May 14, 2019

How secure is emoji-based verification?

11:46 -0400

In February, Riot 1.0 was released, which featured a brand new verification system for device keys. The Android and iOS versions of Riot also recently included the new verification system. Key verification is an important part of ensuring that encrypted messages are only read by the people whom you want to be able to read. With emoji verification, instead of comparing long incomprehensible strings of characters, as in previous versions of Riot, key verification can now be done by comparing seven emoji. The seven emoji are chosen from a pool of 64, chosen to be distinguishable from each other and describable with few words, with the goal that people using different platforms with different emoji renderings should still be able to verify each other. (Riot also supports using a short sequence of decimal numbers when verifying against other Matrix clients that are unable to show emoji.)

The astute reader will note that seven emoji chosen from a pool of 64 (repetitions are allowed) gives you a total of 647 or 242 total possibilities. But surely 42 bits (despite 42 being the answer to the ultimate question of life, the universe, and everything) isn't enough to verify a single encryption key, let alone two!? What's going on here?

The short answer is that it isn't simply verifying the encryption keys directly. Rather, the devices involved are negotiating a shared secret, and the emoji are used to verify the shared secret negotiation. The shared secret is then used by the devices to verify their device keys by way of a Message Authentication Code (MAC). You'll note that if you begin a key verification in Riot, cancel it, and then begin a new verification, then you'll see that the emoji are different each time. This is because the devices negotiate a new shared secret each time you do a verification.

So are 42 bits secure enough to verify the shared secret? Let's look at how it works in more detail.

Basic Diffie-Hellman

Suppose that Alice and Bob wish to verify their keys, and Mallory wishes to attack the verification process so that she can trick Alice and Bob into verifying her own keys. Let's first consider what happens if Alice and Bob simply perform an (Elliptic-Curve) Diffie-Hellman. Elliptic-Curve Diffie-Hellman, once the elliptic curve is established, simply consists of one message in each direction: Alice and Bob each generate a key pair, and send each other their public keys. They use their own private key and the other person's public key, do some math, and end up arriving at the same shared secret. An eavesdropper cannot discover the shared secret, as they only know the public keys, which is not enough to calculate the shared secret. But if Mallory can intercept and alter the communication between Alice and Bob, then she can replace their public keys with her own public key: when Alice sends Bob, Bob instead receives Mallory's public key, and similarly when Bob sends Alice his public key. Alice and Bob then complete the calculations using Mallory's public key, and rather than having a shared secret between the two of them, they each have a shared secret with Mallory. If they do not verify that the shared secret matches, then they will not know that Mallory has attacked their secret sharing.

Alice and Bob perform a Diffie-Hellman
Mallory intercepts Alice and Bob's exchange

Diffie-Hellman with a verified shared secret

In order to detect Mallory's attack on their communication, Alice and Bob try to verify that they have ended up with the same shared secret. They don't want to verify the entire secret, as that would involve comparing a long string, and verifying the secret directly would expose the shared secret to an eavesdropper. So instead, they take some sort of hash of the secret and compare some number n of bits from the hash in some way, say through the use of an authentication string generated based on those n bits. Now Mallory can't just sit in the middle and just send any key to Alice and Bob; she needs to make sure that when they compare the bits from the hash, they'll get the same result, even though the shared secrets are different. If the Diffie-Hellman is done in a way where one person (say Alice) sends her public key first, and then Bob sends his public key after he receives Alice's public key, then Mallory will need to send some key to Alice, and then she will need to try, on average, about 2n-1 different keys to send to Bob before she finds one that has the same n bits of the hash. However, Mallory is able to do this before sending anything to Bob, so if n is small enough, Alice and Bob won't notice anything other than maybe a slight delay. If n is increased, it may increase the delay introduced by Mallory's attack so that it is noticeable, but then it means that Alice and Bob need to compare more bits, making it more work for them to verify since the authentication string that they compare will be longer.

Alice and Bob verify the shared secret; Mallory tries to get around this verification

Diffie-Hellman with a verified shared secret and hash commitment

At this point, we borrow an idea from ZRTP (by Phil Zimmerman (the creator of PGP), and others). Rather than just having Alice and Bob send each other their public keys, the exchange begins with Bob sending Alice a hash of his public key (and some other information). This is called a hash commitment. Alice then sends Bob her public key, and finally Bob sends Alice his public key. By having Bob send a hash of his key first (and by using a strong hash), this prevents Mallory from being able try different keys like she was able to before, because she now needs to find a key that not only results in a collision in the authentication string, but also has the same hash as the hash that Bob sent.

Bob sends Alice a hash commitment before any public keys are exchanged

Let's look at what Mallory would need to do to attack the exchange, step by step. Bob sends Alice a hash of his public key. Mallory intercepts the transmission and can alter it or not; we'll take a look later on at whether she wants to modify it or not, and if so, how she would want to alter it. Alice receives the (possibly modified) hash from Bob, and sends Bob her public key. Again, Mallory intercepts the transmission. Mallory needs Bob to see a public key that she has a private key for, so that she can calculate a shared secret with Bob, so she creates a new key pair and sends the public key to Bob instead of Alice's public key. Now Bob sends his public key to Alice. Again, Mallory intercepts the transmission, and now needs to create a new key pair and send the public key to Alice. However, the public key that she sends must satisfy two criteria: first of all, the authentication string generated by that key pair with Alice's key pair must match the authentication string generated by the key that she send to Bob with Bob's key pair. And secondly, the public key must hash to the same value as the hash that Alice received initially. If Mallory did not modify the hash that Bob sent, then she is stuck trying to find a key that has the same hash, which if the hash is good, should be nearly impossible. So what Mallory should have done was to replace the hash that Bob sent with the hash of a key that she already knows. If she does a birthday attack, she may end up with more than one key with the same hash, but again, if the hash is good, this should be nearly impossible, so we will assume that she only has one key for that hash. Now Mallory has solved the problem of making her key match the hash that Alice received, but not the problem of making the authentication strings match. Just by chance, Mallory may have selected a key that produces the same authentication string on both sides; this has a 1 in 2n chance of happening. If the key that she has already selected doesn't produce the same authentication string, then Mallory is stuck, as she's back to the point of trying to find another key that satisfies the two criteria. So no matter how much computing power Mallory has (short of being able break the hashing algorithm), Mallory only ever gets one guess to successfully attack the secret exchange, and that guess has a 1 in 2n chance of being correct. So by sending a hash first, the authentication string can be much shorter while still giving good security (hence the name Short Authentication String).

Mallory tries to attack an exchange that uses a hash commitment

So given that Mallory has a 1 in 2n chance of successfully attacking a key exchange, how good is that? As noted earlier, by verifying 7 emoji, we're verifying 42 bits, so Mallory has a 1 in 242=4,398,046,511,104 (4 trillion) chance of success, which is quite low for a single attempted attack. What if Mallory attempts multiple attacks at the same time? For example, if I did a key verification with every single person in the world (currently estimated by one site as 7.7 billion people), then how many attacks might be successful? Since each of the attacks is independent, this is known in probability as a Binomial distribution, which due to the numbers involved, is approximated using a Poisson distribution with rate μ=7,700,000,0004,398,046,411,104≈0.00175. For a Poisson distribution with rate μ, the probability of no successes is equal to e, so in this case, the probability that Mallory will have no successes is approximately e-0.00175≈0.998. In other words, if I verify keys with every person on earth and Mallory tries to attack every one of the verifications, there is a 99.8% chance that none of her attempts will be successful.

Now, this is not necessarily the only way that Mallory can attack the key verification process, but this is the most interesting part of the verification method. We have included measures to protect against other possible attacks and we believe that it is secure. We encourage people to review the details of the verification process, and let us know of any potential issues. While the verification process has not been audited yet, we do plan on having it audited in the future, along with other parts of Matrix's end-to-end encryption.

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May 2, 2019
12:24 -0400
Hubert Chathi: I'm discontinuing my microblog here, and switching to https://social.uhoreg.ca/. You can follow me there using Friendica, Mastodon, Diaspora, GNU Social, etc., or any feed reader.
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January 4, 2019

Happy Birthday, XMPP!

14:29 -0500

Today is the 20th anniversary of XMPP, also known as Jabber. I don't remember much about how I first heard about Jabber, but it was likely through the Slashdot post. I've been running my own personal XMPP server for a while (I believe I started with ejabberd, though now it runs prosody), and I've written some XMPP-related software.

Nowadays, I work full-time on Matrix, which you could say is a competitor to XMPP. However, I think that both projects would benefit from co-operation, and I think that a little friendly competition is helpful. At the end of the day though, I'm hoping that an open, decentralized, secure communications protocol will become commonplace, whether it be XMPP or Matrix, rather than having the majority of people on multiple proprietary walled gardens. However, with XMPP and Matrix both having features for interoperability with other networks (through transports in XMPP, and through Application Services in Matrix), I think that it's likely that we'll end up with XMPP and Matrix co-existing in a federation.

So congratulations to the XMPP community on the past 20 years, and I hope that the XMPP and Matrix communities can work together to make our shared dream a reality.

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August 16, 2018
09:51 -0400
Hubert Chathi: Happy 25th birthday, @debian.org! (7 more years until a nice round number)
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July 18, 2018
11:13 -0400
Hubert Chathi: Why do smoke detectors always choose the middle of the night to run low on batteries?
0 Comments
June 7, 2018
14:21 -0400
Hubert Chathi: Ontario, vote today!
0 Comments
May 16, 2018
16:00 -0400
Hubert Chathi: enjoying how easy it is to move files from my tablet to my laptop using KDE Connect
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May 7, 2018

A New Vector for my Career

10:19 -0400

In three weeks, I will be joining the team at New Vector, working with Matrix, an open communications protocol. It's exciting to be working full time on Free/Open Source software again (I used to work for a Moodle partner). Matrix itself is pretty exciting, with features such as federation (the ability to host your own server and communicate with anyone else using Matrix), bridging together different communication networks, and end-to-end encryption.

My tasks at New Vector will be quite varied. At some point I will be working on bridges, but to start with, I'll probably be helping out with some of the more pressing tasks such as spec wrangling (both documenting missing parts of the spec, and working with the community on spec improvements), doing some work on Dendrite, and helping out with some of the outstanding end-to-end encryption UX work.

I've been doing some Matrix-related things in my spare time, and I've been enjoying it, both working with the technology and interacting with the community. But my free time has been quite limited, so I'm really looking forward to being able to work on Matrix full-time starting in a few weeks.

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May 6, 2018
23:49 -0400
Hubert Chathi: Meet Waterloo provincial candidates at the @beavercreek.coop Community Centre, hosted by @cochf.coop
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April 17, 2018
18:29 -0400
Hubert Chathi: Great article about how France will be using @matrix.org for secure messaging
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April 12, 2018
16:07 -0400
Hubert Chathi: Dear NodeJS, how do you manage to run out of memory by just copying a bunch of files?
0 Comments
March 31, 2018
22:26 -0400
Hubert Chathi: Happy 20th birthday, @mozilla.org
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February 3, 2018
17:01 -0500
Hubert Chathi: @slack.com can't support video chat in @firefox.com. Try @riot.im instead.
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October 20, 2017
14:40 -0400
Hubert Chathi: Congratulations to @puri.sm for completely disabling Intel ME on their laptops.
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October 9, 2017
22:07 -0400
Hubert Chathi: congratulations to @puri.sm on the successful funding of the Librem 5 #
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September 21, 2017
15:51 -0400
Hubert Chathi: Congratulations to @emacsair.me for the successful @magit.vc Kickstarter. Looking forward to seeing the improvements.
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September 13, 2017
00:12 -0400
Hubert Chathi: Don't get an iPhone X. Pre-order a phone that will leave you in control.
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September 10, 2017

An introduction to end-to-end encryption in Matrix and Riot

21:10 -0400

Disclaimer

End-to-end encryption in Matrix and in Riot are in Beta, and may be subject to change.

I have made every effort to ensure the accuracy of the information in this post, but this should not be viewed as an official guide to end-to-end encryption in Matrix or Riot.

Introduction

End-to-end encryption is one of the main features of the Matrix communications protocol and of Riot, a glossy client for Matrix. This post provides a high-level overview of what end-to-end encryption is, how it works in Matrix and Riot, and how to use it. It is intended to be understandable to people who are starting with little to no knowledge of encryption, while still being as accurate as possible, and the goal is to help people get a better understanding of end-to-end encryption in Matrix so that they can use it more securely and effectively.

What is end-to-end encryption?

Encryption is a way of ensuring that unauthorized people cannot view information that is not intended for them. Encryption takes the information and, using an encryption key, scrambles the information in a such a way that it cannot be read without the corresponding decryption key. (In some encryption systems, the encryption and decryption keys are the same, whereas in others, they are different.)

In some communication systems that involve a server, the connection between each user and the server is encrypted so that anyone who taps into that connection cannot read any messages. By default, all communication in Matrix is encrypted in this way. However, this still allows messages to be read by server administrators, or anyone who manages to gain access to the server.

End-to-end encryption (sometimes abbreviated as e2e encryption, or simply e2e or e2ee) means that messages are encrypted by the sender in such a way that only the people you are communicating with can read it — none of the servers in between can read the message.

Why do I need end-to-end encryption?

Whether it's our credit card or banking details, health records, corporate strategy, or even plans for a surprise party, we all have things that we would prefer not to be made public. End-to-end encryption helps maintain your privacy.

Using end-to-end encryption even for messages that don't need to be secret also helps increase the security of messages that do need to be secret, as it prevents someone from determining which messages have sensitive information and which ones don't.

Are all conversations in Matrix end-to-end encrypted?

End-to-end encryption can be enabled on each room individually. While encryption is still in beta, all rooms are unencrypted by default. Once encryption is out of beta, then private rooms will be encrypted by default.

If you have sufficient privileges (normally moderator or admin permissions) in a room, you can go to the room settings and enable encryption. Note that once encryption is enabled in a room, it cannot be disabled again.

Riot indicates encrypted rooms with a locked icon next to the message input box, and unencrypted rooms with an unlocked icon.

Why won't all rooms be encrypted?

There are several reasons why some rooms will not be encrypted even after encryption is out of beta. In brief, some of the reasons are that encryption interferes with certain types of integrations (including the bots and bridges hosted by matrix.org), encryption prevents people from reading messages sent before they joined the room (which is useful for some rooms such as rooms used as support forums), encryption can slow down sending messages (which should not be noticeable in small rooms, but could be quite significant in large rooms), and encryption is of questionable value in a room that anyone can join and read.

What's the deal with all these devices?

Matrix encrypts messages to devices rather than to users. This allows for greater flexibility and privacy. For example, if your phone gets stolen, then you can tell your contacts to blacklist your phone, and whoever has your phone will not be able to decrypt any future conversations, without affecting any of your other devices.

Why does Riot complain about "unknown devices" when I send a message in an encrypted chat?

When you try to communicate with someone, Riot will fetch the list of that person's devices from the server, including an encryption key for each device that can be used to encrypt messages so that they can be read on that device. However, Riot has no way of determining whether that the key is legitimate or if it was planted or altered by someone trying to snoop in on your conversations, so it warns you when it encounters a device that it hasn't seen before.

Riot allows you the option to send messages even to devices that you haven't verified, or to verify the key to tell Riot that it is trusted, or to blacklist the device to tell Riot that it should never encrypt messages to that device.

How do I verify devices?

Note that the current device verification process is only temporary and in the future will be replaced by something that's easier to use.

In order to verify someone's device, you need to have some reasonably secure way to communicate with them. It doesn't have to be secret (if someone listens in on the key verification process, it won't make it any less secure), but it has to be something that won't allow someone else to be able to impersonate you or the device's owner. For example, if you know the device owner's voice, you can phone them, or even start a video call with them in Riot. You can also verify someone's devices if you meet them in person.

When you're ready to verify someone's devices, you can click on their avatar in any conversation that you have with them, and Riot will show you a list of their devices. Find the device that you want to verify, and click the "Verify" button under it. This will show the device's name, ID and key.

The other person will then have to go to their user settings on the device that you want to verify, and find the device key there. You can then compare the keys, and if they match, then you can click the button saying so, and their device is now verified.

Repeat this for all of their devices that you want to verify.

This may seem like a lot of work, and it is, but there are plans to improve this in the future, before end-to-end encryption leaves Beta. For example, in the future your devices may be able to vouch for each other so that others will only have to verify one of your devices.

How does encryption work in Matrix?

Conceptually, when you first send a message in an encrypted room, your Riot client generates a random key to encrypt your message, sends the encrypted message to the server, and then sends the decryption key to all the devices in the room that should be allowed to decrypt the message. Of course, the decryption key is sent encrypted (based on¹ the device's unique key, which you verified above) so that it cannot be intercepted. The recipient then fetches the message decryption key and the encrypted message and decrypts the message.

In order to avoid having to re-send decryption keys to every device for every message you send, Matrix's encryption system includes a method for generating a new key based on an old key. So for the next message you send, your Riot client will use that method on your previous encryption key to generate a new key, and the recipients will use the same method and generate the same key, so that when you send a message encrypted using the new key, the recipients can decrypt the message without any extra key exchange. The new key will only need to be sent to any new devices that showed up in between when the first message was sent and when the second message was sent.

Riot will occasionally start from scratch, generating a new random key and sending it to all the devices in the room. This happens, for example, whenever someone leaves a room, after you have sent a certain number of messages, or after a certain amount of time.

As a result of how encryption is done in Matrix, there are several encryption and decryption keys being used. The main ones that you may need to be aware of are the device keys and the message decryption keys. The message decryption keys allow you to decrypt encrypted messages, and device keys allow you to send the decryption keys securely to other devices. Device keys are unique to each device and cannot be copied from one device to another, whereas decryption keys may be sent from one device to another, or exported from one device and imported to another, in order to allow you to read older messages.

¹ The decryption key is not encrypted directly with the device's key, but uses a more complicated method to improve security.

Help! I can't read some encrypted messages!

There are a few main possible reasons for not being able to decrypt a message.

The first possible reason is that you were not a member of the chat when the message was sent. In this case, it is by design that you cannot decrypt the message; decryption keys for messages are only sent to the users that are in the room when the message was sent.

Another possible reason is that your device was not registered at the time the message was sent. When a message is sent, the sender only sends the decryption key to devices that it knows about; when you log into a new device, that device has not yet received the decryption key for the message, and so cannot decrypt the message. (Note that when you log out and log in again, your new session is considered a new device from Riot's perspective.) There are two ways around this. One way is to export the decryption keys from another device that is able to decrypt the message, and import the keys into the new device. Another way is to verify your new device with another device: When Riot encounters a message that it cannot decrypt, it will ask your other devices for the decryption keys for that message. If you have verified that device from your other devices, then they will send the decryption key to your new device. Recent versions of Riot may automatically prompt you to verify new devices.

The final reason that you might not be able to decrypt a message is that you have encountered a bug. If you are interested in the technical details, you can see the tracking issue for encryption bugs, but the short story is that developers are aware of most (if not all) of the bugs and are working on fixing them. Some bugs can be worked around by the sender clearing Riot's cache and reloading (in their user settings), or by leaving a room and rejoining. Other bugs can only by worked around by logging out and logging back in. However, note that this will create a new device that will need to be re-verified by others, and you will probably want to export your decryption keys before logging out and import them after you log back in so that you can read old messages.

When will encryption be out of beta?

Before encryption is out of beta, the developers need to fix some of the remaining bugs that prevent people from decrypting messages that they should be able to decrypt, and to make the device verification process more usable. It is difficult to estimate when this work will be completed as the developers are working on other issues as well.

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