Will encrypted files on a volume still be encrypted when uploaded to a cloud service?

Question

If I encrypt some files in a folder using EFS, and then I upload such files to a cloud service, will they be encrypted on the cloud?

While the logical answer seems to be yes, I think in reality the answer must be no: because when I upload I am logged-in in the system, where these files are manipulated in a decrypted form...

Here's what I know: when you use EFS, which is what I'd like to do since I only use Windows, the encryption is "transparent" to the user. Files always appear decrypted because I am logged in in the system. My question is: if I use, for example, the Dropbox or the Mega cloud services and upload such files (with their app or through the browser), wouldn't the file be uploaded in a decrypted form, since I am logged in the system? Doesn't EFS only protect files locally?

I think this would work only if I uploaded such files from a different system or account where they appear as encrypted. Am I correct? Or totally misunderstanding how this works?

Yet if I use something like VeraCrypt I am back to square one because such files are encrypted only when inside their container. Once they are copied elsewhere, be it the temp working folder of my cloud sync app or just in the ram, they are decrypted and thus uploaded in such form. Is this also correct?

See, my purpose is to encrypt my local mail folder and then upload it to a backup service. It's a big archive and it's not feasible nor good practice to compress it in an encrypted archive because such archive takes days to upload on my normal second-world home connection, and my could sync service cannot just upload the little chunks of it that need to be updated.

Answer 1

In most of the EFS solutions, the file is unencrypted before going to the memory, meaning that if you upload a file to a cloud platform, it will be decrypted in your computer's memory before being uploaded on the website.

What you may do is create an folder in your computer mapped to the DropBox folder you want to use. Then you use a solution such Veracrypt to create an encrypted volume inside the dropbox folder (which is mapped in your computer). In this way, the dropbox will see only a large encrypted file, while you will be able to decrypt it from your computer.

Answer 2

You could use the 'encrypted volume' effect to your advantage. Encrypt the email store and use an email client that decrypts the store when it uses it.

Veracrypt can be used in the same way with an extra step. Create an encrypted volume, put your email store in it, then decrypt the volume before launching your email client.

Then, all you have to do is to backup the already encrypted mail volume.