Project Euler/59

Problem Statement

This question is about a method of encrypting ASCII text.

Each character in ASCII has some representation (e.g., A=65, *=42, k=107)

Encryption technique is to take text file, convert bytes to ASCII, and XOR each byte with a character from a secret key

Password method: use a password in place of a secret one-time pad; shorter than message, so repeated cyclically throughout method.

You are given a file with encrypted ascii codes, knowledge that plain text must contain common English words, and told encryption key consists of 3 lowercase characters. Decrypt message, find sum of ascii values in original text.

Sample file looks like this: 36,22,80,0,0,4,23,...

Link: https://projecteuler.net/problem=59

Approach

Create list of most common bigrams in English language

Sliding window of 2 letters at a time

For each pair of letters in the ciphertext, for each most common bigram, find the corresponding secret key at that position that would cause this bigram to become the most common bigram

Can weight each secret key by how common the bigram is

Part 2:

Create list of most common trigrams in English language

Sliding window of 3 letters at a time

For each triplet of letters, for each most common trigram, find the corresponding secret key at that position that would cause this trigram to become the most common trigram

Part 3;

Create list of most common letters in English language, plus weights

For each "letter" in the ciphertext, for each most common letter, find the corresponding secret key at that position that would cause the ciphertext to become the most common letter