Questions tagged [classical-cipher]

Ciphers invented before the era of computers and modern cryptographic theory. Classical ciphers typically operate on letters instead of bits and are usually designed to be implemented by hand or using simple mechanical devices.

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Possible ways to crack simple hand ciphers?

We had a quiz in class today where we had to break the ciphertext with the key given, but not the algorithm. Suffice to say that I wasn't able to decrypt it within the allotted time of 12 mins and ...
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What is the most secure hand cipher?

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....
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Can an Enigma-style cipher of sufficient complexity be considered secure today?

Regarding the German Enigma machines, if I recall correctly, the reason they were defeated was because the Allies were able to generate a massive database of possible rotor settings, and because the ...
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What are the main weaknesses of a Playfair cipher, if any?

What are the main weaknesses of a Playfair cipher, if any? I know that they depend on none of the letters missing, but that is an easy fix if a letter gets dropped. Besides that, are there any other ...
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How were one-time pads and keys historically generated?

In the 20th century, it was common for various intelligence agencies and military organizations to use ciphering machines and one-time pads. However, no source I've seen ever mentions the process of ...
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Encryption/ciphers/codes in Chinese

I am quite curious as to how you can perform simple encryption for the Chinese language. Saw a similar question related to encryption/Chinese here: About cryptography in a character language, however ...
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Creating your own encryption and decryption algorithm

I'm very uneducated when it comes to cryptography. I have tried to find an answer to my question, but what I've read doesn't quite cover what I'm asking. I have thought up my own encryption algorithm ...
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About Cryptography in a Character Language

Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it. Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem ...
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Which is the simplest cryptographic algorithm which is close to commercial-level security?

An algorithm is simpler than another if It is easier to explain to a noob with basic high-school math knowledge. It is easier to securely self-implement by an intermediate programmer (Yes I know one ...
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Is the Caesar cipher really a cipher?

In this lecture by Dan Boneh on Coursera it was stated at minute 03:37 that The Caesar cipher, actually, is not a cipher at all. And the reason is that it doesn't have a key. What a Caesar cipher ...
Pichi Wuana's user avatar
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How to build an electro-mechanical public key cipher machine?

It is generally assumed that asymmetric encryption schemes were invented in 1973 at GCHQ in Britain and, independently, in 1976 at the MIT. Imagine, if the abstract idea of having a public key and a ...
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How does the index of coincidence work in the Kasiki test?

I'm starting to learn about cryptanalysis and I am having a bit of difficulty understanding the Kasiski test's index of coincidence. I have a book (Cryptography Theory And Practice by Douglas Stinson) ...
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Common Classical Ciphers

When studying cryptography, the first thing every student learns is some historical ciphers. There are way too many of those ciphers to name them all. So my question is: What are the most important ...
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Why was the Navajo code not broken by the Japanese in WWII?

In reading about this topic recently, to my understanding, the encryption schemes used on top of the Navajo language were very simple and definitely could have been broken (my research shows they ...
Michael Goldstein's user avatar
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How many keys does the Playfair Cipher have?

I was just studying the Playfair cipher and from what I've understood, it is just a slightly better version of a Caesar cipher, in that it isn't actually mono-alphabetic but rather the 'digrams' are ...
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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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Historic Authentication Schemes Before Computers

The Bulletin of the Atomic Scientists recently ran an article about an alleged near miss with rockets in 1962, which raises some interesting cryptographic questions: After the usual time-check and ...
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Finding a keylength in a repeating key XOR cipher

In an old cryptography FAQ, I found the following step described for determining a the length of the key a cipher was repeatedly XORed against: Discover the length of the key by counting ...
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How did the cryptographers of Bletchley Park figure out the chi stream of the Lorenz cipher?

How did the Bletchley Park code breakers figure out the chi stream of the Lorenz cipher, that was obscured in the ciphertext, which British code breakers eventually decoded? It's written in The ...
justin's user avatar
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Is a book cipher provably secure?

I've seen ciphers (usually in spy drama shows) that involve taking a book and writing down an index to individual characters. Essentially it's a keyed substitution cipher, where the key is the name ...
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Affine Cipher over an Affine Cipher

I would like to know your view about the title, encrypting a plain text with an affine cipher then encrypting that ciphertext once more using the same cipher, but of course different keys. Would it be ...
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Toy cipher -- does it have a name?

When I was perhaps nine years, I borrowed a book from the library on various maths and CS topics. It outlined various simple ciphers, including one that I used a lot, just for fun. I can't remember ...
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How to attack a classical cipher using known partial plaintext?

I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext. What are the cryptanalysis approaches for ...
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How can a Vigenère cipher be broken if the message is short?

I was watching a Stanford lecture on Vigenère cipher and in it the professor said that – to break the cipher – we assume the length of the key is known. We then break the cipher into groups of this ...
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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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Can a shift cipher attain perfect secrecy?

On a practice question for my intro cryptography exam, it asks the following: Assuming that keys are chosen with equal likelihood, the shift cipher provides:    A) computational security    B) ...
Cat's user avatar
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Alphabetic Substitution with Symbols

I was reading on a site about the Zodiac Killer and how he used a basic substitution cipher, but instead of substituting english letters and characters he substituted symbols. I was wondering, if you ...
Jim's user avatar
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How does cryptanalysis of the Playfair cipher work?

I have a set of Playfair-enciphered data that I'm trying to crack without the key. I know I need to analyse bigrams; I've currently worked out what decrypts to th, <...
helpneeded123's user avatar
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Solving Vigenère Encryption

I'm trying to crack a cypher that I believe is Vigenère encrypted and I'm currently stuck. I calculated the key length by finding repeated sequences in the cypher and calculating the the common ...
vignere_solve's user avatar
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Is the logic for how the enigma machine worked documented somewhere?

I know that there is a formula to calculate the result of any input but is the logic of how the machine actually worked documented? I have seen schematics for the circuitry and even a how to make ...
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Obtaining the key length of a columnar transposition, given a known plaintext word

I'm revising for a cryptography exam at the moment and I'm having some problems with a question. The question looks for the key length of a cipher given that the word "earthquake" appears in the ...
cian1500ww's user avatar
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Example of CHI Square test on Caesar Cipher?

I am trying to get my head round the chi square test, when used with the Caesar cipher. I started off using this formula, $$ X = \sum_{i = 1}^k \frac{f_i · f'_i}{n · n'} $$ Where $...
Lunar's user avatar
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Cracking WWII-era codes - code found on a pigeon's leg in Surrey

A recent BBC article entitled WWII code 'may never be cracked' posted a code: AOAKN HVPKD FNFJW YIDDC RQXSR DJHFP GOVFN MIAPX PABUZ WYYNP CMPNW HJRZH NLXKG MEMKK ONOIB AKEEQ WAOTA RBQRH DJOFM TPZEH ...
Brian M. Hunt's user avatar
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Does adding more characters to the Enigma rotors improve crypto strength?

This seems like an obvious question but I haven't been able to find it, so here goes: Enigma is based on an alphabet of A-Z (26 characters); obviously this directly affects message content (e.g. ...
Adrian K's user avatar
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How to deduce enigma settings given a partial plaintext?

Assuming some large block of text is encrypted with an enigma machine and I only know a small subset of letters before and after encryption, how do I go about figuring out the enigma settings from ...
Chris's user avatar
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How to break a columnar transposition cipher?

I'm currently studying for a cryptography exam. I've been given ciphertext that has been encrypted by a columnar transposition cipher. I've been given no shift key length or key word, the only thing I ...
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Make a Strong, Easy-to-Remember Password Using Classical Cryptography?

Passwords can be tough to remember. For example: H7535637353959595*9608J614625C1313^398583I0397897j^ So Bob wants to make and use a good password for GPG that he ...
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How can frequency analysis be applied to modern ciphers?

I am building a computer program that deciphers Caesar, Vigenere and monoalphabetic substitution ciphers. All of those are susceptible to frequency analysis. However, it does not seem to be real-world ...
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Using a mono-alphabetic substitution cipher with a different language per word

How much harder is it to determine the secret key for a mono-alphabetic substitution cipher, if each word is translated into a different language before the cipher is applied? If somehow computers ...
Scott's user avatar
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How were semagrams encrypted in the pre-digital era?

Historically, messages in languages that use alphabets have been encrypted manually according to some kind of algorithm (e.g. mono- and poly-alphabetic ciphers). But how were messages encrypted in a ...
Doug B's user avatar
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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, ...
Angela's user avatar
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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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Why does ROT13 provide no cryptographic security?

I can understand that ROT13 is not secure for obvious reasons, but I'm looking for the theoretical answer. Wikipedia says "The algorithm provides no cryptographic security.." What does it mean to ...
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Was the Enigma's double stepping mechanism intentional?

It's sometimes refered to as the double stepping anomaly, so was it just a design flaw or was it put in place deliberately?
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Strategy to crack a presumed substitution cipher

The ciphertext given is: ejitp spawa qleji taiul rtwll rflrl laoat wsqqj atgac kthls iraoa twlpl qjatw jufrh lhuts qataq itats aittk stqfj cae I've done ...
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How secure would hand ciphers be using a Block Cipher Mode?

I've been looking into different ciphers that require very minimal computation power (calculator, deck of cards, index cards and a pencil), i.e. hand ciphers. For the most part all methods typically ...
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How to find the keyword of the Playfair cipher, given the plaintext and the ciphertext?

I know how the Playfair cipher works. I would like to know how to find the keyword of the Playfair cipher, given some ciphertext and the corresponding plaintext. For example: encrypted: ...
Brian Brown's user avatar
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2 answers
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Is Chaocipher a secure cipher under ciphertext-only attack?

Chaocipher was invented by John F. Byrne in 1919. The algorithm was recently revealed – see Moshe Rubin's Chaocipher Revealed, the Algorithm (PDF). While a known plaintext attack successfully finds ...
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Does machine cryptography only consist of rotor machines?

For the chronological categorisation of cryptography, I have proceeded as follows: Cryptography by hand (e.g. Caesar, Vigenére, etc., till around 1900/1920) Cryptography using machines (e.g. Enigma, ...
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Is this hand cipher any more secure than the Vigenère cipher?

I know that inventing one's one crypto always sucks, but the problem is that hand ciphers are usually very insecure very slow. This is an attempt to make a relatively secure, keyable, and modular ...
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