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15 votes

Creating your own encryption and decryption algorithm

Edit: I think the edit to the question makes it as vigenere cipher; which invalidates my answer below. @galvatron answer gives the suitable answer why vigenere is not secure. The old answer below ( ...
NuminousName's user avatar
11 votes

Creating your own encryption and decryption algorithm

Encryption is naïvely viewed as a way to send messages from A to B that cannot be deciphered (at least in practice) by an adversary. Sure, encryption does do that, but modern ciphers do so much more......
Dennis's user avatar
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7 votes
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What is the difference between a Vigenère cipher and a one time pad cipher?

The Vigenère cipher is essentially a series of interwoven Caesar ciphers, in which the size of the shift is different for each character and is defined by the keyword. The one-time pad (OTP) is a ...
Dull Unicorn's user avatar
6 votes
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Does Kasisky test for Vigenère cyphers assumption always hold?

There is imprecision in what is stated in your notes. The Kasisky test only works if the corresponding letters in the two segments are separated by a distance that is a multiple of the key length (in ...
puzzlepalace's user avatar
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6 votes

Does a Vigenère cipher with SHA-512 and a little foresight turn into a one-time pad? How safe can it be?

Is this a real one-time pad? No, it's not a one time pad; as the keystream is not generated from a truly random process, hence you don't get the informational security guarrantees that you'd get from ...
poncho's user avatar
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6 votes
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Conditions for Perfect Secrecy with the Vigenere Cipher

As far as I can tell, your original reasoning is correct. To obtain perfect secrecy with a Vigenère cipher, the key must not repeat. The key does not repeat whenever it is at least as long as the ...
Ilmari Karonen's user avatar
6 votes

Is Vigenere's cipher really breakable for very long cipher keys?

Yes, it would become harder, if just because you'd first have to find the size of the key, and then you can probably not use the old technique. Would it be secure? No, of course not. Finding words is ...
Maarten Bodewes's user avatar
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5 votes

Vigenère cipher

Let's consider claim 1 first. If the ciphertext alphabet has, say, 26 letters, then a Vigenère cipher with a 100-character key length has $26^{100}$ possible keys. You may wish to do your own back-...
Ilmari Karonen's user avatar
5 votes
Accepted

Vigenère cipher - sorted order

ADLY is the order those letters appear in the alphabet, so A=1, D=2, L=3, Y=4. But I've never seen such a substitution in a Vigenere cipher, and it's dumb, since it makes a weak algorithm even weaker....
CodesInChaos's user avatar
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4 votes

Cryptanalysis of XOR cipher with repeated key phrase

The first thing you should do is estimate the key length. While you may be able to do this using Kasiski examination, in this modern era of computers I very much recommend instead using the index of ...
Ilmari Karonen's user avatar
4 votes
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How to break a Quagmire 3 cipher?

All the Quagmire ciphers (see e.g. here for definitions) are combinations of a Vigenère shift cipher and a keyword-based simple substitution cipher, where the substitution cipher is used to scramble ...
Ilmari Karonen's user avatar
4 votes

Conditions for Perfect Secrecy with the Vigenere Cipher

2 can't be correct and it's not a trick, message space should be equal or smaller than key space, in 2 message space is much larger, its : $\sum_{i=1}^{6} 26^{i}$
Mairon's user avatar
  • 161
4 votes

Creating your own encryption and decryption algorithm

Sari's answer is very good at explaining why this method isn't particularly good. However I'd like to add that if your array is chosen from a very good random number source (think radioactive decay, ...
MikeS159's user avatar
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4 votes
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Mutual Index of Coincidence

As defined e.g. in Stinson's Cryptography: Theory and Practice (1995, 1st ed.; the earliest source for the term I could find that has even excerpts online), the mutual index of coincidence of two ...
Ilmari Karonen's user avatar
4 votes
Accepted

Are there ready-to-use software that can try many decryption techniques on a ciphertext, with no information?

The project Cryptool does some if not all of what you want. I have not used it extensively, but it seems quite well documented. Below from the webpage: CrypTool 1 (CT1) was the first version of ...
kodlu's user avatar
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3 votes
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Prove Vigenere cipher isn't perfectly secret

Given the ciphertext and the first $n$ characters of the plaintext, you can uniquely determine the remainder of the plaintext. Thus, for each ciphertext, there are only $k^n$ possible plaintexts (...
Ilmari Karonen's user avatar
3 votes

Prove Vigenere cipher isn't perfectly secret

One way to approach this is to look at the decryption process; the plaintext is a function of the ciphertext and the key. The conditional probability says you know the ciphertext (entropy 0), you don'...
poncho's user avatar
  • 148k
3 votes

Does a Vigenère cipher with SHA-512 and a little foresight turn into a one-time pad? How safe can it be?

No, that’s not a “real one-time pad”. As I explained in another answer of mine… Per definition, OTP requires the “key“ to be… a truly random one-time ...
e-sushi's user avatar
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3 votes
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K Friedman - two texts explanation

Just for simplicity lets assume that your short period "password" was just XORed with plaintext. So we have encryption procedure like: ...
CaptainRR's user avatar
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3 votes
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Breaking a XOR cipher of known key length?

After more time on this challenge, here is what I've done to solve it. Divide the text by N columns, where N is the length of your key. Run frequency analysis on each column. Given enough data, you ...
Jay Zus's user avatar
  • 151
3 votes

Modification of vigenere cipher cryptographically secure?

Vigenère Cipher Let's look at some different attacks on the Vigenère cipher and determine how much of a role alphabet size plays. From Wikipedia: The primary weakness of the Vigenère cipher is ...
Jacob H's user avatar
  • 364
3 votes

Need help cracking a Vigenère cypher with a period of 2

Cracking a Vigenere cipher usually involves the below 3 steps: The Friedman Test - The first thing you need is the length of the key (approximately at least). You can do this by finding the Index of ...
alwaysn00b's user avatar
3 votes

Attacking a variation of the Vigenere with very long key length

The important part about the length is how many letters appear on those 17 characters: YVBCXGJRYHHRCJIUL WEDNESDAYTHURSDAY WEDNESDAYSATURDAY We have the plaintext ...
Ángel's user avatar
  • 496
3 votes

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?

In short: it's bad. The main reason is, Vigenere is really bad at hiding any kind of structure. Any kind of non uniform distribution in the data leaks, and frequency analysis can find that. And the ...
tylo's user avatar
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2 votes
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Product of Vigenère ciphers is not a Vigenère cipher with length of LCM

I have tried with some lengths but it worked so that the product was Vigenère with length of LCM. Actually, the statement really isn't "is encrypting with cipher $\mathcal{V}_1$ and the cipher $\...
poncho's user avatar
  • 148k
2 votes

Levels of strength: The Caesar Cipher

Hmm, first about your type of encryption: ...for a Caesar Cipher that encrypts each letter with a different integer as a shift... This actually describes a Vigenère Cipher. The classic approach to ...
jjm's user avatar
  • 166
2 votes

Comparison Vigènere vs. Monoalphabetic cipher

a) The question seems to be about a comparison of the size of the key spaces. The hint already shows the key spaces for Vigenère with a 10-letter key ($26^{10}$) and for simple substitution ($26!$). ...
jjm's user avatar
  • 166
2 votes

Can I make a cipher (ex: Vigenère) harder to break?

You can greatly increase the security of the Vigenere cipher by encrypting a second time with a key whose length is relatively prime to that of the first key.
Meler Lawler's user avatar
2 votes
Accepted

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

The actual "encryption" is done on this line: mysecretmessage[i] ^= ((mysecretvalue>>(8*(i%4)))&255); Clearly, this line XORs every byte (or at least, ...
Ilmari Karonen's user avatar

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