Overview

Cryptanalysis is the study of methods for obtaining the meaning of secret information without knowing the secret information.

Also known as hacking or cracking it utilizes the weaknesses

Attack Methods

Brute Force

Brute forcing involves trying all possible inputs to a cipher and manually inspecting for any possible combination that may be meaningful.

Example

We can brute force the #Vigenère Cipher by generating all the possible keys up to the max key length and decrypting the ciphertext to find a possible plaintext. The decrypted ciphertext may be the plaintext if a word is found that exists in a dictionary.

Frequency Analysis

Frequency analysis correlates the language-specific statistical patterns/distribution from the plaintext to the ciphertext.

Example

When trying to crack a #Substitution Cipher if the language of the plaintext is English, we can use the distribution of English letters to find which letters may have been substituted with each other.

Index of Coincidence

The index of coincidence, or IC/IOC, is the measure of similarity between a frequency distribution and the uniform distribution.

It calculates the probability of choosing two letters at random from some random text where they are the same.

Example

The IC of English is

ICEnglish=∑i=126pi2=0.0686

This can help indicate what language our alphabet belongs to.

If the IC of some ciphertext is close to ICEnglish we can assume the plaintext is English, and that it is encrypted with some monoalphabetic substitution cipher.

For polyalphabetic substitution ciphers, we can create the letter strings that are encrypted by the same key by testing the IC of the string.

Also known as KES,

Definitions

Reference

  1. Li, Fengjun Various, Lectures University of Kansas 2024

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