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I am trying to find out a way to protect files that will be stored on a drive accessible to the public. Using permissions is not an option in this case, so I am planning to encrypt the files using PGP (or GPG).

It is critical that the files be accessible quickly in an emergency (they are backup files for some important databases), possibly two or three years down the road, but only to one of about five people.

This would be easy if these five people were experienced GPG users, but most people in the team are very casual/occasional GPG users.

My general idea is to create a GPG key for that specific purpose, and distribute it to the team.

Here are my concerns:

  • Casual users may simply forget that they even have the key, or forget the pass phrase to their key ring or to the GPG key itself.
  • Our workstations get replaced on a fairly regular basis, and users may forget to transfer their GPG key ring (or the GPG key).

In the context of this question, I am not worried about usability issues (such as not remembering how to use gpg) - I'll address that separately with scripting and documentation in a knowledge base.

We will of course have one additional copy of the key on a USB stick in a vault, but I'm looking for additional ways to keep the key secure, yet keep it available quickly in an emergency.

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Is this in a corporate setting?

If so, make a key without a passphrase, print the key on paper, stuff it in an envelope, and hand it to your HR or financial department. They're probably used to keeping stuff around for long times, and should have proper procedures for maintaining secrecy.

As it's on paper, it's a format we know the archival properties of; good paper easily keeps information legible for ten years. When you need the key, OCR it. If you print it in hexadecimal OCR will be highly accurate. You could even include QR code, which is even quicker to decode. Worst case? Punch it in by hand - this would probably take you an hour or two, but doable.

What you are doing is essentially trading a large, difficult-to-keep secret for a small, easy-to-keep secret.

  • Interesting idea, thank you! One concern here is that most such departments are moving towards scanning paper documents and destroying the originals. But it is a very good idea nonetheless. – Kevin Keane Jun 10 '18 at 3:08
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You can use GnuPG with symmetric keys instead of doing asymmetric public/private key cryptography. There are many upsides -- it can be a simple long passphrase like "correct horse battery staple" that is easy to share and to type in for the restore process (certainly a lot less painful than dealing with private keys).

The main downside is that you have to be careful how your backup jobs are set up, to make sure the symmetric key is not exposed to sniffing by other processes. Software like duplicity handles this fairly well.

  • I wanted to avoid symmetric keys because asymmetric encryption allows me to easily, and securely, distribute the public key to all backup jobs. The task of securing just the private key for recovery seems smaller than the task of securing all the backup jobs as well. – Kevin Keane Jun 10 '18 at 3:02
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Does public mean the world? If it's a private network, you can configure file r/w permissions to the individual users and block access rights to everyone else without necessarily encrypting the files. This will prevent access to everyone else. Changing workstations will not affect access as it's tied to the specific users and not the workstations.

  • OP explicitly states that permissions are not an option, and it sounds to me like they're saying an adversary could have physical access to the drive (or at the very least the ability to access it as a block device rather than having to go through a filesystem driver). – AndrolGenhald Jun 8 '18 at 18:46
  • I do not have control over the permissions. While it's a private network, it is accessible to thousands of people. – Kevin Keane Jun 10 '18 at 3:06

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