I'm having difficulties to find a backup and synchronization solution with the following characteristics:

• Cross-platform: Windows, Linux, Mac
• Offsite backup (so Internet Backup)
• Data deduplication
• Transfer only new/modified bits of modified files
• Secure: Data encrypted before leaving computer
• Maintain multiple versions of files (even deleted files)
• Folder synchronization integrated with backup and across multiple computers connected to the internet (not necessarily in the same LAN)

I think that the Folder Sync feature needs a better explanation.

The use case is this: you have a desktop pc and a laptop. The desktop pc contains a folder with some files and this folder is part of the backup (so it was selected to be backed up). The laptop does not contain that folder or that files at all.

Then you're abroad with your laptop and you need that folder. So you want to be able to open the backup program, select that folder from the backup and download it in your laptop mantaining it synchronized with the backed up version. When you then come back home and switch on your desktop pc you want the folder we're talking about to be updated in the desktop PC.

Does anyone knows any service with all these features?

I've only found SpiderOak to support all the features I've mentioned but I'm not completely satisfied by the time taken to complete a backup. Sometimes it seems to hang for minutes with no reasons at all and folder synchronization occurs only after all files are backed up (instead folder sync should have a separated queue independent from other backup operations and synchronization should occurs frequently... for example every 5 minutes or less, independently from the frequency of normal backup operations)