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I was doing some thinking about what would happen to my computer and passwords if I fell off the earth tomorrow. I wanted to have a message that only they would have the ability to decrypt and I wanted it to be very strong.

Lets say they are the only ones who know our first dog's name, Scout.

How could the decrypt a say RSA-encrypted message using this information? I was thinking they could add it to the end of the private key and but I don't think that is how RSA works. How would you approach this task?

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    $\begingroup$ You could leave a password-encrypted private key behind along with instructions on how to decrypt it given the password (this is probably simpler than the more clean solution to give everyone a private key and encrypt it using their public key). Also note that a dog's name usually makes for a really bad password that any somewhat determined attacker will find / guess within minutes. $\endgroup$ – SEJPM Jun 20 at 11:25
  • $\begingroup$ Thanks that is a great solution. I wanted to avoid them having to remember a password or have encryption keys. The dog's name was just an example. Will probably have a passphrase with 3-4 'security questions' that make it up. $\endgroup$ – nv0id Jun 20 at 11:49
  • $\begingroup$ @SEJPM Just one more thing though, what do you think would be the easiest was for someone with limited computer knowledge to decrypt a RSA encrypted message? $\endgroup$ – nv0id Jun 20 at 11:54
  • $\begingroup$ @nv0id: To passphrases: This is not a good idea. The number of names, cities, addresses, dates, titles (of books, films, songs) is relatively limited. That's why an attacker can relatively easy test all such answers. A reliable password needs to be random. Not a random word from some language, but a random sequence of characters. $\endgroup$ – mentallurg Jun 20 at 12:34
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I was thinking they could add it to the end of the private key and but I don't think that is how RSA works.

Indeed it is not.

How could the decrypt a say RSA-encrypted message using this information?

One solution is to encrypt a private key with the password and encrypt the message under that private key. This also allows you to add new messages / remove old ones w/o having to recover the private key first. Of course if you'd always do that anyways, you could also just forgoe the asymmetric encryption and just use plain password-based encryption.

The cleaner solution would of course be to encrypt the message under everyone's public key, but given that it sounds like the family is not so much up to tech, a simpler solution like the above one will probably be better.

Of course note that the security of passwords like a dog name is very questionable and for a proper security level one would probably have to combine a dozen or so such information pieces (assuming that every question has ~2000 equiprobable answers).

[What] [...] would be the easiest [way] for someone with limited computer knowledge to decrypt a RSA encrypted message?

Using a standard tool and writing a detailed guide (ideally with pictures) on how to use it. A GUI is probably preferred as quite a few people are scared of command lines.

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  • $\begingroup$ Thanks for your answer. I ended up writing instructions to generate a passphrase out of 4 questions. The questions have answers only my family would have all 4 answers to and a they are not just cities so it wouldn't be trivial to brute force with current technology or would require a lot of motivation to do so. I then used the passphrase for a to unlock a private key. This private key encrypts a secret which once decrypted, can be used to decrypt the message. $\endgroup$ – nv0id Jun 20 at 13:15
  • $\begingroup$ Now the USB goes in a safe with a yubi key and instructions and if anything happens to me, they have access to my Keepass database $\endgroup$ – nv0id Jun 20 at 13:18
  • $\begingroup$ For people with limited knowledge of computer maybe a aes protected zip file containing all the messages could be a solution. 7-zip use a password. Now you should find a way to transmit this password... $\endgroup$ – ddddavidee Jun 20 at 14:24
  • $\begingroup$ Maybe you can send the password with something like vice.com/amp/sv/article/zn8e9w/… $\endgroup$ – ddddavidee Jun 20 at 14:25

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