There is some interest in encryption schemes meant for humans to perform without computational aid. (Relevant questions here, here, and here.) People often cite the VIC pen-and-paper cipher, the Schneier Solitaire cipher, or more recently the LC4 cipher. These would be useful in the context of, say, two people using communication devices where one or both may be monitored.

I've seen these referred to as pen-and-paper ciphers, human-computable ciphers, and as low-tech ciphers. They've also been compared to historical ciphers.


I could not find any lit-reviews for these schemes. Generally, I'm wondering if this is a field, and how much it has been explored. More specifically, is there a name for this class of modern, strong, human-computable encryption schemes? Have there been any attempts at defining, describing or measuring what makes a cipher "human computable"?

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    $\begingroup$ AFAIK, this isn't really a field. It's of interest only to amateurs, computers are too widespread and too easy to use to make manual crypto worth the time. The closest you typically get is lightweight crypto, but even that is generally hard for humans to do by hand. Even existing manual crypo tends to be stretching the idea of "strong" quite a lot. $\endgroup$ Jul 11, 2020 at 20:48
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    $\begingroup$ I think hand cipher or pen-an-paper cipher is as good as it gets when it comes to naming... $\endgroup$
    – Maarten Bodewes
    Jul 12, 2020 at 10:13
  • $\begingroup$ @SAIPeregrinus Err, isn't a pen and paper OTP reasonably "strong"? And pretty easy to do? $\endgroup$
    – Paul Uszak
    Sep 10, 2022 at 0:46
  • $\begingroup$ It'd be strong if a human could create an unbiased uniformly random sequence. Since that takes a machine (or nigh-impossibly well made dice) I wouldn't count it as "by hand". $\endgroup$ Sep 12, 2022 at 3:13

1 Answer 1


While I wish I knew a name for the entire field you describe "strong human-computable encryption schemes", here are some names I've encountered for various sub-fields:

  • steganography: Modern steganography schemes are designed to be resistant to highly automated computerized attack. A few are designed to be encoded and decoded by humans -- for example, Richard Bergmair and Stefan Katzenbeisser: "Content-Aware Steganography: About Lazy Prisoners and Narrow-Minded Wardens" 2005.
  • CAPTCHA, aka human interaction proof: systems designed to be resistant to highly automated computerized guessing attacks. (CAPTCHA)
  • mental cryptography:
  • pen-and-paper cryptography:
  • encryption without suspicious tools: a hand-held 4-function calculator, decks of playing cards (Solitaire), pencil, paper, a dictionary/book/magazine/newspaper (but excluding nomenclator codebooks) for use in a running key cipher or a book cipher, etc.
  • nomenclators : my understanding is that some historical encrypted messages composed using codebook ciphers have resisted even computerized attempts at decryption.
  • purely mechanical cryptography: the humans enciphering and deciphering are allowed complicated mechanical machines (for example, Pinwheel machines, aka Pin-and-lug cipher machines ), but this category excludes machines powered by electricity.
  • visual cryptography: an implementation of one-time pad encryption that allows humans to decode the message remarkably quickly. Later variants implement k of n secret sharing. ( "Visual Cryptography is the Coolest Thing Ever" )

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