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Assume the following:

You have to encrypt a file (ZIP-file) containing import information. This file should be password-protected and you want to leave yourself the password encrypted (on paper, maybe bury it somewhere in your yard, hide it behind the walls, you name it). How could you encrypt it so that you might be able to recover it? You might as well assume extreme situations like suffering a stroke (and recovering) or becoming enemy of the state and thus being pursued by government agencies. Apart from a secure physical location (six feet under) it should also be logically secured.

Some thoughts:

  • tabula recta might work -> how long could passwords be (more than 24 characters?)
  • the information to decrypt the password should be self-containing -> hinting to things only you yourself know or something similar
  • or does it make no difference if I buried a flash drive somewhere?

Thank you for your thoughts on this one.

Update 2017/12/31:

I checked the key sharing schemes article on Wikipedia. Actually this is slightly beyond the scope of my question - although I like the idea of splitting the key!

I am more concerned about proper encryption. I would really like to find a way to create a proper key and encrypt a password.

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Key management is always an issue, and you could write books about it.

I'd myself create two keys by splitting one off a normal symmetric key. This key I would print out on paper and store in a secure location such as a physical (fireproof) safe. I'd store the other part in another secure location, such as a bank, referring to the bank from the safe (you know where your own safe is, but will you remember the details of the bank?). To decrypt you can re-establish the key and then decrypt the password.

There are literally hundreds of other ways of accomplishing the same thing though. Which one is most secure depends on the situation and threats. In the end this is about balancing convenience and security. You can have a look at key sharing schemes to have some more ideas for splitting and securing keys. If security is a real issue you should be just using keys, no passwords - they are simply too easy to guess or too difficult to remember.

I'd not store the keys on flash - there is too much chance of it losing too much charge. Burying anything is not a good idea at all; somebody may accidentally or purposefully find it and with the amount of trash we generate, it will not be a great archeological find far into the future - please do not bury your electronics. Besides the chance of damage is just too likely when you bury anything.

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  • $\begingroup$ Thank you for your response. I will checkout key sharing schemes. $\endgroup$ – bash.d Dec 30 '17 at 18:12
  • $\begingroup$ I updated my question. I upvoted your response as a "thank you" - I believe there is no real answer though, what do you think? $\endgroup$ – bash.d Dec 31 '17 at 21:59
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A standard approach is to encrypt a file with a strong random unmemorable key and store the key elsewhere encrypted with a weaker yet memorable key.

Many password managers work this way, you have a random key encrypting the file store it in a password manager with a memorable (yet reasonable) password, further strengthened with a good Key derivation function.

you can go further and instead of a memorable password use a couple of password hint questions. This is obviously much weaker. But you could ask a few personal questions and use the answers to encrypt the key. If the encrypted key is kept secret with non cryptographic measures, like hiding the paper this may be a reasonable trade off between security and risk of losing the key.

I see no reasom to use weak pencil and paper ciphers. Even if you store the encrypted key and security questions on paper you can still type them in to a computer for decryption.

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Use the pencil-and-paper technique described here to split your password into 3 pieces. Hide the 3 pieces in different places. As long as you remember where you hid any 2 of the 3 pieces, you can reconstruct your password. If an attacker finds only 1 piece, it does him no good, and it doesn't hurt you either even if he destroys it.

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