Nextcloud-Tools: Working with the Nextcloud Server-Side Encryption

02.12.2019 yahe administration code security

At the beginning of the year we ran into a strange problem with our server-side encrypted Nextcloud installation at work. Files got corrupted when they were moved between folders. We had found another problem with the Nextcloud Desktop client just recently and therefore thought that this was also related to synchronization problems within the Nextcloud Desktop client. Later in the year we bumped into this problem again, but this time it occured while using the web frontend of Nextcloud. Now we understood that the behaviour did not have anything to do with the client but with the server itself. Interestingly, another user had opened a Github issue about this problem at around the same time. As these corruptions lead to quite some workload for the restore I decided to dig deeper to find an adequate solution.

After I had found out how to reproduce the problem it was important for us to know whether corrupted files could still be decrypted at all. I wrote a decryption script and proved that corrupted files could in fact be decrypted when Nextcloud said that they were broken. With this in mind I tried to find out what happened during the encryption and what broke files while being moved. Doing all the research about the server-side encryption of Nextcloud, debugging the software, creating a potential bugfix and coming up with a temporary workaround took about a month of interrupted work.

Even more important than the actual bugfix (as we are currently living with the workaround) is the knowledge we gained about the server-side encryption. Based on this knowledge I developed a bunch of scripts that have been published as nextcloud-tools on GitHub. These scripts can help you to rescue your server-side encrypted files in cases when your database was corrupted or completely lost.

I also wrote an elaborate description about the inner workings of the server-side encryption and tried to get it added to the documentation. It took some time but in the end it worked! For about a week now you can find my description of the Nextcloud Server-Side Encryption Details in the official Nextcloud documentation.


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