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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Shannon's Diffusion: classical ciphers vs modern ciphers

Is there an easy to understand and intuitive "one fits all" definition for the concept of diffusion that is applicable to both modern binary cryptosystems as well as classical ciphers? For a ...
1 vote
1 answer
205 views

How do the notch and window positions on an Enigma machine correlate?

I am having difficulty visualizing the stepping process. The permutation of Wheel I of Enigma I is as follows: ...
1 vote
1 answer
117 views

How to Calculate Digraphic/Trigraphic Index of Coincidence?

How do you calculate the digraphic and trigraphic IOCs and the same expected IOCs? I'm aware of the formula for calculating the single letter IOC which is given at this formula but I can't find the ...
4 votes
1 answer
142 views

VIC cipher author known?

Does anybody know if the design of VIC cipher is attributed to anybody? I've tried Google and found nothing. There are quite a few sites describing the method in detail, a few implementing it ...
4 votes
2 answers
15k views

How to get the keyword from a keyword cipher?

I was given a ciphertext and now I am trying to break it via looking for the keyword. This is a keyword cipher. So: PlainEnglish: ABCDEFGHIJKLMNOPQRSTUVWXYZ If ...
0 votes
0 answers
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How to cryptanalyze Bifid cipher?

this is my first question. I've been into classical cryptography for about a year or so, but I am getting started with cryptanalysis of these ciphers. I am trying to figure out how to reconstruct the ...
11 votes
4 answers
3k views

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 ...
20 votes
3 answers
2k views

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 ...
3 votes
1 answer
172 views

How do different types of ciphers relate to each other?

I understand that there are symmetric and asymmetric (public-key) cyphers. The first have the same key used for encryption and decryption, while the second use a public key for encryption and a ...
2 votes
0 answers
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For fun puzzle has me stumped on a book cipher [closed]

So originally a puzzle was given out to a community i'm a part of and what was given was this. . .. . ... . . . . . . . ... . .. . . . . . .. . ... . . . . . .... . .. . . . . . .. .. .. . . . . . .. ....
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3 answers
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Kryptos : K1. What is the origin of the "palimpsest" keyword?

I'm studying the Kryptos sculpture with its cryptographic puzzles K1 to K4. I understand that the keyword "palimpsest" was reverse-engineered using the tableau (and brute-force computer processing), ...
2 votes
1 answer
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Interesting and fun facts about cryptology [closed]

We are planning to organize a workshop with the participation of academicians, engineers and graduate students working in the field of cryptology. On the first day, we are planning a fun competition ...
2 votes
1 answer
123 views

What does the normalizing coefficient do in the Index of Coincidence?

In school we only learned the formula without the normalizing coefficient: But Wikipedia has one that says: I am using IC in a cryptography class to find the key length and am checking various key ...
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Could this be a cipher or not? [closed]

I am trying to solve a puzzle, and I think have to solve a cipher. But I'm not actually sure what I have to do ... Example : ...
23 votes
6 answers
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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....
-1 votes
2 answers
156 views

How are the cipher, the key and the initial message (that is not encrypted) are releted?

Suppose that $m$ is a message that someone player $i$ wants to send to a network of other players $j\neq -i$. The player to prevent his message from cheating by others uses an encyrpstion scheme. Say $...
3 votes
1 answer
248 views

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 ...
5 votes
5 answers
493 views

If classical ciphers are used with compressed plaintext, how much does it make frequency analysis attack harder?

Classical ciphers, such as the Vigenère cipher, are weak and no longer used. They can be broken by using frequency analysis, which is a well-known fact. However, frequency analysis often depends on ...
5 votes
2 answers
188 views

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, ...
6 votes
2 answers
349 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 ...
1 vote
1 answer
232 views

Analysis of the Vigenere cipher

Im just starting out learning some cryptoanalysis techniques. I came across an idea which analyzes the vigenere cipher. Essentially the video explains that there is a standard english probability ...
2 votes
1 answer
361 views

Keyless cipher proof

I answered a question on StackOverflow yesterday that was asking why the following keyless encoding/decoding scheme for 16-bit numbers worked: Encoding Note that the "shift" operation is ...
1 vote
0 answers
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Is there any known/standard encoding or cipher method that uses a 64-character alphabet from byte 30 (ASCII `0`) to byte 6F (ASCII `o`)?

Is there any known/standard encoding or cipher method that uses a 64-character alphabet from byte 30 (ASCII 0) to byte 6F (ASCII ...
4 votes
2 answers
190 views

How does the Fialka manage to have a letter encipher unto itself?

The Fialka was an electromechanical rotor machine. How does the Fialka manage to have a letter encipher unto itself? I realize it does something clever by using transistors, but it has a reflector. ...
2 votes
1 answer
441 views

Mathematical definition of scytale

Most cryptographers know the scytale. It is that cipher where you roll a leather strip around a rod and then write text on it. For encryption you roll it off the stick and for decryption you roll it ...
2 votes
1 answer
237 views

Shift cypher, perfectly secure?

I know that if only one character is encrypted using a shift cipher, then the shift cipher is perfectly secure. But what if the space of keys is greater than the space of messages? Would it still be ...
3 votes
2 answers
428 views

Crack a polyalphabetic cipher given a pair of (plaintext, ciphertext) encrypted by it

The original question only states that a classical cipher is used, and I am going to articulate 1) why I think a polyalphabetic substitution cipher is used AND 2) my attempts so far. The Question <...
2 votes
0 answers
60 views

Using ML to detect what classical cipher the ciphertext is encrypted with

I was considering creating an ML project where it is fed some ciphertext by any classical cipher and would return possible ciphers that encrypted the text. I would have to create a sizeable dataset ...
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How to encrypt affine cipher in block?

Assuming the alphabet of capital letters, if an affine linear cipher $v\to Av+b \bmod 26$ with block length $3$ maps plaintext ENCRYPTAGAIN to ciphertext ...
5 votes
2 answers
31k views

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: ...
1 vote
1 answer
169 views

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 The first letter is encrypted using 1, and then ...
1 vote
1 answer
319 views

What are the equations (Gen, Enc, Dec) for the vigenere cipher?

Long story short, trying to understand how I would write the vigenere cipher mathematically when using Gen, Enc, Dec and I can't figure it out. This is what I've come up with so far. $$\mathrm{Gen}: k ...
0 votes
1 answer
152 views

Index of Coincidence Averages List

I am looking for a comprehensive list of the IOC values of any or all classical ciphers. I can't find a list, only pieces. Each cipher has an average IOC value, and I was wondering if there was a ...
11 votes
3 answers
4k views

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 ...
6 votes
2 answers
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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 ...
1 vote
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140 views

Vigenère cipher with switching keys

I am looking for possible ways of attacking a modified Vigenère cipher. Let's say we have two keys e.g. 'stackoverflow' and 'Vigenère'. The V cipher starts with one of those keys but switches as ...
2 votes
0 answers
155 views

Need some help with decryption code for monoalphabetic substitution

I am working on decrypting ciphertext in monoalphabetic substitution algorithm. I tried to decrypt using frequency histogram of English alphabets individually. The percentage of these frequencies can ...
1 vote
1 answer
121 views

Can I use a Friedman test when decrypting a Playfair cipher?

I’d like to decrypt a Playfair ciphertext; however, I need to find the key word first. Is it possible to use the Friedman test here to find the length of the key word? My cipher has no J’s in it would ...
7 votes
2 answers
8k views

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, <...
1 vote
2 answers
2k views

How to decipher an ADFGVX cipher?

I got introduced to the ADFGVX cipher, and it seems very tedious to decipher such ciphertexts. If I do not have access to the key square and the keyword, and I am just given the ciphertext and a ...
6 votes
4 answers
2k views

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 ...
1 vote
2 answers
183 views

Concatenating "words," is it possible to create a key that is indistinguishable from one made in a truly random manner?

By concatenating "words" and adding them Modulo 26, can a key be created that is indistinguishable from one generated in a truly random manner? Let's say the key is more than about ten ...
2 votes
1 answer
290 views

Does the transposition cipher have a network application?

So I'm reading a chapter in my networking book, and it talks about substitution and transposition ciphers. I know that most network security uses public key or symmetric encryption, but I wonder ...
1 vote
0 answers
148 views

Method for cryptanalysis of a keyword-keyletter cipher?

I am a CS student who is in an introductory cryptography course. I am not looking for the complete plaintext of this message, as I am not looking to cheat. I am simply stuck and have no idea what to ...
0 votes
1 answer
804 views

What is a ciphertext only attack and how OTP is unbreakable using this attack

I was wondering that what actually is cipherText-only attack and how the One Time Pad Encryption cant be broken by this attack?
2 votes
1 answer
210 views

Repeated Vigenère Cipher

I was looking into the Vigenère cipher when I thought of something. Would you improve the security of the cipher if you shifted each letter (from the text you want to encrypt) with each letter of the ...
2 votes
1 answer
279 views

Mixing Autokey with Regular Vigenère

It seems to me that cryptanalysis of the autokey cipher relies on patterns being found in key attempts, easy when it's English you're looking for, but how can you do this if you're just getting ...
1 vote
1 answer
591 views

Key space of a substitution cipher

I was going through a cryptography course, and I found a question there that: What is the size of the key space of the substitution cipher with 26 letters? Its answer was 26! I am not sure what this ...
9 votes
1 answer
375 views

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 ...
1 vote
2 answers
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Can all affine cyphers be expressed with this formula

There are $26!$ permutations of the English alphabet. In Modern Cryptography textbook, William Easttom states that the basic formula for any affine cipher is $ax + b \equiv \pmod{26}$. I'm wondering ...

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