Are there any strong manual (i.e. performed only by pen and paper) ciphers for practical use?

It should be:

  1. no less strong than modern cryptography algorithms used in computers,
  2. at least relatively fast/easy to encrypt/decrypt with just a pen and paper (say, no more than 10 minutes per character),
  3. key should be memorable (i.e. it shouldn't be a 256 random character string, because it's nearly impossible for an ordinary human to memorize it and remember for a long time).
  4. not necessary, but if possible, it should be quantum resistant.

I believe that latest progress in mathematics and cryptography made all this possible, but we are just unaware of it, as it stands in the "shadow" of easily accessible and powerful computer cryptography. So, cryptography experts, please enlighten us, lay people.

  • $\begingroup$ Latest progress in mathematics, Cyrptography, Quantum made is impossible. $\endgroup$ – kelalaka Oct 19 '18 at 13:17
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    $\begingroup$ Hi lay person! Click on the one time pad tag and have a browse. There's stuff that will definitely help, especially the WW2 references where it was done pen and paper style. $\endgroup$ – Paul Uszak Oct 19 '18 at 13:18
  • $\begingroup$ I don't think this is a dupe of the question pointed to by @AleksanderRas except for point 4, and that one is optional. $\endgroup$ – Maarten Bodewes Oct 19 '18 at 14:59
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    $\begingroup$ Then again, there are others such as this one that I do consider a dupe (I already retracted my close vote though) $\endgroup$ – Maarten Bodewes Oct 19 '18 at 15:01
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    $\begingroup$ A one time pad violates requirement #3. In fact, requirement #3 may imply that nothing will be satisfactory: even if you were using a proper cipher but your key is not strong enough, it will be breakable. Especially if you want quantum resistance, you must have a large enough key. Instead of saying what size is too big, it would be more helpful to specify what size is acceptable. $\endgroup$ – Ella Rose Oct 19 '18 at 15:59

You may have a look at the Schneier's blog https://www.schneier.com/blog/archives/2018/05/lc4_another_pen.html

There are two ciphers described (LC4 and Solitaire).

I am not sure a card-pack based cipher would fulfill #4, but imho still having good strength for a field hand-only cipher

edit: now I noticed Maarten linked the Solitaire cipher too,

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