Questions tagged [pen-and-paper]

The term "pen and paper" can practically be interpreted as "using no tools like electronic devices", since it targets cryptographic functions, schemes, and procedures which can be handled by humans who have nothing more available than the option to use manual writing. So, "pen and paper" can also mean "pencil and post-it", "chalkboard and chalk", "wall and spraycan", etc.

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Is there a name for strong human-computable encryption schemes? Are there definitions for “human-computable”?

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

I am looking for a hash function that is computable by hand (in reasonable time). The function should be at least a little bit secure: There should be no trivial way to find a collision (by hand). For ...
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Easiest Simple-Key Hand Cipher

I need a little help choosing the right cipher. I'm looking for a simple pen-and-paper cipher that is easy enough for kids to quickly decode by hand, preferably using an alphanumeric decoding table ...
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Authentication over the phone?

Are there any known good methods of performing authentication over the phone in a way that's not susceptible to replay attacks? The specific scenario that comes to mind is having an automated call ...
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What is the strength of this machine? [closed]

This machine has the stator, with one half of the alphabet, and three rotors with replications of the other half in different order and repeated twice. There can have two rotors in use or the full ...
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Strong and Practical Hand Ciphers [duplicate]

I know monoalphabetic ciphers are weak, but my message recipient doesn't like taking the time for the decryption process. I've tried Vigenere, Gronsfeld, Beaufort and Digraphic Ciphers, but he doesn't ...
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161 views

Pen and Paper Authentication

What is the best way to prove that you wrote the encrypted message on a piece of paper? As most of us know, many conspirators have died because of false message implants, Mary Queen of Scots, for ...
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Is there a way to easily encode a message without a computer on either end?

I enjoy making notes and passing them to my friends, and I also like making them encrypted for no reason. My friends enjoy it too, but it's a bit annoying to use the Caesar cipher for everything ...
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Using Alberti Disks to Create a Strong Cipher

Could one use three different disks set at different positions and encrypt a strong message? For example: Wheel 1: A = J Wheel 2: A = E Wheel 3: A = B First letter is encrypted using 1 and then ...
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389 views

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

I have been reading on multiplication over GF for the Inverse Matrix Step in AES, and I feel like converting the hex numbers to binary, multiplying them and then doing the modifications required for ...
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What is the largest key size that could be carried by a shuffled deck of playing cards?

If you are carrying a standard 52 card pack of cards (no jokers) what is the largest key size, or the amount of data the cards can hold? Without the cards being marked or distinguishable from an ...
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Are there any strong manual ciphers? [duplicate]

Are there any strong manual (i.e. performed only by pen and paper) ciphers for practical use? It should be: no less strong than modern cryptography algorithms used in computers, at least relatively ...
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Manual secret sharing?

What are feasible options for an equivalent of Shamir Secret Sharing using small tables, preferably usable with pen-and-paper? We want to share a secret $K$ into $n\ge2$ shares, so that $m$ shares ($2\...
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Pen-and-paper one-way function for externally-anonymous survey

When conducting surveys, an Administrator might send an Enumerator to survey a Respondent. For "sensitive" questions (e.g. about embarrassing behavior), the Respondent may be fine with the truth being ...
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Encrypt password for oneself on paper

Assume the following: You have to encrypt a file (ZIP-file) containing import information. This file should be password-protected and you want to leave yourself the password encrypted (on paper, ...
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Are there cryptographic hash functions that can be computed using only paper and pen without leaking any information about the plaintext?

I am looking for a cryptographic hash function that can be computed by a human using only paper and pen without ever leaking any information about the plaintext on the paper. The cryptographic hash ...
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Is there a pen-and-paper way to securely share a secret via public key encryption? [duplicate]

We have several questions tagged pen-and-paper talking about encryption, hashing, signing, etc. but no question asks about exchanging a secret via public key encryption in a secure way. Does any ...
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While encrypting what can happen between words

From some time I am working with "pen and paper" encrypted text. Keys are not known, algorithm is not known. I managed to find simple algorithm and key that seems to be nearly perfect. Nearly perfect ...
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What is a simple way to “2 out of 3”-secure a mnemonic phrase without computers? [closed]

I'm working on a safe way to store a mnemonic key. It's 24 words long. Each word is 11 bits of entropy I want to store it in 3 locations One plate should not allow you to unlock the code Two plates ...
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1answer
183 views

Whats the most secure way to encrypt a string without the use of a computer? [duplicate]

I want to decrease attack surface by preventing the plaintext string or encryption key from ever being typed or otherwise entered into a computer. Ideally the decryption key would be something that ...
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1answer
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Given alphabetic key and cipher text decrypt the cipher text [closed]

I've been at this for a while. I've been given two things Ciphertext = bsaspp kkuosp Key = rsidpy dkawoa Now seeing the domain that the characters are in. ...
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540 views

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

I know about VIC and Solitare but as I understand they are not quantum resistant ciphers. Is there any quantum resistant pen-and-paper or mind cipher?
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Pencil & paper spyfall [closed]

As a followup to previously asked question: What's a good strategy for the game "spyfall"? I was wondering if we could build a pencil & paper solution for the problem resistant ...
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2answers
733 views

Apply a permutation cipher by hand

I did the following problem from the book "Cryptography Theory and Practice" as I am doing some self-study. Problem: a) Suppose that $\pi$ is the following permutations of the set $\lbrace 1,\dots,8 ...
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Simple digital signature example that one could compute without a computer?

I am working on a document to explain Bitcoin to students. But I am having a hard time translating the principle described in §2 of the Bitcoin whitepaper in layman's terms. There is a great question ...
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830 views

Toy encryption system that provides “hints”

I'm looking for a toy encryption system that has a specific property that when decrypting it with a key that is close enough to the key used in the encryption, it will produce some of the original ...
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1answer
259 views

Is there a password hash function with a human component?

Sort of Related: Is there a cryptographic hash function that can be performed with pencil and paper? One problem with hashing passwords is that the attacker has more of everything it seems. More ...
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Toy hash algorithm that can be broken

I'm looking for a toy hash function, where the idea is to have high school students break (i.e. find a collision) a hash function by hand, in order to teach them how one way functions and hashing ...
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5answers
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Is there a secure cryptosystem that can be performed mentally?

I, myself, do not plan on getting into a situation where I would be unable to use a computer in order to communicate securely. However, I can think of many practical situations in which mental ...
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1answer
81 views

Are digital signatures using preprocessed paper pads possible?

My idea was, given that a computer could produce some sort of lookup tables on paper, could then a human use those lookup tables to create a system for digital signatures? This was really an idea for ...
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1answer
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How hard is the Solitaire cipher to crack?

Assuming your deck is completely random and that it is not retrivable, how hard would it be for a casual person to decrypt a message encrypted with the Solitaire cipher?
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Fast mental encryption algorithm? [duplicate]

Are there any algorithms that exist to encode messages written with pen and paper such that the messages can be encrypted/decrypted mentally? For example, Bob thinks of the text "This is my plaintext",...
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1answer
212 views

Do hand-based hash functions / MACs exist?

Is there research on a hash function or a MAC that can be executed without computers? I know there's a lot of history on hand-based ciphers and the OTP can easily be excecuted by hand. (By using ...
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1answer
228 views

Which algorithm can be performed by humans?

Suppose I want my grandmother to perform encryption without any help from a computer, and she must encrypt the word 'HELLO' for me. I could ask her to instead of typing the real keys, that she always ...
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1answer
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How would you encrypt-then-MAC when using pen-and-paper and a Caesar cipher?

I'll probably get shot for asking this, but I've got some kids (aged 8-10) in my neighbourhood that I've been showing/teaching the simple pen-and-paper Caesar cipher and they're successfully playing ...
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Good challenges for a crypto competition for teenagers

I'm holding a cryptography workshop for teenagers (around 16 years old) at our university. As part of the workshop, I'm planning to run a crypto competition with prizes: there will be different tasks, ...
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Is there any strong enough pen-and-paper or mind cipher?

Some ciphers are talked about at “Is there a secure cryptosystem that can be performed mentally?”, but (at the time of writing) I don't see an answer. Are they strong enough, or are non-computer ...
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3answers
447 views

Would it be possible to create a game where one would need to decrypt a simple encryption?

This might sound pretty weird, but I was thinking of a game a person could play alone with a pen and paper. I got this idea where the person would write a random four letter string on a paper, and ...
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Assuming that we had a way of finding primes, could RSA be used without computers?

I once asked a question "Would RSA make sense if we used no computers?". The answer was negative, because finding primes that would make secure keys would prove too hard. Assuming that we had found ...
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1answer
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Is there a cryptographic hash function that can be performed with pencil and paper?

Imagine I'm signing up for the 99th new web site this month. I somehow take my secret key (which I have written down on a card in my wallet) and the domain name of the site and feed them both into ...
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1answer
270 views

Can someone tell if my Hand Cipher is secure? [closed]

In easy steps, this is how it works: Convert txt to numbers. mod 1-26. Generate random numbers (by my other cipher) equal to plain txt. Write random numbers under txt numbers. Like this: ...
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1answer
679 views

Can you help me understand how RC4-52 is done using pen-and-paper?

I've played a lot with VIC and enjoy its complexity and effectiveness as a field cipher. However, RC4 has caught my attention lately and I've read there is a RC4-52 variant that can be done by hand. ...
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Would RSA make sense if we used no computers?

I was recently wondering - would RSA be useful if we brought it to, say, medieval times? Could you choose the keys so that you could encrypt/decrypt messages quite easily, but factoring the private ...
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1answer
858 views

How would I make a secret notation alphabet more secure?

Disclaimer: I was looking for a place to ask this question on SE and this site seems to be the most fitting. If this question doesn't belong here, feel free to redirect me. Just out of boredom and ...
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computing inverses in truncated polynomial rings manually for NTRU encryption [duplicate]

Can someone explain how to find inverses in truncated polynomial rings manually (i.e. on pen and paper)? As an example from the tutorial: Example. Take $N=7$, $q=11$, $a=3+2X^2-3X^4+X^6$. The ...