Playfair Cipher Attack Playfair Cipher Attack
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This is the fifth and final (for now) mission inspired by classical cryptography. In this mission we'll try to break the Playfair cipher using known plaintext attack. To learn more about this cipher, check out Playfair Cipher mission.

Playfair Cipher is a manual substitution cipher invented in 1854. Unlike other substitution ciphers that existed at the time, Playfair doesn't encrypt single letters - instead it works with pairs of letters, or bigrams . Let's remind ourselves of the encryption algorithm:

Playfair cipher used a 5x5 key table filled with 25 letters of English alphabet in random order (letter J is omitted). Message is broken up into pairs of letters (each pair must consist of two different letters, otherwise letter X is inserted between them). Then each bigram is encrypted according to following rules:

1. If both letters are in the same row of key table, each is replaced by the letter to it's immediate right (the last column wraps around to the first);

2. If the letters appear in the same column - each is replaced with the one below it (the bottom row wraps around to the top);

3. If the letters are at diagonally opposite corners of a rectangle, they are replaced with letters at other two corners. The order... as the first letter of the plaintext pair

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CheckiO Extensions

CheckiO Extensions allow you to use local files to solve missions. More info in a blog post.

In order to install CheckiO client you'll need installed Python (version at least 3.8)

Install CheckiO Client first:

pip3 install checkio_client

Configure your tool

checkio --domain=py config --key=

Sync solutions into your local folder

checkio sync

(in beta testing) Launch local server so your browser can use it and sync solution between local file end extension on the fly. (doesn't work for safari)

checkio serv -d

Alternatevly, you can install Chrome extension or FF addon

checkio install-plugin
checkio install-plugin --ff
checkio install-plugin --chromium

Read more here about other functionality that the checkio client provides. Feel free to submit an issue in case of any difficulties.

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