8 votes
Accepted

Efficient pen-and-paper calculation of the Galois Field multiplication?

Probably the easiest way to do finite field multiplication by hand is using discrete logarithm and antilogarithm tables. For example, here's a pair of such tables for the AES field (using 3 as the ...
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8 votes
Accepted

How hard is the Solitaire cipher to crack?

Of course, it strongly depends on what exactly you mean when writing “casual person”. The cipher algorithm works similar to a shift cipher. As long as the attacker does not know the original order of ...
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  • 17.4k
7 votes

Is there any quantum resistant pen-and-paper or mind cipher?

You are not likely to find such a construction. One problem you will run in to is that of size: In order to be secure against brute force search with Grover's algorithm, you will need to use at least ...
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  • 19.3k
7 votes
Accepted

Are there cryptographic hash functions that can be computed using only paper and pen without leaking any information about the plaintext?

Check out Manuel Blum's human computable hash function. He calls it HCMU for Human Computable Machine Unbreakable. He claims you have to spend an hour memorizing the technique and then you can apply ...
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  • 202
5 votes

Good challenges for a crypto competition for teenagers

Perhaps you could do something with Visual Cryptography. Maybe something like: Gather a few low-resolution images (symbols or short text phrases), perhaps a few more images than you have kids Use ...
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  • 5,288
5 votes

What is the largest key size that could be carried by a shuffled deck of playing cards?

Any set of $N$ distinguishable objects can be permuted in $N!$ different ways, thus giving you the ability to represent $N!$ different keys, resulting in a representable keylength of $\log_2(N!)$ bits....
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  • 6,424
4 votes
Accepted

What is the largest key size that could be carried by a shuffled deck of playing cards?

Assuming that we can encode information with: the order of the cards; which face is up; rotation of cards by 180° as clearly distinguishable on a standard deck of cards as pictured below, that is ...
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  • 125k
4 votes

What is the largest key size that could be carried by a shuffled deck of playing cards?

The question relates to the amount of information that can be encoded within a standard pack of cards. The cards can be face up or face down. It is reasonable to assume therefore that they can also ...
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4 votes

Is there a simple hash function that one can compute without a computer?

How about using something similar to Zobrist hashing and generate the look-up table by coin flips? Let's say you want to commit to a 64-bit integer and you are able to deliver the look-up table in ...
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  • 181
4 votes

Is there a simple hash function that one can compute without a computer?

I don't think RC4-based constructs were mentioned yet, it would be fairly trivial to implement with a deck or two of playing cards or self-made cards. It has only modular sums and swaps, and only two ...
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  • 181
4 votes

Good challenges for a crypto competition for teenagers

You could challenge them to devise low-tech, physical zero-knowledge proofs (of knowledge) for games like "Where's Waldo?" and Sudoku, then show them some methods that really work and why. I've done ...
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4 votes

What's an algorithm for laypeople to make personal passwords

No, because any easy to memorize algorithm would fail the Kerckhoff's principle. You'd basically have one secret algorithm that if it's leaked, all your passwords would fail. Very likely multiple ...
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  • 85.9k
3 votes

Where can I find information on the speed of basic hand-operated ciphers?

You’ll likely have to do your own analysis. First of all, most of existing literature focuses either on asymptotic speed or specific implementations (such as in the case of hardware based encryption). ...
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2 votes

Are there cryptographic hash functions that can be computed using only paper and pen without leaking any information about the plaintext?

I'm not sure how secure this is (posting here will probably reduce its security at least a little), or whether it would necessarily even be called a hash function, but in terms of input/output (...
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  • 129
2 votes

Do hand-based hash functions / MACs exist?

It depends on the time you want to spend. But most likely, there is nothing with reasonable efficiency. For arithmetic operations, humans are really bad compared to computers, and the difference is at ...
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  • 12.3k
2 votes

Simple digital signature example that one could compute without a computer?

The main difference between what you want an example for (digital signatures) and secure communication is this: the roles of the public and private keys are reversed. Also, the content being ...
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  • 186
2 votes
Accepted

Which algorithm can be performed by humans?

Every classical cipher can be used without a computer's assistance; while simple mechanical ciphers can fall into the "classical cipher" category, in general classical ciphers are pen-and-paper ...
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  • 3,567
2 votes

Encrypt password for oneself on paper

Key management is always an issue, and you could write books about it. I'd myself create two keys by splitting one off a normal symmetric key. This key I would print out on paper and store in a ...
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  • 85.9k
2 votes

Manual secret sharing?

It is known that (m,n)-threshold schemes are equivalent to n-1 m-dimensional mutually orthogonal latin m-hypercubes. For m=2, there is a basic construction (Latin Squares) for prime n. For m > 2 you ...
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  • 21
2 votes

Are there any strong manual ciphers?

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 ...
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  • 1,192
2 votes

Pen and Paper Authentication

You have asked about the authentication of a hand-written message, but let's also include an integrity check as a desired service. Let's say that your message was encrypted with a one-time pad. The ...
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  • 3,067
2 votes

What's an algorithm for laypeople to make personal passwords

A common technique is Diceware with one of the new Electronic Frontier Foundation's word lists. But you clearly need dice. And that's the thing, you need some 'device' that can provide a good degree ...
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1 vote

Using Alberti Disks to Create a Strong Cipher

You have described the Alberti cipher, outlined in Leon Battista Alberti's 1467 treatise on encipherment, De Cifris (Alberti, Leon Battista, A Treatise on Ciphers, trans. A. Zaccagnini. Foreword by ...
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  • 3,067
1 vote

Efficient pen-and-paper calculation of the Galois Field multiplication?

Let's multiply 0x84 and 0x5a using keyboard and monitor (paper and pen works the same): Converting into binary you can do digit by digit independently and should get 10000100 and 01011010. The ...
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  • 1,468
1 vote

Encrypt password for oneself on paper

Use the pencil-and-paper technique described here to split your password into 3 pieces. Hide the 3 pieces in different places. As long as you remember where you hid any 2 of the 3 pieces, you can ...
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1 vote

Encrypt password for oneself on paper

A standard approach is to encrypt a file with a strong random unmemorable key and store the key elsewhere encrypted with a weaker yet memorable key. Many password managers work this way, you have a ...
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  • 10.7k
1 vote

While encrypting what can happen between words

Keys are not known, algorithm is not known. Wrong initial assumption. The only secret must be the key by Kerckhoffs's_principle. but in order to make it harder I could add consecutive letters to ...
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  • 9,829
1 vote

Manual secret sharing?

From page 45 of this very interesting paper. What about other m-out-of-n schemes? In particular 2-out-of-4, 2-out-of-5, 3-out-of-4? I don't know about 3-of-4, unfortunately.
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1 vote

Apply a permutation cipher by hand

Your $\pi^{-1}$ is correct but for decryption you can do this: $\pi^{-1}\begin{pmatrix} T&G&E&E&M&N&L&E\\ 1&2&3&4&5&6&7&8\\ N&N&T&D&...
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