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Published on 28 May 2020
Submitted on 8 August 2020
Reading encryption papers so you don't have to
The fediverse has been buzzing with talk about end-to-end encryption. In this (quite long) article, I’ll try to explain what E2EE is, what some common approaches to it are in the instant messaging space, and what I think would be the best way to use it on the fediverse.
Pretty much all fediverse servers use encryption during transport of your message. That means that when you write “#cofe” and smash that ‘Submit’ button, your browser encrypts the message and sends it to your server. Your server can decrypt your message, and it sends it on to other servers. This connection is, again, encrypted. So nobody between you and your server, or between your server and other servers, can read your message.
Still, if you send a private message to your friend on another server, who can read it? You and your friend can, of course, but also your two servers and server administrators. The messages live in their database in clear text, ready to be read by a curious admin.
End-to-end encryption solves this problem, making the message readable by only you and the recipient. How does that work? Your client encrypts your message with a key that only you and your friend know. Then, you send the encrypted message to the server. The server can’t read it and just sends it on. Finally, it arrives at your friend’s client, is decoded there and can be read (‘suya..').
But how do you two exchange that secret key? Let’s quickly review two popular approaches.
PGP (Pretty Good Privacy) and similar cryptographic approaches are called
asymmetric, because they have two keys instead of one. One of those is the
private key that you don’t share with anyone, the other
one is the aptly named
public key which you can share freely
with anybody who should send you an encrypted message. In this system, the
message can be encrypted with the public key, but only the private key can
Now anyone who wants to send you a message can encrypt it (just for you!), send it through any channel, and you can decrypt it on your end. Seems easy enough! So why is this not the preferred approach for messaging systems? Well, it has a big drawback. The same key is used to decode every message. If that key gets stolen, every message ever sent can be decrypted. It’s also not easy to switch to a new key, as you then have to give out the new public key to everyone. So these keys both tend to be a very valuable and long-lived, both properties you wouldn’t want in a cryptography system.
This one goes by a lot of names. It’s used by Signal, WhatsApp, Matrix and many other messaging systems. Double ratchet cryptography is one way to make a stolen message key less problematic. It’s also a lot more complicated than the rather simple PGP. First, let’s explain what a ‘ratchet’ is in this context.
A ratchet is a function that can generate a series of values based on previous values, but you can’t get back from later values to previous values. These are used to generate the keys. Let me illustrate this. (Note: The ratchets here are a bit simpler than how the are actually used in the protocol. The idea is the same.)
f(x) = x * 2
Here’s a function that’s not a ratchet.
If you start with the value
1, how does this develop when we
always feed the next step with the output of the last? Well, I’m sure
you can remember this much math:
So we do get a new value each time, and it even is unique. But if you see a value like 16, it’s trivial to go back in time to any previous step - just divide by two again! So we can not use this function as a ratchet.
f(x) = md5(x)
MD5 is a hashing function. You can look it up on Wikipedia if you want to know details, but for now it’s enough to know that it generates a value for any input. Let’s try our game again.
Now if you manage to somehow steal the key with the value
ca745939d5fe6f6b781c74dee3c3153c from somebody, you’ll
have no way to figure out the earlier keys. Thus, we can use these keys to
encrypt our messages, and nobody can go ‘back in time’ to decrypt
(Note: don’t use MD5 in any cryptographical context. It’s not actually secure anymore. Use something like SHA-3 instead.)
Now to send a message, we need a key from the ratchet. But to get the
ratchet, first me and my friend will need to exchange some starting value for
it that only we two know. Luckily, there’s an almost magical procedure
Diffie-Hellman key exchange which allows exchanging a
secret key over a public channel. It’s also during this step that your
identity is checked so that nobody can impersonate you. Yes, this is actually
possible, and you’ll just have to believe me because this introduction
is already too long.
Now that you two have a secret, you can feed the ratchet with it and start sending messages with the keys it generates, one after the other. You have both a sending and receiving ratchet (and no, this is not the ‘double’ of the double ratchet), so when you send a message, you advance your sending ratchet and encode a message with the key. When you receive message, you advance the receiving ratchet and decode it with the key. Your sending ratchet is your friend’s receiving ratchet and vice versa.
As you see here, every single message is using a new key for encryption. If you can steal the encryption key at any point it time, you can only decode exactly one message with it. That’s a lot better than the ‘decode everything’ we had with PGP!
But what if an attacker can steal the ratchet? They can’t decode old messages, because those keys are already gone. But they decode every new message that’s being send. Not so good! That’s where the second ratchet comes into play.
While sending a message, you advance your key ratchet and send the encrypted text. But with it, you also send another try at doing a Diffie Hellman key exchange. As soon as the other side sends you a message back, you two can complete the DH exchange and have a new secret key that only you two know! This one is then used to advance the DH ratchet (this is the ‘double’ ratchet), which generates a value that in turn is used to generate new sending and receiving ratchets.
So the ratchets that generate your keys are thrown away every time a send receive cycle is completed! What this means is that, should an attacker get access to the key ratchet, they can only read your messages until you send and receive a message from your friend. In this sense, the algorithm is self-healing and will make the once-broken encryption work again after a short amount of messages (unless it’s a very one-sided conversation)…
Let’s see how these systems work out in practice.
It’s often said that the best way to use encryption is to enable it by default. If possible, you should even make it impossible to send unencrypted messages. This is because, by making even non-secret conversations encrypted, you make secret and non-secret conversations look the same, so an attacker has a harder time to find ‘suspicious’ people to attack.
But is this true in every case? It surely would be if encryption was free, but, depending on the kind of encryption, it comes with huge tradeoffs.
Let’s refresh our knowledge of the nice properties of the Double Ratchet algorithm:
This is indeed very secure. But let’s look at a very natural usage pattern for messengers.
I’m meeting a friend for cofe. We’re talking about this cool new encrypted messenger, we install it and add each other. On sending a message, the app asks us to verify that the other person is indeed the person we’re talking to. We do that (we’re sitting next to each other, anyway). I send a message to my friend from my phone. It is encrypted and she reads it on her phone. We send a few messages back and forth and everything works fine. Later that day, I’m at my PC and want to answer her. Opening the chat, there are no messages. Why?
The server didn’t save them after the clients acknowledged the receipt, because they are only decryptable by the client anyway. So I don’t get any history.
I send a message to her from my pc. But then, I see my phone buzzing with a message, asking me to verify the new participating device in the conversation. Why that? Well, someone could have stolen my password and added a new device with it. In that case, I absolutely wouldn’t want to trust any new devices! Alright, I verify that it is indeed me. Suddenly, I get a call from my friend. She’s asking me if the new device is really me. Just like me, she too has to verify the identify of the new device. End-to End encryption is not user-to-user encryption, it is device-to-device encryption, which means that every device has to verify each other device.
Now imagine a group chat with 200 people, and then you add one new device. Now you have to verify 200 people. And they have to verify you.
I could spin the story further, but I think the point is made: encryption is not free. It comes with usability issues that can make it essentially unusable for many cases. How many times will non-crypto-nerds actually verify another device? Many apps have a ‘send unverified’ button for these cases. Use that a few times, and you’re looking at this.
So how do existing systems deal with this issue? WhatsApp and Line take a simple approach: You can only have one device. When you want to use another device, your old one has to give. You can’t even move your messages, generally. The identity is guaranteed by the phone number, so you can skip the ‘verify identity’ check. The rules for using the chat systems make it so that the issues mentioned above never occur. The focus on a one-device-policy also makes the system easy to grasp for users.
“But Lain”, you will say, “I’ve been using Matrix and I do have message history, and I don’t have to verify stuff all the time either!". Very true. The current Matrix system is quite user friendly. You can chat away, have message history, add new devices without verification, and you still have that nice lock symbol (or, in Matrix’s case, a shield) that tells you you are encrypted.
How do they do it? Well, by trading away security for convenience. The basic encryption algorithm in Matrix is Olm. This is pretty much the same as the Signal double ratchet I talked about and has the same security properties. But no actual chat messages are sent over this protocol. Instead, they have a separate protocol called Megolm. It only has one sending ratchet per device that is supposed to be sent around to recipients so they can decode your message. This ratchet is NOT reset on every send-receive cycle. The clients are supposed to keep the ratchet around in its earliest state, so that they can decode each message from that point in time on.
Now imagine you’re adding a new device to a chat. Your existing device just sends over the earliest Megolm ratchet it has to your new device. Your new device can then decode all the messages that your earlier device sent. That way, you can keep your history synced across devices.
But what about verification? The excellent XMPP client “Conversation” has an approach called Blind Trust Before Verification. It means that by default, you never verify identities at all. You just trust them blindly. But then, as soon as you verify even one device, you’ll have to verify each new device before sending messages to them after that.
The idea behind this is a casual user will never encounter the dreaded ‘verify the new device’ screen. The security-conscious user will verify once and have a secure system ever after. Now, this solves the problem for users who ignore everything and just don’t care (a questionably secure approach). But for users who still verify, they still have that 200-device problem.
Matrix came up with an extended approach. It is also Blind Trust Before Verification. When nobody is verified in a chat, you get a black shield. That means ‘we have encryption, but you don’t know who you’re talking to and also don’t care’. Let’s say you verify your friend’s phone. You now get a green shield, which means ‘encrypted and everyone is verified’. Now you add another device of yours, and your shield changes to red. That means: ‘encrypted, and you care about verification, and not all devices are verified!'. DANGER!! No biggie, you verify your device, shield is green again. But what about your friend? Don’t they have to verify your new device?
Turns out, they don’t! By verifying your new device with your old one, your old device vouches for new one. Your friend’s device will accept this verification and not ask for any additional proof that your new device is indeed yours.
With the new shields, we no longer nag the user whenever a new device is added in their conversations - instead we expect them to see the room’s shields flash red everywhere if an untrustworthy device is added.
This solves the 200-device-problem. When a new device by an existing user is added, only one person has to verify it, not 200. But why doesn’t everybody do it like this?
Well, you delegate device verification to another user that you might or might not trust with that. Let’s say you’re in a chat with a user who already has 10 devices. On the user’s laptop, someone installed a keylogger and stole his password. They also know the victim personally and can get access to their phone for a short amount of time. While the victim is making room for more cocktails, the attacker logs in on their own device with the victim’s credentials. The attacker joins the existing chat. There are now 11 devices for that user, one is not verified, and the shield turns red. Now they verify the new device with the victim’s phone and voila, green shield again. Will anyone actually count the devices to see how the number changed? The attacker can also read all the old history, because the megolm ratchet was sent to their new device.
This scenario is impossible with WhatsApp, for example. So, in some situations, WhatsApp is more secure than this FOSS solution.
Now, I’m not saying that the Matrix team is wrong in doing this. The security / convenience tradeoff might well be worth it, overall. What I am saying is that these subtle effects of different cryptological algorithms are nearly impossible for the end user to grasp. I had to read a few papers during a sleepless night to come to a somewhat proper understanding of the implications. I’m quite sure that even explaining how the double ratchet works will take an hour or two, if you’re explaining to a smart person who’s not versed in cryptography. And they just wanna chat!
Making these tradeoffs does not quite lead to security theater, but it’s getting dangerously close. “End-to-End encryption” becomes a feature to check off your list. The importance of the actual algorithm and user experience is often swept under the rug. As long as you have a shield or lock symbol, you’re good.
What can we do then? Existing fediverse servers are all multi-device, so we can’t go the WhatsApp way of using just one device. We also don’t want verification hell, so that’s out, too. Copy what Matrix is doing? I don’t really think this is a good idea. There are too many subtle issues with it that I wouldn’t want to expose casual users to. So what else can we do then?
Non-technical end users will always have problems understanding the properties of encryption when there is a mismatch between the UX of the application and the underlying encryption system, and it’s not easy to understand for techies, either. This is the case when you use something like the double ratchet and then pretend to users that these are user-to-user encrypted systems. They are not! They are actually device-to-device encrypted, but sometimes a user has multiple devices. This is the part were the confusion comes in! And worse, the identity and key management…
So let’s just match the encryption to the user experience. Encrypted chats should give a user this:
In this system, every encrypted chat is explicitly initiated between two devices. If I’m at my PC and want to write a secret message to my friend, I can do that by selecting one of her devices (for example her phone) from a menu and use that to start an encrypted conversation. I can also write her from my phone, but that’s a separate conversation!
This is pretty much the system that WhatsApp or Line use, just that they also enforce the use of only one single device. In my conversations about this with non-technical people, they usually had difficulty to understand all the trade-offs made in multiple-device-per-user encrypted chats (like matrix uses), but understanding device-to-device encryption was straightforward. There was also a higher acceptance of losing features like shared message history, because now it was clear why this was happening.
I think that this is a good first paradigm to try out when implementing E2EE chats on the Fediverse. It would give users an extremely secure encrypted chat that is easy to use and easy to understand. It also makes it explicit just what you lose when you go the encryption route. Additionally, keeping encrypted chats per-device will prevent less-secure devices (like browsers) from mixing with secure devices (like phones or applications) any more than necessary. There are only two possible weak links in the chain for every chat, not potentially hundreds.
It seems that I didn’t make it clear enough that I expect encrypted chats to NOT be the default. They shouldn’t even be too easy to initiate, and probably come with a warning. This is an important point! It’s not possible to keep the conveniences of unencrypted chats and a very high level of security at the same time. In the model I’m thinking of, encrypted chats become something you choose to use explicitly, fully knowing that you have to give up some convenience for the gained security. Just like whispering in real life is less convenient than talking normally, so it is with an encrypted chat. Making this trade-off explicit, visible and understandable to the user is the key aspect here.
Will Fediverse users eventually want encrypted group chats? I don’t doubt it. At that point, it will be interesting to figure out what they want more: The better-than-nothing security that a system like Matrix/Megolm provides, or the very hard security of the double ratchet and manual device verification?
For most groups, being unencrypted is just fine. People want server side search. They want to share history with new participants. Often they even want things to be completely open, free for anyone to see. This is how most IRC chats have worked for decades now.
Still, let’s cross that river when we get there. If we have a good solution for 1-on-1 chats, we can use that user knowledge to extend the UX to group variants. Until then, let’s do our best and make it possible to share birthday gift tips on the Fediverse.
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