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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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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Tranposition Ciphers

I am studying classical ciphers and have the following doubts: Is the row and columnar transposition cipher the same? It is similar since the process of encryption and decryption is the same for ...
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How to attack a general polyalphabetic cipher?

I am able to decrypt Vigenere ciphertext using the index of coincidence and chi-squared test. However, out of interest, how do you go about attacking ciphertext that was encrypted using a mixed ...
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An unbreakable book cipher?

In a recent press interview, a former terrorist of the 1970s described the cipher his group used to communicate. He claimed that method was unbreakable and I wonder if cryptographers today would agree....
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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 ...
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Does the Richard Sorge cipher still provide good security?

Is the cipher used by soviet spy Richard Sorge still sufficiently secure nowadays?
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How can I determine the number of times I have to reapply the Scytale until the original plaintext comes back?

Given "①②③④⑤⑥⑦⑧⑨⑩⑪⑫" as plain text and a 4 rows Scytale inscription with 3 as the length of the longest row, the original plaintext is back after 5 runs: ①④⑦⑩②⑤⑧⑪③⑥⑨⑫ ①⑩⑧⑥④②⑪⑨⑦⑤③⑫ ①⑥⑪⑤⑩④⑨③⑧②⑦...
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How was the 5 digit random number in the VIC cipher generated?

Knowing that VIC was a "spy cipher" it is unlikely that the agents used a cryptographic device to genreate the 5 digit number but how did they do it?
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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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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 ...
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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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How were codes in WW1 reciphered (to enhance security levels) without them turning meaningless?

In WW1, the Germans made their communications secret by encoding their messages. This meant that the message's sender would have a codebook with all possible words and phrases he might use listed in ...
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Does a one-time pad of digits 0-9 have any weaknesses?

I'm working on one of those unsolved puzzles, and one theory I have is that one or more sections of it are using digits of pi, e, or some other irrational number as a one-time pad. (The obvious ...
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Mixing Autokey with Regular Vignere

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 ...
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How to determine which type of transposition cipher I have?

I have some ciphertext that I have determined using frequency analysis to be most likely encrypted using a transposition cipher, as the letter frequencies are similar to plain English language text. I ...
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Does anyone have any hard ciphers to be cracked for fun?

The title says it all. Do you have any ciphers or encoded messages that you could post as a brainteaser? Thanks!
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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) ...
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How to go about decrypting a cipher with known input and output? [closed]

I have a known input and a known output but how would I go about working out what the cipher was used. Below are a few examples of input and output. On the last one I noticed a repeating pattern. ...
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How can I break a Vigenère cipher with partial plain key? [closed]

I've got the cypher text: cöimamiçknsyznhaczstş and a partial plain key: ?ğre??i I've used "?" instead of unknown ...
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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 ...
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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 we create a key that cannot be distinguished from one that was generated in a truly random manner? For example, let's say we use three ...
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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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What is the formal name for determining the key to a Vigenere cipher using frequency analysis once the key length has been found?

In short, I am writing a paper about cracking the Vigenere cipher. There are supposed to be three parts to it and each is supposed to be related to an academic article (or anything formal). I chose ...
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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 ...
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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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LAT of an SBox, values are even

This is probably a silly question, but I could not find any reference. For the DDT of an SBox, it is easy to see why all the values are even. Is there any related result for LAT (i.e., all its values ...
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Can someone help me understand this?

In the book Serious Cryptography It says for a cipher's permutation to be secure "Different keys should result in different permutations. Otherwise it becomes easier to decrypt without the key: ...
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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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How do you determine the recorded value of a cipher when using the shift key?

I am taking a cryptography class and having a hard time understanding how to utilize Ceasar Cipher and how to determine the shift key values. Is it based on many characters are in the cipher or is it ...
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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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How to “score” partially decoded ciphertext?

I am attempting to write a basic codebreaking program to break simple ciphers (think caesar cipher & railfence) such as these here https://en.wikipedia.org/wiki/Classical_cipher. The program ...
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How to decrypt a short ciphertext that is said to be immune to attacks?

I need to decrypt a ciphertext which is short, and I have the first word of the plaintext. I thought of doing a probable-word attack, but I already know the position of the known word. In addition, ...
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Packing technique for large data storage

Packing multiple plaintext messages into a single ciphertext block is known to increase the effeciency of homomorphic encryption. Can someone let me know if this technique can support large data ? For ...
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How to find/determine the Polybius Square Key and Additive Key, if only the Cipher Text is known, in Nihilist Cipher?

I was solving a Crypto Challenge in a CTF, and I got a Nihilist Cipher Encoded String. Luckily the additive key was given as a hint, and for the polybius Square key, I kinda bruteforced or I just got ...
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219 views

Please prove distinguishability given a non-perfectly secure cipher

I'm trying to prove that a perfectly secure cipher yields indistinguishability. I already know and can prove that a perfect cipher => indistinguishability by the following proof: \begin{align} \Pr(...
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Why was it easier to decode messages that you had several copies of?

During WW1 the German ambassador in Washington, von Bernstorff, blamed their numerous transmissions of the same messages as the cause for the successful decoding done by the British. I don't see the ...
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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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Prove monoalphabetic cipher is a valid symmetric cipher

I should prove mathematically that: $D_k(E_k(p))=p$ I know $E_k(p)=c=(p+k)\bmod26$ and $D_k(c)=p=(c-k)\bmod26$ Substituting the two formulas to the initial assertion I have $((p+k)\bmod26-k)\...
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Hill cipher. Any way around the not invertible matrices [duplicate]

Question: The plaintext JULY is encrypted using Hill Cipher with m = 2, to give the ciphertext DELW, then determine the encryption and decryption matrix My approach: We will form a 2x2 matrix of ...
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Vigenère cipher - sorted order

i'm new to cryptography so i feel sorry for this newbie question! Can anyone tell me how Vigenère work with sorted order? For example i have this in my book: LADY(KEY) 3124(SORTED ORDER) 312431243(...
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How to make this cipher strong?

Suppose I have an arbitrary 256 bit number $m$ another secret number $k$ of the same bit length, and then I multiply them both modulo a 256 bit prime number $p$ to get $c$ as follows: $$ c = (m\cdot k)...
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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 ...
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Classic cipher method [closed]

I am struggling with these two cipher texts, I cannot identify which king of cipher was used. First: otnpeetrnneepeufofutisrtrllohcdmetoeheeatyngonrmrm And second: ...
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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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Is it possible to crack the trifid cipher without the keyword?

I was learning about the classical ciphers and was looking up the methods to break them without the keywords and when I searched up "cryptanalysis of the trifid cipher" I couldn't find anything. I ...
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Running Key Gibbs Sampling

I am trying to implement the algorithm described in this paper. It uses blocked Gibbs sampling to perform a ciphertext-only attack on text encrypted with a running key cipher. However, I am confused ...
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Applications of machine learning in classical ciphers?

Machine learning is definitely applicable in analyzing simple shift ciphers like Caesar and affine ciphers, as well as substitution ciphers like Vigenère, but is it possible for machine learning to ...
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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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Is there a way to make a non-specific algorithm to decypher substitutions with “anti-frequency analysis” measures?

There's a code I'm trying to decipher. What I know is that it's a simple substitution, with just a few added rules intended to make frequency analysis and other very common methods a little less ...
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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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