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The Security Behind the Birth of Zcash

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“How would you feel about donating your phone to science?”

When Zooko Wilcox posed this question to me in October, what I heard was: Can I take your phone and hand it over to a hacker to riffle through its contents and sniff all over your data like a pervert who’s just opened the top drawer of a lady’s dresser?

At least, that’s how it felt.

“I think I’d rather donate my body,” I said.

What Wilcox really wanted to do with my phone was to run forensic analysis on it in the hopes of determining whether someone was using it to spy on us. Wilcox is the CEO of a company called Zcash which designed and recently launched a new privacy-preserving digital currency of the same name. On the weekend he asked for my phone we were both sitting with a two-man documentary film crew in a hotel room stuffed with computer equipment and surveillance cameras.

A secret ceremony was underway. Before the company could release the source code of its digital currency and turn the crank on the engine, a series of cryptographic computations needed to be completed and added to the protocol. But for complex reasons, Wilcox had to prevent the calculations from ever being seen. If they were, it could completely compromise the security of the currency he had built.

Over the course of the two-day event, everything went pretty much as planned. Everyone and everything did just what they were supposed to do, except for my cellphone, which in the middle of the event exhibited behaviors that made no sense at all and which planted suspicions that it had been used in a targeted attack against the currency.

The story of Zcashhas already been roughly sketched by me and others. The currency launched 28 October onto the high seas of the cryptocurrency ecosystem with a strong wind of hype pushing violently at its sails. On the first morning that Zcash existed, it was trading on cryptocurrency exchanges for over US $4000 per coin. By the next day, the first round of frenzied feeding had subsided and the price was already below $1000. Now, a month later, you’ll be lucky if you can get $100 for a single Zcash coin. Even in the bubble-and-burst landscape of cryptocurrency trading, these fluctuations are completely insane.

Some hype was certainly warranted. The vast majority of digital currencies out there are cheap Bitcoin imitations. But the same cannot be said of Zcash. The project, which was three years in the making and which combines the cutting edge research of cryptographers and computer scientists at multiple top universities, confronts Bitcoin’s privacy problems head on, introducing an optional layer of encryption that veils the identifying marks of a transaction: who sent it, how much was sent, who received it. InBitcoin, all of this data is out in the public for anyone to see.

However, with digital currencies, everything is a trade-off, and the improvement in privacy that Zcash brings comes with a risk, one that has gotten much less attention since the currency launched. Obscuring data on theblockchain inevitably complicates the process of verifying the validity of transactions, which in Bitcoin is a simple matter of tracking coins on a public ledger. In Zcash, verifying transactions requires some seriously experimental computation, mathematical proofs called zk-SNARKS that are so hot-off-the-presses that they’ve never been used anywhere else. In order to set up the zk-SNARKS in the Zcash protocol, a human being must create a pair of mathematically linked cryptographic keys. One of the keys is essential to ensuring the proper functioning of the currency, while the other one―and here’s the big risk―can be used to counterfeit new coins.

If it’s not immediately clear how this works, you’re in good company. The number of people who really understand zk-SNARKs, and therefore the Zcash protocol, is probably small enough that you could feed them all with one Thanksgiving turkey. The important thing to get is that, given the current state of cryptographic research, it’s impossible to create a private, reliable version of Zcash without also simultaneously creating the tools for plundering it. Let’s call those tools the bad key.

Prior to launching Zcash, the developers who invented it had to create the bad key, use it to make a set of mathematical parameters for the zk-SNARKS (the good key), then dispose of the bad key before any nefarious individual could get hold of it. And they had to do it all in a way that was both secret enough to be secure yet public enough that anyone who wanted to use Zcash felt well-assured of the technology’s integrity.

The Zcash developers, whose work is funded by over $2 million raised from private investors in the Zcash Company, chose a strategy that relied heavily on the secrecy part of this equation. Nearly everything about the ceremony―where and when it would be held, who would be involved, what software would be used―was kept from the public until a blog post about it was published this afternoon.

Instead of building real-time transparency into the ceremony design, the Zcash team opted to meticulously document the event and save all artifacts that remained after the bad key was destroyed. This evidence is now available for analysis to prove the process went as it was described.

As an extra measure, they decided to invite a journalist to bear witness―me.

Two weeks before the ceremony,I got a vague invite on Signal, an encrypted messaging app, from Wilcox without any specifics about what to expect. A week later he told me where I would have to go. And a week after that―two days before the ceremony―I was told when to arrive. On 21 October, I walked into a coffee shop in Boulder Colorado where I met up with Wilcox and a documentary filmmaker who had been hired to get the whole thing on tape. From there we headed to a computer shop in Denver to buy a bunch of equipment and then returned to a hotel in Boulder, where I stayed for the next three days.

The headquarters in Boulder was one of five “immobile” stations, all of which were participating in the ceremony from different cities across the planet. One mobile station was doing its part while making a mad dash across British Columbia. The generation of the keys was decentralized such that each station would only be responsible for creating a fragment of the bad key. For the ceremony, a cryptographic algorithm was custom designed that created a full version of the zk-SNARK parameters while keeping the pieces of the bad key segregated, a process that took two days of relaying data back and forth among the six stations.

I’ll hazard an analogy in order to explain more generally how this works: Let’s say you have a recipe and you want to use it to make a single cake that is going to feed everyone in the world and that’s the only cake that anyone is allowed to eat, ever. You have to have a recipe to bake the cake, but you also have to make sure no one can ever make it again. So you split the recipe up into six parts and you design a baking process that allows each participant to add their ingredients and mix them into the batter without the others (or anyone else) seeing what they’re up to. After pulling the cake out of the oven, you burn all the pieces of the recipe.

In this analogy, the recipe is the bad key; the cake is the zk-SNARK parameters; and the person hiding the ingredients and doing all of the mixing is a cryptographic algorithm.


The Security Behind the Birth of Zcash
Photo: Morgen Peck

Zooko Wilcox, Zcash CEO, with a DVD containing part of a record of the cryptographic ceremony.

The way this

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