015

Real World Crypto 2015: “Designers: ask not what your implementer can do for you, but what you can do for your implementer” Dan Bernstein, Real World Crypto 2015

Last year at Real World Crypto you may remember my blog post about how the conference went about the business of lamented the downfall of theoretically secure cryptographic primitives based on practical implementation issues. Well this year at real world crypto things have gone in a similar direction. An example of this is Bernstein's talk on Wednesday entitled “Error-prone cryptographic designs.” Much of the talk has already been described in a previous post by David B here which helpfully gives us the “Bernstein principle” which I'll follow up on, perhaps from a more applied point of view.

As the title suggests the talk was about error prone cryptography, but not as you might expect. The talk was not a frustrated outburst at how the practical implementation side of things was letting the side down and needed to get it's act together, the tone of which you may be familiar with from other talks with similar titles. The talk did contain many of the ingredients of this sort of talk. There was a look at how the problems with cryptosystems seem always to come from the implementation of secure primitives not the primitives themselves, backed up by a number of horror stories showing this for instance. But this talk had one major twist.

The main point of the presentation was made near the beginning in one of the horror stories (see blog post), when he referred to the breaking of the encryption used by Sony Playstation. Relating to this he observed how the blame for the attack got distributed. The attack was a practical attack and so naturally the designers of the primitives remarked how their secure designs had been made vulnerable by the terrible implementers who seem to keep getting things wrong and making all their hard work meaningless. But wait a minute, asked the implementers, if we keep getting what you designs wrong, perhaps the problem is with you? Perhaps the designs of the primitives are so hard to implement properly that the designers are really to blame for not taking implementation practicalities seriously enough in their design choices.
So who was to blame for this and many other security slip ups that have occurred through implementation errors? Well the answer to the question still remains unresolved, but the fact that the discussion is taking place throws a different kind of light on the practice of designing crypto systems. The current system of designer make something secure; implementer implement the design securely, may well be where many of the problems are coming from and should be replaced by something more like designer make something that's secure and easy to implement; implementer … you can't really go wrong!

Although unlikely to have drawn the agreement of the whole audience, the talk didn't have the feel of a dig at designers but an appeal to them to wake up to their responsibility to the implementers in making designs that are easy to implement. It neither had the air of trying to get implementers off the hook, but sought rather to look at the question as to why in a world where secure systems seem to break because of practical attacks against implementations rather than the security of the primitives aren't the primitives ever singled out to blame for being hard to implement rather than the people given the task of implementing them.
It's true, designers have a hard problem on their hands making sure what they design is secure, but often being from a more mathematical, theoretical background, have they tended to overlook the practical aspect of what they are concocting? Many designers are highly skilled in the art of theoretical design but is this at the expense of not knowing the real engineering world in which they work as well as they should?
The talk examined these and other similar questions and was summarised with an appeal to designers to “think of the children, think of the implementers” and ended with the words given as the title for this post.

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