In the absence of computing power if we are reduced to using tiny QRP [low power] transmitters for communication, then there may come a time where some messages require heavy duty encryption. This is the easiest method I know of the Solitaire card deck encryption method. A group could even generate one time pads which would be starting order for a deck and store them in a secure location. See: http://www.schneier.com/solitaire.html Here is a snip from this site:
"In Neal Stephenson's novel Cryptonomicon, the character Enoch Root describes a cryptosystem code-named "Pontifex" to another character named Randy Waterhouse, and later reveals that the steps of the algorithm are intended to be carried out using a deck of playing cards. These two characters go on to exchange several encrypted messages using this system. The system is called "Solitaire" (in the novel, "Pontifex" is a code name intended to temporarily conceal the fact that it employs a deck of cards) and I designed it to allow field agents to communicate securely without having to rely on electronics or having to carry incriminating tools. An agent might be in a situation where he just does not have access to a computer, or may be prosecuted if he has tools for secret communication. But a deck of cards...what harm is that?"
[See the URL cited above, for the details on this enciphering
JWR Replies: Thanks for sending that, David. In the near future I plan to post a brief article about "book codes" --using two identical books as one-time pads. This method is called a Buchspiel ("book game") by the German spymasters that perfected it.