The index of coincidence (IC) is a measure of the relative frequency of letters in a cipher text sample, which facilitates cryptanalysis of poly-alphabetic ciphers by allowing determination of the period t, length of the key, as an alternative to Kasiski’s method.
Questions tagged [index-of-coincidence]
17 questions
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How does the index of coincidence work in the Kasiki test?
I'm starting to learn about cryptanalysis and I am having a bit of difficulty understanding the Kasiski test's index of coincidence. I have a book (Cryptography Theory And Practice by Douglas Stinson) about it that I'm going through, but it seems to…
norman
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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 values of $K(C,C^n)n$, where $C^n$ is the…
PRVS
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Hamming Distance vs. Index of Coincidence
When analyzing a repeating-key xor cipher to find the key length, I've read about two key methods (assuming there aren't just repeating chunks of ciphertext for Kasiski's method), for some assumed key length $l$, checking the Hamming distance of…
Brownbat
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How do I exactly use the Index of Coincidence in Cyphertext?
I came in contact with a ciphertext, which is:
KCCPKBGUFDPHQTYAVINRRTMVGRKDNBVFDETDGILTXRGUD
DKOTFMBPVGEGLTGCKQRACQCWDNAWCRXIZAKFTLEWRPTYC
QKYVXCHKFTPONCQQRHJVAJUWETMCMSPKQDYHJVDAHCTRL
…
João Víctor Melo
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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 (a) in the whole sentence or what ? I am trying to solve…
ARG
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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 12 characters
make a list of the distances between…
Stephan Muller
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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 lengths to see which one results in an IC closest…
DarkFireGuy
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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 means
$$\textit{MI}_c(\mathbf x, \mathbf y) =…
Wei Wen
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Avoiding doubled letters in encrypted text
I have an encrypted string based on an "alphabet" of 29 runes and access to successfully decrypted (runes->English) plaintext. Testing for a Vigenere cipher, I tried a Friedman attack (aka Kappa test, Index of coincidence test) to try and find a…
Pishi
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Index of coincidence for completely random text over k alphabets
I'm confused about /Friedman's method 2 (using a table with column of k size of key, and n/k rows, n is the total size for cipher text)
\begin{equation}
m = \Bigl(\varphi(l)-\varphi(o)\Bigr)/\Bigl(\varphi(T)-\varphi(o)\Bigr)
\end{equation}
P.S.…
atom2ueki
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Expectation Value of the Index of Coincidence
Can Anybody tell me why the expectation value of the Index of Coincidence is the following?
For a cipher text string of length $L$, where $n$ is the number of
alphabet characters, $k_r = 1/n$, and $k_p = \sum_{i=1}^n p_i^2$
$$ E(IC) = …
Matteo
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For calculating the index of coincidence for each sequence
I was learning about the finding the key length reading the following web site...
http://practicalcryptography.com/cryptanalysis/stochastic-searching/cryptanalysis-vigenere-cipher/
and I really don't understand some part of the example shown in the…
user3708299
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How to Calculate Digraphic/Trigraphic Index of Coincidence?
How do you calculate the digraphic and trigraphic IOCs and the same expected IOCs? I'm aware of the formula for calculating the single letter IOC which is given at this formula but I can't find the formula for the higher orders.
Is it simply the sum…
firefly
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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 comprehensive list of these values for every…
Neo1009
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Calculating Maximum Index of Coincidence
I'm trying to understand how the maximum Index of Coincidence (IC) is found. This is the only description I have for it:
"For a given period $P$ you can divide a cipher into $P$ groups, calculate the IC for each group, then take the average of these…
mr c r
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