How secure internet connections work is often a mystery, even for fairly technical people – let's rectify this!
Although some fairly complicated mathematics lays the foundation, it's not necessary to grasp all of it to get a good high-level understanding of the secure web. In this post, I'll give an overview of the major components of the secure web and of how they interact. I want to shed some light on the wizardry that both your browser and webservers do to make secure communication over the internet possible. Specifically, the focus is on authentication: how the public key infrastructure (PKI) guarantees you that you are really talking to your bank not to some scammer.
This is going to be a high-level overview with a lot of handwaving. We'll gloss over some of the nitty gritty details, and focus on the big picture.
Let's start off with the basic building block of public-key cryptography, the public/private keypair. It consists of two halves: the public part can be shared with the world, the private half must stay hidden, for your eyes only. With such a keypair, you can do three pretty nifty things:
- People can encrypt data with your public key, and only you can decrypt it.
- You can prove to other people (who have your public key) that you know the private key. In some sense, they can verify your identity.
- You can sign data with your private key, and everyone with the public key can verify that this data came from you and was not tempered with.
Let's look at an example to see how cool this is: let's say your bank has a keypair and you know their public key (it could be printed in large letters on the walls of their building, it's public after all). You can then securely communicate with your bank, without anyone being able to listen in, even if they can intercept or modify the messages between you and the bank (encryption). You can be sure you really are talking to the bank, and not to a fraudster (verification). And the bank can make statements that anyone who has their public key can ascertain is genuine, i.e. it really came from the bank (signing).
The last part is nice because the bank can sign a statement about your balance, give it to you, and you can forward it to your sleazy landlord who wants proof of your financial situation. The bank and the landlord never directly talk with each other, nevertheless the latter has full certainty that the statement was made by the bank, and that you didn't tamper with it.
So it's cool that we can securely communicate with our banks. We can do the same with websites: once we have the public key of e.g. Google, it's easy to setup an encrypted communication channel. Via the verification function of keypairs, it's also easy to prove we really are talking to that Google, and not to some kid who's trying to steal our password to post dog pictures in our cat groups.
How do we get Google's public key? — This is where things start going downhill.
In the bank example, we'd gotten the public key personally from the bank (written on its front wall). With Google, it'd be kind of difficult to travel to Mountain View just to get get their public key. And we can't just go and download the key from google.com, the whole point is that we're not sure that the google.com we're talking to is the real Google.
Are we completely out of luck? Can we communicate securely over the internet only if we manually exchange keys before, which we usually can't? It turns out we are only sort-of out of luck: we are stuck with certificates and the halfway-broken system of certificate authorities.
A certificate contains several parts:
- an identifier (a hash) that uniquely identifies a keypair
- metadata that says who this keypair belongs to
- signatures: statements signed by other keys that vouch that the keypair referenced here really belongs to the entity described in the metadata section
It's important to note that certificates don't need to be kept secret. The keypair identifier doesn't reveal the private key, so certificates can be shared freely. The corollary of this is that a certificate alone can't be used to verify you're talking to anyone in particular. To be used for authentification, it needs to be paired with the associated private key.
With that out of the way, let's look at why certificates are useful. Say someone gives you a certificate and proves they have the associated private key. You've never met this person. However, the certificate carries signatures from several keys that you know belong to close friends of yours. All of those signatures attest that this person is called "Hari Seldon". If you trust your friends, you can be pretty certain that the person is really called that way.
When you think about this, it's kind of neat. A stranger can authenticate to you (prove that they say who they are) just because someone you trust made a statement confirming the stranger's identity. That this statement is really coming from your trusted friend is ensured, because it's signed with their private key.
The same concept can be applied websites. As long as there's someone you trust and you have their public key, they can sign other people's certificates to affirm that identity to you. For example, they can sign a certificate for Google that says "This really is the real google.com". When you see that certificate and verify that the other party has the associated private key, you'll have good reason to believe that you really are talking to Google's google.com server, not some scam version by a North Korean hacker.
So how do you find someone you can trust? And how does that person make sure that the certificate they are signing really belongs to Google? They face the same problems confirming that fact as you did! Does this even improve the situation in any way?
It does – let's take the questions in order. The reality on the internet is: it's not you trusting someone, it's your browser that does the trusting. Your browser includes public keys from so-called "certificate authorities" (CA's). You can find the list of CA's trusted by your own browser in its options, under Advanced / Security / Certificates / Authorities. If the browser sees certificates signed by any one of these keys, it believes them to be true. It trusts CA's not to sign any bogus certificates.
Why are these keys trustworthy? Because CA's are mostly operated by large companies that have strict policies in place to make sure they only sign stuff that's legit. How do they do that? After all, as an individual you'd have a pretty tough time verifying that the public key offered by google.com really belongs to Google. Don't CA's face the same problem?
Not really. There are billions of people accessing google.com. There are only about 200 CA's that are trusted by the common browsers. And Google needs a signed certificate by only one of them (one signature is enough to earn the browser's trust). So Google can afford to prove it's identity to a CA: by sending written letters, a team of lawyers, or whatever. Once Google gets a certificate for google.com signed by any reputable CA, it is recognized by pretty much every device in the world.
Similarly, I, as a private person, can get a certificate for caichinger.com by proving my identity and my ownership of this domain to the CA. The identity part is usually done by submitting a scan of a driver's license or passport. Ownership of the domain can be shown by uploading a file supplied by the CA to the webserver. Once the CA confirms that the file is there, it knows I have control of that domain.
So instead of me having to prove my identity to every single user visiting this website, I can prove it once to a CA, and all browsers coming here will recognize this as good enough. This way, CA's make the problem of authentication of servers ("I'm the real google.com, not a cheap fake") tractable. It's a system that has made the large-scale deployment of secure internet traffic via HTTPS possible.
The half-broken part
Let's get back to the analogy of a stranger authenticating to you via a certificate signed by someone you know. What if the signature wasn't from a close friend of yours, but from a seedy guy you meet occasionally when going out? Would you still have full confidence in the certificate? Hopefully not.
What does this mean for the web?
Not all of the CA's included in the common web browsers are the equivalent of a trusted friend:
- They may be in control of some government who wants a certificate for gmail.com, so it can read dissident's emails
- An employee with access to the certificate authority key may create certificates for bank websites and sell them on the black market
- The computer network where the CA keys are stored could have been hacked
I'm pretty sure all three of those have actually happened in the past. Given that a single forged certificate can be used to attack millions of users, CA's are juicy targets. As soon as forged certificates are detected in the wild, they tend to be blacklisted (blocked by browsers) very quickly, but there is still a window of vulnerability.
For this reason, the whole CA system has been questioned over the last few years, but replacing it does not seem feasible at the moment. There are techniques (such as public key pinning) to augment the CA-based authentication, but it takes time for them to be picked up by website owners.
While this is a problem, it mostly affects the largest websites (obtaining a forged certificate is difficult and costly). Together with browser vendors, they are developing new mitigation techniques against forged certificates. In the meantime, the rest of us is still pretty well served by the current CA system, even though it is not perfect.
So, this is it for an overview of the public key infrastructure that enables secure connections to internet sites, from the basics of public key cryptography to certificate authorities. If you want to dig deeper, I recommend starting with the Wikipedia articles I linked throughout the article. If you are interested in cryptography in general, I highly recommend Bruce Schneier's book Applied Cryptography. It's 20 years old now, and still enormously relevant today.
I hope this text helps a bit to clear up the confusion associated with public key cryptography and the secure web. If you liked it, or if you have any suggestions for improvement, please let me know in the comments!