What is a good hand cipher?

Here are descriptions of relatively simple hand ciphers. I’m not sure what exactly you want to encode or why but these seem to be a good place to start. You can use any of the as is, or one or more can be modified to varying degrees to create your own cipher.

Personally, I rather like the Solitaire cipher but anybody who really knows crypto knows that one. Still, those who don’t know crypto probably don’t and might have a hard time with it even if they did.

Are you trying to keep it from the eyes of people with some skill, or just casual observers who are good at the word puzzles in the daily paper? I think something with an output-driven keystream really is the easy to go regardless though.

@jerv I know about Solitaire Cipher from reading Cryptonomicon a few years ago. Quoting from its designer at your link:

”Of course Solitaire can be simulated on a computer, but it is designed to be implemented by hand…Solitaire may be low-tech, but its security is intended to be high-tech. I designed Solitaire to be secure even against the most well-funded military adversaries with the biggest computers and the smartest cryptanalysts…”

As it happens I’m currently reading Quicksilver by the same author but set in 17C England, where Leibnitz & others are shown using hand ciphers, keyed to I-Ching symbols, hiding messages inside longer pieces of text written in plain English.

Either way a long message can take an entire evening to decrypt by hand.

@PhiNotPi Take a look at Codes, Ciphers and Secret Writing by the late Martin Gardner, a classic from 1972. It includes some simple ciphers kids can use. Less than 5 bucks at Amazon.

I have read all of Neal’s works. FYI, The Baroque Cycle is great despite dragging a little, especially in the second book, and Anathem gets weird near the end ;)

You are correct that it can take a while to do a long message with Solitaire, but you asked for ”...something more complicated than a monoalphabetic substitution cipher…” and you have to admit that Solitaire fits the bill :D

If you are looking for something simpler and security is less of a concern then the Double Transposition should suffice.

@jerv The Double Transposition… is that what would have been used on a destroyer escort maybe during WWII? My dad was a Radioman on destroyer escorts during the war and the First Officer or whoever it was who would have done it was too lazy to decode the messages so he had my dad do it. As the radioman he received and recorded the morse code and then he decoded that message from a list or book of keys that the ship had been provided, but the keys changed at least every day, I think, maybe more often. I guess that was how it worked but I’m not sure. I think the keys were predetermined rather than sent to the ship in a separate message. That would have been very secure, would it? Do you know how that might have worked?

@lillycoyote In addition to my own hobbies/interests, I had a few buddies up in the “spook shack” and elsewhere on both ships I was on. That includes Radiomen, Signalmen, Cryptologic Technicians of various specialties, and more. As such, I can think of so many ways that it could have worked that I can’t guess which one is correct in this instance.

@jerv Well, I thought I’d give it a try since you seem to know about these sorts of things. I’m thinking it must have been a relatively low level or at least standard level of code/cryptograpy because my dad certainly wasn’t on any secret missions during the war as far as I know. He served on an Evarts class destroyer escort, the smallest of the DEs escorting convoys across the Atlantic and then got transferred to another ship, a PCS/minesweeper for what had to be some of the cushiest duty a sailor could get in WWII and that was patrol and escort duty between southern Florida and Guantanamo Bay. Not exactly the kind of missions they needed high levels of cryptography or Navaho Code Talkers for, I would imagine. :-)