Offline, multi-machine, 2-factor authentication information vault?

Question

I think this should be the right SE, apologies otherwise

I have been researching ways to be more careful with how I handle important documents and credentials, but everything I found sounded incredibly inconvenient to use so I would like to get a confirmation of the simplest way to meet my use case - which I think is rather common.

Use case:

1. I may not be a network or even IT engineer, but as a robotics engineer I am comfortable with pretty much any solution I have seen - it's just a matter of daily convenience
2. I need to be able to store my personal administrative documents (including important ones) and various account credentials somewhere safe where I can be confident they will not be accessed nor damaged (within reason)
3. I need to read/write my credentials from my personal computer (Windows), my mobile phone (Android), and my work laptop (usually Windows, sometimes Linux). It's okay if I can't access my documents on my mobile phone.
4. I don't want a solution based on a cloud service, I'm really not comfortable depending on anyone for something as critical as this. I'm not a fan of a self-hosted server solution either but if that's what it takes I'm ok with it.

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Edit: taking your advice into consideration, here is what I came up with. What do you think?

All terminals have the veracrypt container (for documents) and the keepass database (for credentials) 2-way synced using SyncThing (peer to peer). I can access the credentials and the documents using 2 factor auth, the master password (something I know) and the Yubikey (something I have). Both encrypted containers are 1-way-synced with file versioning ("staggered", i.e. with timestamp) to 2 NAS, each set up as RAID1 and in different locations. Incidentally, both NAS will receive non-critical backups. And since I'm not arrogant enough to consider myself more reliable than Google, each NAS will 1-way sync to my Google Drive in their own subfolder (I'm not sure it's safe to target the same folder). Once it's set up, I will plug my yubikey in the machine I want, open either encrypted container with the master password, and for credentials use the browser's Keepass plugin to auto-fill login & password for me. When I need to add an account, I generate a random one from Keepass.

The reason I didn't use Google Drive instead of SyncThing is that I don't rely on my devices being auto-connected to Google Drive to access my credentials, and if my devices get stolen I don't rely on a third party to provide me with a backup.

Oh, and I've covered added malware protection because that's obviously a prerequisite to Keepass and Veracrypt. Not too sure whether magikeyboard is not a vulnerability still here.

Answer 1

If you consider carrying a USB stick around convenient, then a VeraCrypt container is a good starting point. You could additionally use hardware encryption, but I wouldn't rely on it alone. Open-source software tools like VeraCrypt can be and have been audited, whereas proprietary hardware encryption is largely a blackbox – we don't know how secure it really is. In the past, major vulnerabilities were found in, for example, hardware encryption of SSDs.

However, VeraCrypt only protects the data at rest. As soon as you open a file with a program, the data is loaded into RAM as plaintext. Even worse, it may also be written to disk as plaintext. For example, the program may create temporary files, or the operating system may swap RAM content to disk. This is particularly problematic in your case, because you have so many different client devices.

Therefore, encrypting only the USB stick isn't going to be enough. Ideally, you should use full-disk encryption on all client devices. Additionally, use tools which don't create random files and don't let the data be swapped to disk. For passwords, KeepassXC is a good choice. For documents, you should at least make sure that no temporary files are created outside of the VeraCrypt container.

Note that VeraCrypt uses the XTS mode for encryption, which means it does not provide any integrity protection. If you want to ensure that files cannot be manipulated, you need additional message authentication codes.

While you've explicitly ruled out a cloud solution, I don't think this is warranted. If you encrypt the data before uploading and only decrypt it after downloading, then you don't have to depend on the security of the cloud provider at all. You just use the cloud as “dumb storage”. If you're worried about leaking metadata, then a self-hosted Seafile or ownCloud/Nextcloud might be a good alternative.