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By "hand cipher", I mean a symmetric cipher for which encryption and decryption can can both be performed with a pencil on graph paper, consuming about 10-20 seconds per character by a proficient user. Additional simple tools, e.g. a deck of cards or a multiplication table, may be used to expedite the process. No mathematical proficiency is required other than mental addition, subtraction, and maybe multiplication.

Basically, it should be a cipher that someone of average mathematical abilities can master, and it should require no digital technology or cryptographic tools that cannot be constructed out of paper.

For example, four-square in CBC mode seems secure to me because CBC is a well established mode of operation and four-square seems to be a good block cipher as it accommodates large keys and seems to produce a random-looking output. However, the fact that four-square in CBC was never widely adopted before digital encryption became a consumer technology leads me to believe that someone broke the cipher and thus discouraged people from using it.

So is there any hand cipher that the best supercomputers would take years to cryptanalyze? If not, then for which hand cipher is the best known attack the most complex?

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    $\begingroup$ This is very similar to crypto.stackexchange.com/questions/844/… $\endgroup$
    – mikeazo
    Commented Jan 13, 2012 at 2:20
  • $\begingroup$ @Jordan RC4 would take a long time to set up for a given key, but after the 256-byte state is ready, it is very easy to do and a proficient user could easily output more than one byte every 10 seconds. $\endgroup$
    – forest
    Commented Dec 22, 2018 at 5:07

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Solitaire by Bruce schneier is probably your best bet. It has a few issues but it will work well for most things. It ends up having a small bias, but it takes about 15 seconds per character after the initial key stream has been generated.

It is not nearly as widely studied a field as most people are assumed to be able to get a computer or other computation device. The only other one I can think of is VIC. It is based on a lagged Fibonacci generator which is a bit of a different place to start from. See this other answer for more details on that cipher.

I'd strongly suggest looking long and hard at Solitaire. It has really a cool design.

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The VIC cipher and for something not as secure but easier for encryption and decryption, the double transposition cipher.

VIC cipher: The VIC (short for VICTOR) was used by the Soviet spy Reino Häyhänen - a pencil paper cipher. To quote the wikipedia page:

Although certainly not as complex or secure as modern computer operated stream ciphers or block ciphers, in practice messages protected by it resisted all attempts at cryptanalysis by at least NSA from its discovery in 1953 until Häyhänen's defection in 1957.

A good article on the Double Transposition cipher can be found on the pbs site, from where I quote:

This was one of the most secure hand ciphers used in the Second World War.

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The 'One Time Pad' can be considered a secure cipher that may be executed by hand as long as you meet the same security requirements. Yet, you should ask yourself why you are interested in such a method in this wonderful age of high speed digital electronics.

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    – e-sushi
    Commented Jun 19, 2017 at 21:02
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The short answer is that there are no algorithms that are really feasible to do by hand and are really secure (unless maybe you are really patient and really proficient).

See Is there any strong enough pen-and-paper or mind cipher? and Is there a secure cryptosystem that can be performed mentally? for more details.

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A paper by Hopper and Blum discusses the possibility of carrying out by hand cryptography based on the hardness of learning parities with noise. Since their paper several other works have explored carrying out other cryptographic tasks, including symmetric-key encryption, with similar efficiency.

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I would go with the Chaocipher. To date, no known ciphertext-only breaks have been identified. The cipher can be operated using a deck of standard 52 cards. It's a bit time consuming and somewhat error prone but is rock solid.

https://aarontoponce.org/wiki/crypto/card-ciphers/chaocipher

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    $\begingroup$ Security against ciphertext only attacks is a pretty terrible security guarantee. $\endgroup$
    – Maeher
    Commented Dec 21, 2018 at 12:52
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I like a simple alberti disk. 3 settings, index, rotation and number. Index is how the disks align at start. Rotation is obvious. Number could be a section of pi or a cube root, perhaps a phone number. Each step adds another variable and the number can be message length forming a running key. But it is probably only good for 48 hours max. Keep the message short, don't reuse the number.

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