When not encrypted, they can indeed
each other. All you have to do is mount the other drives, or partitions. If you have firmware-level malware, it's a trivial task to get your encrypted disk key. Even when encrypted, you can still access data each way; it just won't be useful until you decrypt it.
Without encryption, you don't even need the malware to reside at the firmware-level; the right malware will be able to read everything. Data can leak to and fro.
Most Linux distributions are open source, so finding out how the data is encrypted is a trivial task if you know what you're doing. That doesn't make it much easier to outright decrypt it, though.
Knowing how the full disk encryption work is not equivalent to being able to decrypt it. You still need the master key which is not stored. That's the FDE password that you get prompted for on boot.
Getting the password can be trivial with the correct malware.
Here's how an attack could work (I've removed the ordered list since some steps can be done in different orders):
- You could exploit a vulnerability on either system, Windows or Linux. When you have elevated access to either machine, their encryption is moot at this point. You can do many things, limited only by your skill set and imagination.
- Now that you have elevated access to their machine, you can try to create and/or analyze
hibernation files /
ram dumps /
memory dumps, and analyze these on the user's system, or send them to yourself (depending on the size and network speed, it could take a long time) for offline analyzing purposes.
- Using either machine, find a way to alter the firmware of the user's keyboard, or modify the login. You can now grab the key that the user types in. "But it's not stored!" Moot point if you can steal it as the user types it in during boot. It's even funnier when users store their key files on their drives, or on a mounted volume such as a USB stick or CD/DVD, and it's accessible.
- Even better, you can manipulate the boot loader to steal the key.
- Analyze the hibernation / ram dumps / memory dumps for the Master Key.
But is this sort of attack easy? Maybe not. It takes a lot of time and effort, unless there are tools I'm unaware of which automate much of the process. Is it possible? Yeah, but implausible. The question is: are you important enough to warrant this kind of attack against you? I doubt it.
Is it possible for someone to enjoy the functionality and entertainment of Windows and privacy of Linux?
Yes. If you're genuinely worried about security, use encryption and two separate computers. If you're not, then dual boot. I'd still recommend encryption either way.