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Currently I keep sensitive information on multiple Mac OS encrypted USB drives with a very long password. They should all be identical except for the one that is kept in a bank safe deposit box, following the usual suggestion to have at least one remote copy. The drawback is that the remote drive is always somewhat out of date and requires a trip to the bank to update, so I am considering using a cloud storage such as Proton Drive.

The cloud drive is itself encrypted, with Proton not being able to access its contents, and I would only ever store an encrypted disk image on it. I read an older thread that asked about the safety of version histories on an encrypted cloud drive, which I didn't understand, but anyhow I would only ever keep one copy there.

How safe would this be? Given the nature of the contents I would have to overcome some degree of concern/paranoia. If someone stole one of my USB drives I would at least know about it rather soon. If someone downloaded my encrypted disk image I wouldn't know about it and they would have a lot of time to try to open it.

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  • You might want to consider using duplicity, which is available for MacOS as well as Linux. Duplicity can be used with any cloud storage provider to make incremental encrypted backups. The cloud provider only sees the encrypted backup files, and the provider does not have the decryption key, so the provider has zero access to the unencrypted files.
    – mti2935
    Apr 2 at 2:12

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... they would have a lot of time to try to open it.

It depends on how you encrypt.

Use a state of the art encryption like AES-256 GCM or ChaCha20-Poly1305.

Use a standard software, don't implement it. Recommendation of particular tools is off-topic here, because it changes over the time.

Standard software will usually make sure that the encryption scheme is implemented correctly. Also it will usually offer a reasonable (resource intensive) key derivation that will prevent brute-forcing.

Protect your password. E.g. use HSM, if possible.

Protect your system. If it is get compromised, encryption will make no any sense.

Then, even if the attacker obtains the encrypted image of your disk, it will be practically impossible to decrypt it. Brute-forcing, even if the computing power of the entire World is used, would take longer than the Universe exists.

Modern encryption algorithms/schemes make sure that even if the attacker has a number of encrypted and original files, it will not be possible to compute a password. Also, even if the attacker makes you to encrypt some specially crafted file, it will still be impossible to compute the password.

How safe would this be?

It depends on who and why may be interested in the information you have, and how much value it has for them.

If this is someone who occasionally obtains your encrypted image, then decryption will be not feasible.

But if there is someone who knows you have this information, and if the information can give them let say 1 000 000 USD benefit, then it may make sense for them to spend a few thousands USD to plant malware on your PC or plant some hardware, so that they will get information before you encrypt it. Or they can install video cameras and see your passwords, or get your HSM device with password for a short time, and decrypt the encrypted images.

Only you can decide, what is sufficient for you.

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  • "Modern encryption algorithms/schemes make sure that even if the attacker has a number of encrypted and original files"...I do all the encrypting on a second, older Mac that is never connected to the internet. The machine that is connected doesn't have the original files stored on it except only for a few minutes if they are downloaded. Apr 2 at 17:01
  • One can take this further - suppose the encrypted disk image I put on the cloud drive has as its content the encrypted disk image (using different passwords) that actually contains the sensitive information? I know there are no guarantees but I'm just trying to determine what is a reasonable level of confidence. Apr 2 at 17:07
  • Encryption of encrypted image doesn't give any practical benefits, because even "single" encryption is practically nor breakable. And if somebody believes your information cost much, then as I said they will not waste time for brute-forcing, but will come to your home and e.g. plant spying hardware in your PC and intercept unencrypted data. Or plant a key logger. Or install malware. This will be much cheaper than trying to brute-force the password.
    – mentallurg
    Apr 3 at 16:07
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Proton claims all the data you upload to them is encrypted by your encryption key and they have no means of accessing it:

https://proton.me/blog/protondrive-security

They have been in the business for over a decade now, so there's no reason not to trust them but you have to remember there's always a possibility they could get hacked or extorted into making encryption void and null.

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  • That article is a little over my head, but even if Proton gets hacked the file I am considering keeping there is itself encrypted with a very long password. Apr 2 at 16:59
  • The article specifically says that your data on Proton is encrypted with your key and Proton staff or anyone else but you can't access it. Apr 2 at 17:10
  • What is "my key?" It's not something known to me, is it? I've put encrypted files on Proton Drive without specifying any particular "key." Apr 2 at 20:19
  • Your login password is basically your encryption key (it's more complicated than that, but for simplicity sake, it's like that). Apr 3 at 8:36

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