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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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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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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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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Substitution cipher with variable key

Given that one of the main disadvantages of a polyalphabetic substitution cipher like Vigenere is that the key length can be determined by comparing the frequency distributions to the plaintext ...
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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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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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How were codes in WW1 reciphered (to enhance security levels) without them turning meaningless?

In WW1 the Germans made their communicating secret by encoding their messages. This meant that the sender of the message would have a codebook with all possible words and phrases he might use listed ...
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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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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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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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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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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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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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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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Using a classical method, is it possible to create a key that is indistinguishable from one made in a truly random manner?

Is there a classical method that will allow us to 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 "words" to create ...
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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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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 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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In cryptography, what is “fractionation”?

I understand how fractionation is used in the straddling keyboard, which is an important feature of the VIC Cipher, and in the Polybius Square of the ADFGVX cipher, but I am not sure how to describe ...
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Can frequency attack be successful on Vigenère cipher?

Here is my question when learning Vigenère cipher, can we perform frequency analysis attack toward this cipher? My first impression is no, because each column of its matrix has exactly 26 English ...
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Is there a cipher that avoids double bigrams?

Are there any known ciphers that whose ciphertext (given any plaintext) avoids bigrams of the form "aa, bb, cc, etc."? What about known ciphers that make bigrams of form "aa, bb, cc, etc." rare ...
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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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Most secure but feasible encryption running on WWII technology

Imagine you are transported back to ~1940. What is the best (i.e. most secure, but technologically feasible) encryption you can think of for widespread (i.e. military) use? How would you implement it?
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About LFSR based M209 Cipher Machine

Can some one please express M209 Cipher Machine in terms of LFSR with Diagram or any simple method?
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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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How can the Enigma's plugboard settings be found with partially known plaintext?

I don't quite understand the procedure described on wikipedia, is there a generalized procedure you can follow to rule out a pairing on the plugboard? For example, let's say the cipher text is ...
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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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Enigma rotation example

According to wikipedia: BDZGO is the expected outcome when doing the enigma chipher on the letters AAAAA with rotors I,II,III and reflector B. ...
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Ciphers based on Keywords

I am looking for different ciphers that use a key word (like "hello" or "awesome") to encrypt a given text. I know about the Keyword Cipher already; I'm looking for others.
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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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Shuffling alphabet cipher [closed]

Disclaimer: this is actually a coding challenge therefore I am expressly not looking for complete solutions. The way the cipher works is as follows: Start with ...
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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 <...
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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 ...
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Codes of “SAS und Chiffrierdienst”

The german site "SAS und Chiffrierdienst der DDR" http://scz.bplaced.net has some interesting content under point 11.3 in the category "manuelle Verfahren" I found ciphers named from A to D. But I ...
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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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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 secure is a book derived key and what can be done to strenghten it? [duplicate]

If you have a manual encryption system where the keying is done by extracting a key from a book, a major weakness is that an adversary can search trough all existing books to find a perfect Match and ...
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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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Repeated Vignère Cipher

I was looking into the Vignère 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 ...
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How can I determine the Hill cipher key in this case?

I have been struggling with a Hill cipher problem for many days, without any luck. I have the following ciphertext, which I know is an encrypted excerpt from an Edgar Allan Poe story: ...
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Vigenere Cipher Methods

Is there already a cipher which uses a method like vigenere yet the shifted encrypting alphabets are mixed? i.e. ASDFGHJKLZXCVBNMQWERTYUIOP BNMQWERTYUIOPASDFGHJKLZXCV CVBNMQWERTYUIOPASDFGHJKLZX etc....
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flaw in perfect secrecy of shift cipher?

Why do we say shift cipher is perfectly secure when it is easy to break it (source)? Let's say I have a plaintext. "Australia is a big country"; I encrypt it using a shift of 2; That ciphertext can be ...
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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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Can a message be encrypted with BOTH mono-alpha and poly-alpha ciphers?

For my class assignment, I was given this text; Qpuvu vo vv stazpuykzh Hhycmlmv asb. Ad dhyaqva wh uyo uzb jpwww Avlw. Tkpjllfdmbp zoatz vzjavfvna ZLHqh. Amva luy yyvlz yu bhk zua zlhvvalaz yow ...
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Cryptanalyze strategy for cipher text [closed]

I have the following ciphertext that I want to decipher: bnmkogt vgyhni uioopkm xswercvdf yuiogvbnm ghjy cftyhng tgeyjn xdrtygvc zsefx xdrgbhu tgbnmko tgbnmhyui yhnmko vbnmgyuiok So far, what I ...
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Does there exist a Cipher / Hash identification tool?

Let's suppose I have two strings One string is plaintext The other is the result of running the plaintext through an unknown cipher. Does there exist a tool where I can enter the pre and post-...
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Fastest / easiest way to find rotor wiring without physical inspection on Enigma Machine emulator

We have an Enigma machine emulator we are trying to replicate. Machine has 3 rotor positions and 6 rotors that can be selected. The input goes through 3 rotors then is reflected back to start as is ...

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