Amir Chaudhry

thoughts, comments & general ramblings

The Bitcoin Piñata!

Last summer we announced the beta release of a clean-slate implementation of TLS in pure OCaml, alongside a series of blog posts that described the libraries and the thinking behind them. It took two hackers six months — starting on the beach — to get the stack to that point and their demo server is still going strong. Since then, the team has continued working and recently presented at the 31st Chaos Communication Congress.

The latest example goes quite a bit further than a server that just displays the handshake. This time, the team have constructed a Xen unikernel that’s holding a private key to a bitcoin address and are asking people to try and break in. Hence, they’ve called it the Bitcoin Piñata!*

What the Bitcoin Piñata does

Bitcoin Pinata

The Piñata unikernel will transmit its private bitcoin key if you can successfully set up a TLS connection but it’s rigged so that it will only create that connection if you can present the certificate it’s expecting to see — which has been signed appropriately. Of course, you’re not being given the secret key with which to do that signing and that means there should be no way for anyone to form a TLS connection with the Piñata. In order to get the private key to the bitcoin address, you’ll have to smash your way in.

Helpfully (perhaps), things are set up so that you can make the Piñata talk to itself, allowing you to eavesdrop on a successful connection and see the encrypted traffic. In addition, all the code and libraries are open-source so you can look through any of the codebase. There isn’t anything that anyone will have to reverse engineer, which should make this a little more enjoyable.

This contest is set to run until mid-March or whenever the coins are taken. If someone does manage to get in, please do let us know how!

The Rubber-hose approach

Of course there are many other ways to get at the private key and as many people like to comment, the human element is sometimes the weakest link — after all, a safe is only as secure as the person with the combination.

In this case, there is obviously a secret key or certificate somewhere that could be presented so it may be tempting to go hunting for that. Perhaps phishing attempts on the authors may yield a way forward, or maybe just straight-forward Rubber-hose cryptanalysis! Sure, these options might provide a result but this is meant to be fun. The authors haven’t specified any rules but please be nice and focus on the technical things around the Piñata. Don’t be this guy.


What’s the point of this contest?

Even though the Bitcoin Piñata is clearly a contest, nobody is deluding themselves into thinking that if the coins are still there in March, that somehow the stack can be declared ‘undefeated’ — while pleasing, that result wouldn’t necessarily prove anything. Contests have their place but as Bruce Schneier already pointed out, they are not useful mechanisms to judge security.

However, it does give us the chance to engage in some shameless self-promotion and try to draw vast amounts of attention to the work. That, and the chance to stress-test the stack in the wild. Ultimately, we want to use this code in production but must take a lot of care to get there and want to be sure that it can bear up. This is just one more way of learning what happens when putting something ‘real’ out there.

If the Bitcoins do end up being taken, then there’s definitely something valuable that the team can learn from that. Regardless of the Piñata, if we have more people exploring the TLS codebase or trying it out for themselves, it will undoubtedly be A Good Thing.

Responsible sidenote

For clarity and to avoid any doubt, please be aware that the TLS codebase is missing external code audits and is not yet intended for use in any security critical applications. All development is done in the open, including the tracking of security-related issues, so please do consider auditing the code, testing it in your services and reporting issues.

* If you've never come across a piñata before, hopefully the gif in the post gives you an idea. If not, the wiki page will surely help, where I learned that the origin may be Chinese rather than Spanish!

Of course, I'm not suggesting that anyone would actually go this far. I'm simply acknowledging that there is a human factor and asking that we put it aside.

Edit to add: After seeing Andrea's tweet I should point out that any part of MirageOS, including the networking stack, OCaml runtime etc is a legitimate vector. It's why there's a manifest of the libraries that have been used to build the Piñata!

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