does bc change the whole file after minor changes?

lindenlauf

Hi,

does anybody know, whether or not a bc-encrypted file changes *entirely* after a partial change within the file?

Background of my question: dropbox normally uploads only the changed part of a file. Will it still be able to do so when combined with boxcryptor, or does bc change the whole file when re-encrypting after (minor) changes? I want to backup a database file every night via dropbox, but the upload probably will take to long if the *whole* file has to be uploaded every night...

Best Regards from Germany,
Martin

Robert

Hello Martin,

The delta sync feature of Dropbox works perfectly with Boxcryptor. Encrypted files are "divided" into 4 KB chunks and each chunk is encrypted separately. So if only plaintext data within one block changes, only 4 KB of the encrypted file change.

Best regards,
Robert

PS: This only applies when an existing file is modified. If a file is copied, synced, deleted and copied again, Dropbox needs to sync the whole encrypted file again because every file has its own unique file key and the encrypted files looks completely different. Without encryption the files are exactly the same and Dropbox does not need to sync it again. If this is a problem you might want to look at Boxcryptor Classic which uses the same encryption key for all files within one Boxcryptor Classic folder (thus the encrypted files would look the same in this case). More information is here: https://www.boxcryptor.com/en/classic

lindenlauf

Hello Robert,

ok, thank you for the information, and for the hint to bc classic!

Indeed my current setup is to *sync* all (changed) files to the bc-folder -> they will be re-encrypted and thus uploaded again completely, as I've learned now.
I might change my setup so that the original files reside within the bc-folder by sharing the mounted bc-volume instead of the original location - I wanted to switch over to that setup anway, because I don't need the sync process then (just wanted to run some tests first). BUT: For the database this is no option, because it runs inside a virtual machine, and I don't want to move (and thus encrypt and upload) the whole VM-file. Currently I run a script inside the VM copying the database-backup-file to a subfolder which is backed up like all the other files. But even if only some blocks of the 1.5 GB file have changed and I use the same name, bc 2.0 will re-encrypt the whole file -> dropbox will upload it completely. Right? So let's have a closer look at bc classic...