If you need to communicate with someone offline, how do securely conduct a conversation?<p>Same place, same time? Easy (relatively speaking). How about when you're separated by distance and time?
This is not offline, but offline seems silly given today's technological options:<p>> Both of you generate public/private key pairs<p>> Both of you spam your public keys to the world (including each other)<p>>> You might meet offline once, to make sure the public keys you gave each other were indeed yours (and not a MITM's sending theirs to both of you while suppressing your actual messages to each other)<p>>>> This is probably where your actual question's parameters would come into play, but it only needs to happen once<p>> You encrypt (using a good cipher) some message into a binary blob with your friend's public key and send the encrypted message<p>>> Only your friend can decrypt this, using their private key which only they have<p>> Your friend encrypts their reply with your public key and sends the encrypted message to you<p>>> Only you can decrypt this, using your private key which only you have<p>E2EE.
I haven't had a need to do this, but I suppose you could print out an ASCII-armored PGP message and snail-mail it to somebody. Of course this still requires each of you has a computer, just not necessarily an internet connection.
Just as a thought exercise, I would consider:<p>- Mailing encrypted USB sticks or messages on paper<p>- Some kind of steganography: hidden/encoded messages disguised in another form, like a painting..?