# Questions tagged [vigenere]

The Vigenère cipher is a classical cipher that shifts each plaintext character by the value of the key at the same position modulo the key-length.

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### Is there a way to defeat statistical information leak in Vigenère cipher when the ciphertext is reasonably larger than the key size?

I read in this answer here that if the ciphertext is reasonably larger than the key size, Vigenère cipher is easily breakable. Is there a way to repeat the key if it is reasonably smaller that the ...
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### Finding key using Mutual Index of coincidence / Mg from vigenere ciphertext [duplicate]

I'm new to cryptography and I was trying to find key to an intercepted vigenere ciphertext using ciphertext-only attack, I'm following book "cryptography and network security" by Forouzan. ...
1 vote
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### Breaking a Vigenere from nouns rather than text?

I have what I believe to be a Vigenere or similar polyalphabetic cipher (IC = 0.0459). However, the ciphertext is further encoded by 'Latin-like' letters, meaning I don't have the ciphertext alphabet (...
1 vote
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### Vigenere Cipher Collision

I have been reading the book "Serious Cryptography" by Aumasson to begin learning about Cryptography. There was a part where the author suggested how encryption using permutation should have ...
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### Is Vigenere's cipher really breakable for very long cipher keys?

It is argued that the Vigenere cipher is easy to break using the Babbage's solution. But this solution counts on finding the repetetive occurences of letter groups(words if you will). What if my key ...
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### Vigenere applied to encrypted data

What happens if you encrypt data with a good cypher that produces a cyphertext that looks random (say, AES but avoiding ECB mode), and then apply a Vigenere cypher to the encrypted random-looking data?...
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1 vote
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### 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 ...
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### How do I exactly use the Index of Coincidence in Cyphertext?

I came in contact with a ciphertext, which is: ...
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### Does using ECB mode of Vigenère cipher on files (no plain text) with a key of 10,000 byte(char) is secure in today's standard?

Hi I'm fairly new to studying cryptography and I'm just curious about the security of Vigenère cipher. Given today's powerful super computers, how secure/unsecure does Vigenère cipher can get? let us ...
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### Vigenere Decryption with key length

I have a question. If I have 2 ciphertexts given and the same key has been used on both. They key lenght is the same as the plaintext and cipherrtext length. How do I decrypt them and find the key. So ...
1 vote
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### 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 ...
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### 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 ...
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### trying to identify the key of a vigenere cipher via brute force

I'm working on a bit of an ARG game type thing that uses the vigenere cipher. what im wondering is the chance a player could brute force said cipher by simply checking all possible key sequences of ...
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### how can identify which cryptanalysis is used ( Indistinguishability cryptanalysis against classical ciphers) [duplicate]

I have 10 ciphertexts and 10 plain texts I want to know which cipher is from which plain text how can I know this I know the name of substitution cipher, transposition cipher, Vigen`ere cipher and ...
1 vote
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### Question about variations on Vigenere cipher

I'm new here and by no means an expert on cryptography. I like to dabble, and one of my favorite classical ciphers is the Vigenere. I understand that the normal method of Vigenere encrypting is to ...
1 vote
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### How to break a vigenere cipher with a period of 2, and knowing a part of the plaintext

the encrypt code is "ARJHNRIUCGQHPFGQFHTXPDEHTLNOC", and the part that i ​know of the plaintext is "DERUNA". The key is period of 2, so the key are 2 letters. Each letter of the ...
1 vote
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### How does the frequency work with Vigenère Cipher?

So I have an unknown key but with the length of 6. I understand that I calculate my own English letter frequency of my letters. I then compare it with the standard English letter frequency and ...
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### Pencil & paper encryption, some thoughts on polyalphabetic techniques [closed]

I have an idea for a simple cipher that builds on old techniques that I thought I would share with this group. It has probably been done but I have not been able to find a reference to it. Given two ...
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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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### decode Vigenere cipher that was encoded with Affine cipher

How do I decode some text that has been encoded like this: affine(vigenere(text, vigenere_key), *affine_key)? I don't know keys to either of them. At first I ...
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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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### 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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### Need help finding keysize for Vigenere cipher

I am doing a crypto challenge for breaking a Vigenere style repeating XOR encryption (https://cryptopals.com/sets/1/challenges/6). I have looked at similar questions asked here, mainly this one: ...
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### Can you break a Vigenere cipher with key length of 2 and message/cipher text length of 4?

I am currently working on a problem where an attacker knows that a user's password is either "pqrs" or "jmlo" and also knows that the encryption scheme is using the Vigenere cipher ...
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### Attacking a variation of the Vigenere with very long key length

I am to decrypt the ciphertext YVBCXGJRYHHRCJIUL into its plaintext message. The information that I am given is that the key used in the cipher is very long (equivalent to the length of an entire book)...
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### Identifying correct cipher

I am trying to decrypt a text(~900 characters) and I started with the assumption that its a Vigenère cipher as the one of the identifier tool identified it as Vigenère cipher and also the Index of ...
1 vote
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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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### Plaintext with 'T' and 'E' alphabets missing and encrypted with Vigenere Cipher

I wrote the normal code to decrypt a Vigenere cipher encrypted text without knowing the key. But that won't work for this are two of the alphabets are missing and that would mess up the frequency ...
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### Decryption of Vigenère cipher by hand using Kasiski method

My task is to decrypt the following message, which was encrypted with a Vigenère cipher of length 4: ...
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### Need help cracking a Vigenère cypher with a period of 2

How do you crack a long Vigenère cypher when the key is unknown? I have tried frequency analysis, but I'm not making any progress.
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### Using same key twice with Vigenere cipher

Could someone please give a walkthrough of how this works: Decrypting a vigenere cipher with a reused one-time pad. It was never fully answered with the example given in the comment. e.g Plaintext ...
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### Are there official test vectors for historic ciphers? [closed]

In many cases when cryptographic algorithms are published the paper also provides a set of test vectors that can be used by developers to verify their implementations. For historic ciphers such as ...
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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 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 ...
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### Are there ready-to-use software that can try many decryption techniques on a ciphertext, with no information?

Let's say I receive a 1 MB encrypted file, but I get no information about the encryption method, and no password. The only thing I can guess is that the person probably used a weak encryption method ...
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### Modification of vigenere cipher cryptographically secure?

After looking for ages online for a python implementation of the Vigenere cipher, and finding few, I decided to implement it myself. After ironing out all of the flaws, it's now a working (and secure) ...
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### Is there an English translation of Vigenère's "Traicté des Chiffres"?

I Googled Vigenère's "Traicté des Chiffres": no English translations, not even a reference to one. Perhaps someone's Google-wu is better than mine; I can't find it. I'm slogging my way ...
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### Mutual Index of Coincidence

I can't seem to find a simple explanation of Mutual Index of Coincidence. I am finding explanations of Mutual Index of Coincidence not Index of Coincidence. I don't understand what the formula below ...
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### Vigenere with multiple encryption [duplicate]

I have a school project about cryptography. The question is the following (will only give a short phrase as example, just want to know the method). A text has been encoded with Vigenere 2 times. The ...
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### What is the difference between a Vigenère cipher and a one time pad cipher?

I'm doing a decryption competition at a nearby college. We are allowed one cheat sheet so I'm trying to make one. I realized that the Vigenère cipher was very similar to the one time pad and wanted ...
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### 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 ...
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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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### 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 ...
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### Decrypting a vigenere cipher with a reused one-time pad

I understand that when a plaintext is encrypted with a key of the same length, it is referred to as a one-time pad. Now when this key is reused to encrypt another plaintext, there is potential for ...
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### Cracking short ciphertexts made by Vigenère cipher

I've encrypted a couple of 10 letters words from https://www.bestwordlist.com/10letterwords.htm using a key of 10 characters. This is done using Vigenere cipher. Decrypt the cipher and give the key ...
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### How to crack Vigenere ciphertext if multiple keys of coprime length are used?

I came across the idea of using multiple Vigenere keys of coprime lengths here. The idea is, what if you use multiple keys, whose lengths are all co-prime? Then your effective key length would be ...
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### Basic Vigenere Cipher

I'm taking my first cryptography course and having trouble deciphering some Vigenere encrypted text. I was able to crack the first one: I'm having difficulty with the other two. I think the ...
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### Probability of Vigenere cipher

I am trying to figure out this quiz question on a course I am doing but keep getting it wrong. Consider the Vigenere cipher over the lowercase English alphabet, where the key can have length 1 or ...
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### Why is Vigenère Autokey self-synchronizing?

According to the Wikipedia, and to several of our school study materials, Vigenère autokey ciphers are considered to be self-synchronizing stream ciphers. If I understand it correctly, the main idea ...
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### Are some combinations of Substitution + Transposition more vulnerable than others?

Iterating multiple substitution ciphers is not mathematically different from using a single substitution cipher (3 Vigenere iterations with a 31-, a 32-, and a 33-letter key would essentially be a ...
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