October 25, 2013

Namecoin, A Replacement For SSL

At cryptic.io we are creating a client-side, in-browser encryption system where a user can upload their already encrypted content to our storage system and be 100% confident that their data can never be decrypted by anyone but them.

One of the main problems with this approach is that the client has to be sure that the code that’s being run in their browser is the correct code; that is, that they aren’t the subject of a man-in-the-middle attack where an attacker is turning our strong encryption into weak encryption that they could later break.

A component of our current solution is to deliver the site’s javascript (and all other assets, for that matter) using SSL encryption. This protects the files from tampering in-between leaving our servers and being received by the client. Unfortunately, SSL isn’t 100% foolproof. This post aims to show why SSL is faulty, and propose a solution.

SSL

SSL is the mechanism by which web-browsers establish an encrypted connection to web-servers. The goal of this connection is that only the destination web-browser and the server know what data is passing between them. Anyone spying on the connection would only see gibberish. To do this a secret key is first established between the client and the server, and used to encrypt/decrypt all data. As long as no-one but those parties knows that key, that data will never be decrypted by anyone else.

SSL is what’s used to establish that secret key on a per-session basis, so that a key isn’t ever re-used and so only the client and the server know it.

Public-Private Key Cryptography

SSL is based around public-private key cryptography. In a public-private key system, you have both a public key which is generated from a private key. The public key can be given to anyone, but the private key must remain hidden. There are two main uses for these two keys:

  • Someone can encrypt a message with your public key, and only you (with the private key) can decrypt it.

  • You can sign a message with your private key, and anyone with your public key can verify that it was you and not someone else who signed it.

These are both extremely useful functions, not just for internet traffic but for any kind of communication form. Unfortunately, there remains a fundamental flaw. At some point you must give your public key to the other person in an insecure way. If an attacker was to intercept your message containing your public key and swap it for their own, then all future communications could be compromised. That attacker could create messages the other person would think are from you, and the other person would encrypt messages meant for you but which would be decrypt-able by the attacker.

How does SSL work?

SSL is at its heart a public-private key system, but its aim is to be more secure against the attack described above.

SSL uses a trust-chain to verify that a public key is the intended one. Your web browser has a built-in set of public keys, called the root certificates, that it implicitly trusts. These root certificates are managed by a small number of companies designated by some agency who decides on these things.

When you receive a server’s SSL certificate (its public key) that certificate will be signed by a root certificate. You can verify that signature since you have the root certificate’s public key built into your browser. If the signature checks out then you know a certificate authority trusts the public key the site gave you, which means you can trust it too.

There’s a bit (a lot!) more to SSL than this, but this is enough to understand the fundamental problems with it.

How SSL doesn’t work

SSL has a few glaring problems. One, it implies we trust the companies holding the root certificates to not be compromised. If some malicious agency was to get ahold of a root certificate they could listen in on any connection on the internet by swapping a site’s real certificate with one they generate on the fly. They could trivially steal any data we send on the internet.

The second problem is that it’s expensive. Really expensive. If you’re running a business you’ll have to shell out about $200 a year to keep your SSL certificate signed (those signatures have an expiration date attached). Since there’s very few root authorities there’s an effective monopoly on signatures, and there’s nothing we can do about it. For 200 bucks I know most people simply say “no thanks” and go unencrypted. The solution is creating a bigger problem.

Bitcoins

Time to switch gears, and propose a solution to the above issues: namecoins. I’m going to first talk about what namecoins are, how they work, and why we need them. To start with, namecoins are based on bitcoins.

If you haven’t yet checked out bitcoins, I highly encourage you to do so. They’re awesome, and I think they have a chance of really changing the way we think of and use money in the future. At the moment they’re still a bit of a novelty in the tech realm, but they’re growing in popularity.

The rest of this post assumes you know more or less what bitcoins are, and how they work.

Namecoins

Few people actually know about bitcoins. Even fewer know that there’s other crypto-currencies besides bitcoins. Basically, developers of these alternative currencies (altcoins, in the parlance of our times) took the original bitcoin source code and modified it to produce a new, separate blockchain from the original bitcoin one. The altcoins are based on the same idea as bitcoins (namely, a chain of blocks representing all the transactions ever made), but have slightly different characterstics.

One of these altcoins is called namecoin. Where other altcoins aim to be digital currencies, and used as such (like bitcoins), namecoin has a different goal. The point of namecoin is to create a global, distributed, secure key-value store. You spend namecoins to claim arbitrary keys (once you’ve claimed it, you own it for a set period of time) and to give those keys arbitrary values. Anyone else with namecoind running can see these values.

Why use it?

A blockchain based on a digital currency seems like a weird idea at first. I know when I first read about it I was less than thrilled. How is this better than a DHT? It’s a key-value store, why is there a currency involved?

DHT

DHT stands for Distributed Hash-Table. I’m not going to go too into how they work, but suffice it to say that they are essentially a distributed key-value store. Like namecoin. The difference is in the operation. DHTs operate by spreading and replicating keys and their values across nodes in a P2P mesh. They have lots of issues as far as security goes, the main one being that it’s fairly easy for an attacker to forge the value for a given key, and very difficult to stop them from doing so or even to detect that it’s happened.

Namecoins don’t have this problem. To forge a particular key an attacker would essentially have to create a new blockchain from a certain point in the existing chain, and then replicate all the work put into the existing chain into that new compromised one so that the new one is longer and other clients in the network will except it. This is extremely non-trivial.

Why a currency?

To answer why a currency needs to be involved, we need to first look at how bitcoin/namecoin work. When you take an action (send someone money, set a value to a key) that action gets broadcast to the network. Nodes on the network collect these actions into a block, which is just a collection of multiple actions. Their goal is to find a hash of this new block, combined with some data from the top-most block in the existing chain, combined with some arbitrary data, such that the first n characters in the resulting hash are zeros (with n constantly increasing). When they find one they broadcast it out on the network. Assuming the block is legitimate they receive some number of coins as compensation.

That compensation is what keeps a blockchain based currency going. If there were no compensation there would be no reason to mine except out of goodwill, so far fewer people would do it. Since the chain can be compromised if a malicious group has more computing power than all legitimate miners combined, having few legitimate miners is a serious problem.

In the case of namecoins, there’s even more reason to involve a currency. Since you have to spend money to make changes to the chain there’s a disincentive for attackers (read: idiots) to spam the chain with frivolous changes to keys.

Why a new currency?

I’ll admit, it’s a bit annoying to see all these altcoins popping up. I’m sure many of them have some solid ideas backing them, but it also makes things confusing for newcomers and dilutes the “market” of cryptocoin users; the more users a particular chain has, the stronger it is. If we have many chains, all we have are a bunch of weak chains.

The exception to this gripe, for me, is namecoin. When I was first thinking about this problem my instinct was to just use the existing bitcoin blockchain as a key-value storage. However, the maintainers of the bitcoin clients (who are, in effect, the maintainers of the chain) don’t want the bitcoin blockchain polluted with non-commerce related data. At first I disagreed; it’s a P2P network, no-one gets to say what I can or can’t use the chain for! And that’s true. But things work out better for everyone involved if there’s two chains.

Bitcoin is a currency. Namecoin is a key-value store (with a currency as its driving force). Those are two completely different use-cases, with two completely difference usage characteristics. And we don’t know yet what those characteristics are, or if they’ll change. If the chain-maintainers have to deal with a mingled chain we could very well be tying their hands with regards to what they can or can’t change with regards to the behavior of the chain, since improving performance for one use-case may hurt the performance of the other. With two separate chains the maintainers of each are free to do what they see fit to keep their respective chains operating as smoothly as possible. Additionally, if for some reason bitcoins fall by the wayside, namecoin will still have a shot at continuing operation since it isn’t tied to the former. Tldr: separation of concerns.

Namecoin as an alternative to SSL

And now to tie it all together.

There are already a number of proposed formats for standardizing how we store data on the namecoin chain so that we can start building tools around it. I’m not hugely concerned with the particulars of those standards, only that we can, in some way, standardize on attaching a public key (or a fingerprint of one) to some key on the namecoin blockchain. When you visit a website, the server would then send both its public key and the namecoin chain key to be checked against to the browser, and the browser would validate that the public key it received is the same as the one on the namecoin chain.

The main issue with this is that it requires another round-trip when visiting a website: One for DNS, and one to check the namecoin chain. And where would this chain even be hosted?

My proposition is there would exist a number of publicly available servers hosting a namecoind process that anyone in the world could send requests for values on the chain. Browsers could then be made with a couple of these hardwired in. ISPs could also run their own copies at various points in their network to improve response-rates and decrease load on the globally public servers. Furthermore, the paranoid could host their own and be absolutely sure that the data they’re receiving is valid.

If the above scheme sounds a lot like what we currently use for DNS, that’s because it is. In fact, one of namecoin’s major goals is that it be used as a replacement for DNS, and most of the talk around it is focused on this subject. DNS has many of the same problems as SSL, namely single-point-of-failure and that it’s run by a centralized agency that we have to pay arbitrarily high fees to. By switching our DNS and SSL infrastructure to use namecoin we could kill two horribly annoying, monopolized, expensive birds with a single stone.

That’s it. If we use the namecoin chain as a DNS service we get security almost for free, along with lots of other benefits. To make this happen we need cooperation from browser makers, and to standardize on a simple way of retrieving DNS information from the chain that the browsers can use. The protocol doesn’t need to be very complex, I think HTTP/REST should suffice, since the meat of the data will be embedded in the JSON value on the namecoin chain.

If you want to contribute or learn more please check out namecoin and specifically the d namespace proposal for it.