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The prison example is clearly the most difficult. Under oppressed governments, you may still be able to use digital help to create offline communication parcels

The prison example is clearly the most difficult. Under oppressed governments, you may still be able to use digital help to create offline communication parcels. When you're in prison, you have your brain, a pen, paper and if you're lucky, a secure destruction mechanism that doesn't immediately raise suspicion.

pyuser583 7y ago
You also have lots of time ... that’s definitely an advantage.
shrimp_emoji 7y ago
>you have your brain, a pen, paper and if you're lucky, a secure destruction mechanism that doesn't immediately raise suspicion. It would be absurdly difficult, but you could manually implement Diffie-Hellman key exchange to generate a secret with the other party without actually ever communicating it. Just find a large prime, p (at least 600 digits to be safe). Then pick an integer g between 1 and p and a random integer x. Then compute g^x mod p. Send p, g, and (g^x mod p) to your friend (you can even include these instructions for them -- it won't affect whether this works). They take your p, g, and (g^x mod p) and choose a random integer y They compute g^y mod p and send that back to you. You take their (g^y mod p) and compute (g^y mod p)^x mod p. This equals g^xy mod p. They take your (g^x mod p) and compute (g^x mod p)^y mod p. This also equals g^xy mod p. You now share a secure secret key, (g^xy mod p), that you can use for encryption in a cipher. This is secure if you both chose a random x and y, which you never shared, and which any attacker would need in order to derive (g^xy mod p) from the four pieces of information they have: p, g, (g^x mod p), and (g^y mod p). The discrete log problem makes it computationally-infeasible to calculate x or y from those values. Buuuuut then would come the even harder part: encrypting and decrypting messages (being a human cipher suite) with that secret key. :3 It's just rote math at that point, but... let's hope you have a lot of time and paper.

pyuser583 7y ago

You also have lots of time ... that’s definitely an advantage.

shrimp_emoji 7y ago

>you have your brain, a pen, paper and if you're lucky, a secure destruction mechanism that doesn't immediately raise suspicion. It would be absurdly difficult, but you could manually implement Diffie-Hellman key exchange to generate a secret with the other party without actually ever communicating it. Just find a large prime, p (at least 600 digits to be safe). Then pick an integer g between 1 and p and a random integer x. Then compute g^x mod p. Send p, g, and (g^x mod p) to your friend (you can even include these instructions for them -- it won't affect whether this works). They take your p, g, and (g^x mod p) and choose a random integer y They compute g^y mod p and send that back to you. You take their (g^y mod p) and compute (g^y mod p)^x mod p. This equals g^xy mod p. They take your (g^x mod p) and compute (g^x mod p)^y mod p. This also equals g^xy mod p. You now share a secure secret key, (g^xy mod p), that you can use for encryption in a cipher. This is secure if you both chose a random x and y, which you never shared, and which any attacker would need in order to derive (g^xy mod p) from the four pieces of information they have: p, g, (g^x mod p), and (g^y mod p). The discrete log problem makes it computationally-infeasible to calculate x or y from those values. Buuuuut then would come the even harder part: encrypting and decrypting messages (being a human cipher suite) with that secret key. :3 It's just rote math at that point, but... let's hope you have a lot of time and paper.