Client-side, server-side and zero-knowledge end-to-end encryption in Nextcloud

Android encryption in action

Yesterday, Nextcloud published plans, designs and code for end-to-end encryption in the clients. A recent report from Forrester named data encryption as one of the top global Cybersecurity trends for 2017, so this is a big deal! I thought it'd be good to go over the differences between client-side, server-side and end-to-end encryption and position what we developed.

End-to-End Encryption

End-to-end encryption is usually associated with communication channels, think chat or video calls. Signal, Telegram and lately WhatsApp employ end-to-end encryption. So what does it mean?

The main property of end-to-end encryption is that the data is encrypted on the one end and decrypted on the other end, so only the sender and receiver can read it.

As an example, take the Nextcloud Video Calls app. When you share a link for a call and the other person joins, the Nextcloud Server essentially gives the other user the address of your computer and helps you connect to each other. It will also send signals when others join the call, when you mute and so on. But the actual call takes place between your system and that of your conversational partner, directly. And before it is sent out, your browser will encrypt the data for the other side to decrypt. This way, nobody in between, like your internet provider, can listen in!

The main downside of the term is that the definition of that 'end' can vary: you can say that normal https encryption is 'end-to-end', defining one end as the browser and the other end as the server. So this brings us to the difference between server-side and client-side encryption.

Server-side encryption

Server-side encryption serves to protect data on or going through a server: as soon as the data arrives, the server encrypts it. When you use a cloud storage like Amazon S3 or a Dropbox account or a FTP at another office with Nextcloud, our Server-side Encryption encrypts the data before it gets sent to the other storage and decrypt it only after it has been retrieved. Server-side encryption thus protects your data from access by a third party storage solution.

Note that encryption in the browser is essentially server-side encryption: the code that does the work comes from the server and thus the server controls what is going on. If a server is compromised, the attacker (or evil system administrator or government agency) can simply make a minor modification to that code so it gives them access to your private key!

When the data is on the server itself, however, the server admin has access to it as the keys are managed by the server. In case of Nextcloud, we encrypt that key with your login, making it impossible for the server to decrypt the files unless you are logged. That is called "protection of data at rest", but, of course, you log in all the time if you have the client running.

So the limitation of server-side encryption lies in what it does not protect you from: the server being compromised.

Client-side encryption

This is why server-side encryption is often contrasted with client-side encryption, which is what is employed by the end-to-end encryption Nextcloud introduced today. Where server-side encryption happens after transmission to the server, we encrypt the data on the Android, iOS or desktop client already. Then, only at the receiving end, it is decrypted again. That receiving end can be another device owned by the same user or a device owned by another user who has been given access to the data. But not anyone in between or otherwise not authorized! This is sometimes also called a zero-knowledge privacy: at no point in time can the server have any knowledge of the data.

The process of creating keys

A little bit about keys

Usually, public and private keys are used for the encryption and decryption. How does that work?

Everybody has a set of closely related keys, a 'public' and a 'private' key. They work a bit like a mail box works: anyone can throw a letter in the box, but only the mailman (or woman) can open and empty it. The public key is, as the name implies, public. Anyone can use it to encrypt something. But to decrypt the result, the private key is needed!

Nextcloud generates a public and private key pair the first time a user enables end-to-end encryption in their client. The public key gets signed by the server with 'certificate' to verify the user identity (our Cryptographic Identity Protection feature) and stored there for other users to encrypt files to that they wish to share. The private key gets encrypted with a locally, app-generated very secure and very long (12 word!) passcode which is displayed to the user and then the key gets stored on the server as well.

Another device owned by the user can download the private key, the user can enter the 12 word passcode to decrypt it, and this device will then also be able to encrypt and decrypt files. That is how you add your phone and desktop after you enabled end-to-end encryption on your laptop.

You can learn much more about exactly how the encryption and decryption work on the end-to-end encryption web page we made as well as the whitepaper you can download there.

Enterprise challenges

While consumers use end-to-end encryption for chat for years, larger organizations like companies and governments have struggled to find solutions that protect their sensitive data without disrupting productivity and legal requirements for audit logs.

Indeed, many solutions create a new layer over existing solutions like Box and Dropbox or have poor key management, making sharing cumbersome and less secure. A real enterprise solution needs to take the burden of complexity away from users and, if it needs to be anywhere, put it on the shoulders of the system administrators. Nextcloud has done exactly that, not even allowing users to create their own, potentially insecure password and making adding devices easy. The 12-word passcode can be recovered from any of the devices an user owns and it is possible to enable a system wide recovery key. If enabled, users will get warned of this and the system administrator gets to see and note down the key once and only once. After that, the code is destroyed and no attacker would be able to steal it, provided the server administrator puts it in a safe location: we'd recommend an actual safe.

Another important feature for enterprises is support for a Hardware Security Module which could be used to generate user certificates. Without it, our design does not allow users to switch identities (as this could be abused by an attacker with control over the server) but this restriction can be relaxed if a secure HSM is in the mix.

You can learn more on our webpage about end-to-end encryption!

Privacy, self-hosting, surveillance, security and open source in Berlin

August 22-29 we're organizing a conference to discuss and work on privacy, self-hosting, security and open source in Berlin: the Nextcloud Conference. We expect some 150-200 people to participate during a week of discussing and coding and, especially on the weekend, presenting and workshopping. So I thought I should blog about why should you be there and what can you expect?

If you care about protecting people from the all-pervasive surveillance, re-gain privacy and security of data and believe in self hosting and open source as solution for these issues, this is the place to be. Our event is special for two reasons:

The team that started ownCloud

We're doing it. And most of us have been, for a decade or more, in KDE, GNOME, SUSE, Ubuntu, phpBB and other earlier projects. The code we wrote has influenced millions of users already and we will go further and wider! Expect to meet people with a can-do attitude.

Second, Nextcloud has got a huge momentum, name recognition and has become one of the largest ecosystems in the open source privacy/self hosting area. It isn't just about us! Large companies, small startups and innovative individuals and small communities all over are building on and around Nextcloud. A few examples:

• Our keynote speaker is former Mozilla president Tristan Nitot, who is currently CTO of Cozycloud, another private, open source self-hosted cloud solution.
• We're working with the GNOME community on deeper Linux desktop integration (we hope we can bring those also to KDE's Plasma!) and lots of projects built on our technology, like the super cool Mail-in-a-box
• Many third party apps like the buttercup password manager, DAVdroid android app, PrivacyIdea 2FA and Gluu SSO, Collabora Online and the innovative open source Blockchain storage SIA provide Nextcloud integration. And more are coming, expect announcements at the Nextcloud conference!
• We have well over 70 different providers like CiviHosting
• You can find over 80 apps on our app store offering all kinds of additional capabilities and integrations, from Kanban or time planning apps to the mentioned authentication and external storages to music players to drawing apps to full text search to...
• One organization after another implements or moves to Nextcloud, with the TU Berlin  (where our conference takes place, like last year) and the capital of Albania merely the last in the long list.

We are doers

So the Nextcloud conference is where you can find a wide range of individuals with interest, skills and ideas in the area of privacy and freedom activism, and they are doers! There is a reason we say "bring your laptop" on our conference page, though with that we don't mean we only want coders there!

Designers, activists and advocates are just as welcome. That is because Nextcloud is about more than technology. Frank is somebody who sometimes asks the hard questions and obviously it his vision is a strong diver, but we're all long time open source and/or privacy activists and deeply and personally motivated. Our entire community is built on drive, passion and a will to take on the challenges our society offers in the area of privacy, self determination, freedom.

That is the why you should be there. To help make a difference.

Now the what.

Getting Stuff Done

Our goal is to get work done; and facilitate communication and collaboration in our community. During the week, we simply provide space to talk and code (with wired and wireless network, Club Mate & other drinks, and free lunch). In the weekend, we have a program with talks & workshops. The setup is simple:

In the morning, everybody is in one room. First, we all hear from long time privacy activist and former Mozilla president Tristan Nitot. After that, community members working on a wide variety of interesting things around privacy/self hosting/open source and of course Nextcloud talk, shortly, about what they do. Just 3-8 minutes to give the audience an idea of their project, their plan, their idea, how to get involved, a call for action. Now again, everybody is in the room, so in the break, everybody has heard the same talks and has the same things to discuss! If you have something to add, be it about TOR, protests, encryption or anything else that is related: SUBMIT A TALK!

Collaboration & sharing ideas

Last year we announced the Nextcloud Box.
This year - be there and find out!

People can look up the speakers, join the meetings proposed and so on, in the afternoon. Because after lunch we have 2 (or more, not sure yet) tracks of workshops as well as hacking, coding and meetings in the coding rooms. Unconference style, so to say.

We now have several dozen talks and workshops already submitted and well over 100 people have registered but we are looking for more input in all areas so consider to be a part of this event!

It is free and open, supported by the TU Berlin which offers us a free location; and Nextcloud GmbH which sponsors drinks & practical stuff; and SUSE Linux which sponsors the Saturday evening party!

Learn more and register!

Happy Birthday ownCloud

Seven years ago at Camp KDE in San Diego, Frank announced a project to help people protect their privacy, building an alternative to Dropbox: ownCloud.

I was there, sharing a room with Frank at the infamous Banana Bungalow. Epic times, I can tell you that - there was lots of rum, lots of rain and loads of good conversations and making new friends.





Since then, a lot has changed. But the people who started building a self-hosted, privacy protecting alternative in 2010 and 2011 are still on it! In 2011, a first meetup was held, and the 5 participants at that meetup recently got on stage at the Nextcloud conference to recall some good memories:



Of course, today we continue the work at Nextcloud, that just yesterday published its latest bugfix- and security update. It is great to see so many people have stuck with us for all these years - just this month, the KDE sysadmins migrated their ownCloud instance to Nextcloud!

We'll keep up the good work and you're welcome to join, either if you're looking for a job or just want to code. In both cases I can promise you: working with such a motivated, dedicated, professional team is just plain amazing.

I also published a blog on our Nextcloud blog about this milestone.

EDIT: By the way - there's a meetup tonight in C-Base, B'lin, 19:00 - would be fun to drink a beer on ownCloud's birthday and talk about the future! Join! It will be at least until 10 or so, so if you can't be there before then - still come! ;-)