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I'm working on a safe way to store a mnemonic key.

  • It's 24 words long.
  • Each word is 11 bits of entropy
  • I want to store it in 3 locations
  • One plate should not allow you to unlock the code
  • Two plates should allow you to unlock the code

Now to the interesting challenges:

  • If I die, I want my loved ones to be able to extract the mnemonic keys. They know nothing of cryptography.
  • To protect it from fire, I'm etching it on metallic plates so the instructions for unlocking can't be to long or complicated.
  • I want it to last for at least 100 years, so I can't rely on any current computer software.

My fist idea was to just leave 8 different words out of each plate, but that is just 88 bits of entropy, a bit to little to crack(or is it). There are also tools to do Shamir's Secret Sharing Scheme, with mnemonic phrases, but it's not really something you do without a computer. Is there some other good way to increase the bits of entropy without to much complication? Maybe mixing up the words or add extra words or something?

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  • $\begingroup$ Crossposted on security.se $\endgroup$ – CodesInChaos Oct 13 '17 at 15:45
  • $\begingroup$ I’m putting this Q on hold, as copy-and-paste cross-posting is deprecated at the StackExchange network. $\endgroup$ – e-sushi Oct 13 '17 at 18:32
  • $\begingroup$ I didn't know you were not allowed to do that... which community do you think is more suitable for this question? $\endgroup$ – Himmators Oct 14 '17 at 11:05