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I am a prepper. I am preparing for a situation where there will be no rule of law and where I can't rely on the government for all my needs. This naturally involves storing food, water, and items for personal defense. I would like to know how to communicate during a time where the Internet and normal means of communication isn't working. With generators one can have a computer to do some computations that will allow for some advanced cryptography. But my concern is that long termed I don't want to rely on electricity.

I am part of a prepper community and we have established ways to communicate using amateur radios. We also have methods that do not depend on electricity.

I am looking for recommendations on how to set up secure communication assuming that there actually is a way to communicate. Preferably the method shouldn't rely on electricity (even though we expect to have access to simple battery powered calculators). I understand that it is hard to set up secure communication without the help of computers, but I am preparing for situations where this isn't available. I understand that an adversary might have access to computers. A couple of other thoughts about what I am thinking:

  • The method doesn't have to be extremely fast as we don't expect to have large volumes of messages.
  • We have a high level of trust, but I would like the system to have something built into it so that a party can be cut off from the communication without anyone having to meet up. From what I can guess, this would eliminate the idea of having a joint code book.
  • Messages should be short, maybe no more that 100 words.
  • I should also say that out group expects to be able to physically meet once in a while. But we will probably have periods where we can't physically meet.
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  • How long would the messages be? Length is relevant, not just the number.
    – cpast
    Jul 8, 2015 at 1:00
  • Can one assume your adversary will not have computers as well? Jul 8, 2015 at 1:03
  • @cpast: I edited. I am thinking that we should have relatively short messages. Maybe about 100 words.
    – John Doe
    Jul 8, 2015 at 1:06
  • @GeraldDavis: In most cases, yes, we assume that the adversary doesn't have computers, but we can't be sure in all cases, so we assume that they do.
    – John Doe
    Jul 8, 2015 at 1:07
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    I'll add that if you plan to use ham radio to communicate, you're not legally allowed to use encryption on the ham bands. Maybe you don't care in a SHTF scenario, but if one of your of your scenarios means resisting government corruption/takeover, you probably don't want to so visibly violate the law, which gives the government an excuse to come visit your compound.
    – Johnny
    Jul 9, 2015 at 1:28

2 Answers 2

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A OTP can be done by created, copied, and used to encrypt and decrypt by hand. There is however no revocation so if 4 parties shared the same OTP there is no way to kick one party out short of destroying the pad and making a new one.

There are also some significant security limitations:

  • The pad must be random. This would be the hardest thing to do without high speed electronics but a small pad could be done using dice for example or using PRNGs prior to the SHTF event.
  • Each party will need a copy of the OTP which if not made prior to SHTF may require a lot of manual copying.
  • Each symbol in the pad can only be used once (1 letter in pad = 1 letter in plain text).
  • If one copy of the pad is compromised then all are compromised.
  • Some coordination is necessary to ensure two parties don't both encode a message using the same page from the OTP. In a two party system a simple way is for one party to encode starting from the first page and the other to encode starting from the last page.

The word comes from the fact that original OTPs were small notepads with multiple sheets containing a couple hundred letters per sheet. When a message was encoded (1 letter from OTP per letter of message) the current sheet was torn off and destroyed to ensure it wouldn't be used again or used by an adversary if the agent was caught.

Due to the limitations you aren't going to want to use an OTP by hand to encrypt novels but for brief urgent messages it would be completely unbreakable by cryptanalysis even if the adversary still had high speed computers. Actually it would even be unbreakable even against quantum computing.

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A classic hand-encryption system, "Solitair", was designed by Bruce Schneier for Neal Stephenson's book Cryptonomicon. It uses identically shuffled decks of cards, one per communicating party, to encrypt text. According to Bruce (who is perhaps not a disinterested party), it is a very strong cipher.

Unfortunately, there's no way to revoke someone's access to the cipher (unless you sneak into their bunker and re-shuffle their copy of the deck).

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  • Cryptonomicon was by Neal Stephenson.
    – mcgyver5
    Jul 16, 2015 at 20:16

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