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Let's say you have a single sensitive document. You're traveling to somewhere high risk and are advised not to bring your data, but you also don't have safe storage at home. So you encrypt your sensitive file and upload it to the cloud.

I'm a noob as will soon be clear. My misconception was that you could just get the file at a later date and decrypt it with your passphrase that you've memorized or written down on a slip of paper. Little did I realize that the passphrase that you enter e.g. in gpg is not the key, and that the actual key is too long to be sensibly written down on a piece of paper. So of course you have to store the key on some media, like a thumb drive. But in that case couldn't you just as well keep the original file in plaintext on the thumb drive? Is the only advantage that you can keep the encrypted file and the key on two separate devices?

Tldr; Alice at time 0 is on a trusted device and wants to send a document to Alice with another trusted device at time 2. But at time 1 Alice does not own a computer and is traveling somewhere adversarial. Is encryption useful to her and how?

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    Are you using GPG with symmetric or asymmetric encryption? For passphrase-based single-file encryption, you should be doing symmetric encryption, in which case the passphrase is actually directly converted into the key (which itself is stored nowhere). – forest Mar 22 at 6:26
  • That's great, I didn't realize that. I'm guessing I need a stronger passphrase than I would for, say, a login to a webpage right? Since the server can time out between attempts. How many random digits 0-9 would make a reasonable secure passphrase for this symmetric encryption? – Perron Aeed Mar 22 at 7:01
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    @PerronAeed you should aim for 128bit entropy for a cipher with an 128bit key. If you use totally random digits (generated by a RNG, not yourself) you would need 39 digits – Josef Mar 22 at 9:24

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