# 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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### 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 ...
12k views

### 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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### K Friedman - two texts explanation

I read the $K$-Friedman test to determine the period of a Vigenère ciphertext, and I understand something like this: $K$-Friedman test tries to find out what period of the cipher by calculating some ...
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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 ...
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### Relative security of a Vigenère cipher

Within a closed computer network, I am ciphering some plaintext data as an added security measure. This is below several other layers of protection. For various technical reasons, I am restricted to ...
18k views

### 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 ...
3k views

### Conditions for Perfect Secrecy with the Vigenere Cipher

Please forgive my ignorance here as I'm far from a math major and I've never looked into cryptography before so if this seems stupid - it probably is. There is a question in a quiz I'm attempting that ...
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### Assistance Cracking Classical Cipher

Below is the cipher text I am trying to break and as you can see its rather short which is why I am having so much trouble. ...
1k views

### A “one time pad” can be thought of a Vigenere cipher with

A "One Time Pad" can be thought of a Vigenere cipher with... An infinitely long key A secure symmetric key Multiple ciphertext alphabets A columnar transposition I had this question come up ...
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### When does a multiple Vignere cipher become intractable?

Multiple Vignere ciphers do not repeat the key until the message gets longer than the LCM of the cipher lengths. Are messages shorter than that safe? If not, what is the attack strategy? I believe ...
3k views

### How can I break a Vigenère cipher with partial plain text?

I have an exercise to do regarding Vigenère encryption. I've got the cipher text: WNZTNVIEEGTJYKRRWYUELWNZTNV and a partial plain text: ...
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### Prove Vigenere cipher isn't perfectly secret

As part of a homework assignment I have to prove/disprove that a Vigenère Cipher with a key of length n, uniformly distributed in the alphabet, on a plain text of length 2n, also uniformly distributed ...
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### Can I make a cipher (ex: Vigenère) harder to break?

The Vigenère cipher can relatively easy be broken when the key size is small compared to the size of the message. One first finds the length of the key, and then uses frequency analysis to actually ...
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### What is the limit of plaintext required to break the Vigenère encryption?

A theoretical question about the Vigenère cipher: Without any knowledge about the key (not even it's length) can we tell how much known or chosen plaintext is needed for the adversary to completely ...
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### How to break a Quagmire 3 cipher?

What would be a good way to go about attacking a Quagmire 3 cipher? I understand that it is polyalphabetic with a key and an indicator. I have started by taking the cipher text, taking every n-th ...
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### Comparison Vigènere vs. Monoalphabetic cipher

I have a question for a class of secure communication, but I have no idea about how to proceed in order to answer it. I'll be very grateful to whom replies to me. Assuming that brute force attack ...
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### Vigenère cipher with sha512 and a little foresight turns to one-time pad. How safe can be?

I'm doing some exercises and of course I have read about the weaknesses of Vigenère cipher because it repeats the key. But what if I use a different hash every time? The hash of the "key + an ...
335 views

### 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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### Plaintext XOR'd with keylength-shifted plaintext

While solving a (easy...) Project Euler cipher problem this week I repeatedly saw instructions for breaking ciphers after identifying the keylength as: 'shift the ciphertext by that key length and ...
334 views

### 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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### Vigenere cipher

I am attending cryptography course on Coursera. I am stuck at one question which I cannot answer right even after few tries. Which of the following are true about Vigenere cipher? (Check all that ...
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### Cryptanalysis of XOR cipher with repeated key phrase

My assignment is to cryptanalyze cipher text that we know was encrypted with a stream cipher (bitwise XOR of plain text and key stream), and decrypt to plain text. We know the key stream is a repeated ...
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### Vigenère cipher: Security when key length and plaintext length are the same

I have read the Vigenère cipher is secure as long as the key length is the same as the length of the data to be ciphered. Is this true when the same key is used multiple times? In such cases, after ...
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### Friedman Index of Coincidence, pruning multiples of the keylength

I'm trying to write a Vigenere decipher script. I defined a couple of possible keylengths using the Kasiski method, using roughly this method: find recurring strings in the ciphertext between 3 and ...
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### How would multiple uses of a vigenère cipher be decrypted on paper (classic)?

As far as I can tell (from reading the Wikipedia article about it) decrypting a message encrypted with the vigenère cipher is possible on paper. But what would happen if the message is encrypted ...
376 views

### Words in “index of coincidence” in relation to the Vigenère cipher

I am trying to interpret the formula for index of coincidence from here… I am not understanding $f_i$. How can I calculate $f_i$ in this context? Is this the appearance of single word like (...
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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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### Is there an English translation of Vigeneré's “Traicté des Chiffres”?

I Googled Vigeneré'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 through a PDF ...
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### Why is Vigenére Autokey self-synchronizing?

According to the Wikipedia, and to several of our school study materials, Vigenére autokey ciphers are considered to be self-synchronizing stream ciphers. If I understand it correctly, the main idea ...
161 views

### Breaking an addition-based cipher with two ciphertexts but no known plaintext

I'm attempting to break a system which uses decryption code functionally equivalent to the following: ...
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### Vigenere ciphertext encrypted with another vigenere cipher

I have been learning about Vigenere ciphers and then thought of this scenario: if a cryptographer encrypts a plaintext English message with a Vigenere cipher and then another cryptographer, who wants ...
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### How to decode a (Vigenère?) substitution cipher? [closed]

I am trying to decode a message that I suspect has been encrypted with a Vigenère cipher: Utl zluuocp zsc rvzu hbcp ztvibfz vrabzz utl ilry. Utlal flal cb fbaiz, qszu zbsciz. Utvu cllihl vhzb dvil ...
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### Find the key to a Vigenère cipher, given known ciphertext and plaintext

I have the ciphertext and the plaintext. Is it possible to know the key? If not, what do I need to find it?
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### Finding length of a key for a given vigenere cipher using Index of Coincidence

I'm developing an algorithm that takes in a vigenere cipher and produces the key length automatically. However, I'm having trouble understanding how to detect the key length when there are multiple ...
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### Using Chi-Square for Vigenère Cipher

I am writing a program which will use Chi-Square to determine which is the correct keyword for a ciphertext via Vigenère cipher. I came across a website that describes the Chi-Square statistics in a ...
2k views

### Attack XOR encryption of binary data compressed by zlib with known key length (very short key)

I am trying to break a packet format. The packet format simply packs several files into one big file. The file contents are plain. But the index data which contain offsets, file sizes and filenames ...
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### Finding Vigenere's keys with part of decrypted text?

I have a text that was encrypted twice with two Vigenere keys. However, I know the seven first characters of the decrypted text. I have already implemented a function displaying for the five first ...
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### Can a Vigenère cipher be solved if the alphabet is modified (custom)?

Let's say I have about a statistically good sample of characters of text that appear to be uniformly distributed, but the composition of the text is [A-Z], [a-z], [0-9], +, and / — constituting ...
2k views

### Reversing a known encryption algorithm (4-byte repeating key XOR)

I've been practicing for an upcoming CTF competition, and I've finally reached the cryptography stage. I'm a complete newbie in this field (my knowledge is essentially that bcrypt is awesome and md5 ...
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### Playfair Cipher + Vigenere Cipher?

I'm researching cryptography for a school project, came across the above two ciphers, and something occurred to me. Would combining these two schemes give a considerably stronger encryption than ...
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### Does the plaintext have to be at least as long as the key in Polyalphabetic substituion cipher

In polyalphabetic substitution cipher, if we have a key $k$ (a string of letters) of length $n$ does the plaintext $m$ (also a string of letters) have to satisfy $|m| \geq n$? Also, side question, ...
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### Frequency analysis of Vigenere Cipher

For Vigenere cipher, I understand Find period first (say we have p = 6) Find highest frequency of letter in each group (from 1 to 6) and assume it is "e" in plaintext What if we have more than one ...
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### Autocorrelation in Autokey

I know that Vigenère can broken using auto-correlation (as explained at cryptool-online.org). Now my question is: Why does this also work with autokey (tested it using crypttool)? To prove that I ...
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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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### How to decode a caesar that goes to transposed plaintext?

When decoding vigeneres, I am left with Caesars. Often when the key is longer, these are shorter so normal frequency analysis won't work. Is there another method I can implement without adding a load ...
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### Is the beaufort cipher just as secure as the Vigenere cipher?

Does the Beaufort cipher have the same strength as the vigenere cipher assuming you use the same complexity of key, or because of it's reciprocal nature make it less secure?
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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 ...