Tool to compute coincidence index. Index of Coincidence is a cryptanalysis technique studying the probability of finding repeating letters in an encrypted text. A text in English language has a, index of coincidence of 0.0667.

Index of Coincidence - dCode

Tag(s) : Cryptanalysis

dCode is free and its tools are a valuable help in games, maths, geocaching, puzzles and problems to solve every day!

A suggestion ? a feedback ? a bug ? an idea ? *Write to dCode*!

Sponsored ads

Tool to compute coincidence index. Index of Coincidence is a cryptanalysis technique studying the probability of finding repeating letters in an encrypted text. A text in English language has a, index of coincidence of 0.0667.

The **index of coincidence** is an indicator used in cryptanalysis which makes it possible to evaluate the global distribution of letters in encrypted message for a given alphabet.

**Index of coincidence** uses the formula:

$$ IC = \sum_{i=A}^{i=Z} \frac{n_{i}(n_{i}-1)}{N(N-1)} $$

with $ n_i $ the number of occurrences of the letter $ i $ in the text and $ N $ the total number of letters.

For a given ciphered message, the value for the **index of coincidence** allows to filter the list of ciphering methods to use. It is one of the first cryptanalysis technique.

If the **Index of coincidence** is high (close to $ 0.070 $), i.e. similar to plain text, then the message has probably been crypted using a transposition cipher (letters were shuffled) or a monoalphabetic substitution (a letter can be replaced by only one other).

If the **Index of coincidence** is low (close to $ 0.0385 $), i.e. similar to a random text, then the message has probably been crypted using a polyalphabetic cipher (a letter can be replaced by multiple other ones).

The more the coincidence count is low, the more alphabets have been used.

Example: Vigenere cipher with a key of length 4 to 8 letters have an IC of about $ 0.045 \pm 0.05 $

For an non encrypted text, coincidence indexes are

English | 0.0667 | French | 0.0778 |
---|---|---|---|

German | 0.0762 | Spanish | 0.0770 |

Italian | 0.0738 | Russian | 0.0529 |

A text where each letter has the same probability of appearance than another, IC is then of $ 1/N $ (where $ N $ is the number of letters in the alphabet)

Example: $ IC = 0.0385 $ for $ N=26 $

Examples of codes in programming languages `// Python`

def ic(self):

num = 0.0

den = 0.0

for val in self.count.values():

i = val

num += i * (i - 1)

den += i

if (den == 0.0):

return 0.0

else:

return num / ( den * (den - 1))

dCode retains ownership of the source code of the script Index of Coincidence online. Except explicit open source licence (indicated Creative Commons / free), any algorithm, applet, snippet, software (converter, solver, encryption / decryption, encoding / decoding, ciphering / deciphering, translator), or any function (convert, solve, decrypt, encrypt, decipher, cipher, decode, code, translate) written in any informatic langauge (PHP, Java, C#, Python, Javascript, Matlab, etc.) which dCode owns rights will not be released for free. To download the online Index of Coincidence script for offline use on PC, iPhone or Android, ask for price quote on contact page !

index,coincidence,counting,ic,cryptanalysis,friedman,kullback,polyalphabetic,monoalphabetic,key,repetition,letter,probability,ioc

Source : https://www.dcode.fr/index-coincidence

© 2019 dCode — The ultimate 'toolkit' to solve every games / riddles / geocaches. dCode

Feedback

▲