Rotating Grille Cipher
Simple
English RU

This is the third mission on classical cryptography. In this mission we will encrypt a message using a rotating grille.

Rotating grille cipher is a transposition cipher that uses a special square sheet of cardboard (the grille ) with holes cut in it. To encrypt a message, sender must place the grille on an empty sheet of paper and write letters of the plaintext into the holes left to right, top to bottom. After filling all the holes, the grille is rotated 90 degrees clockwise (the holes move to empty spaces) and the process continues. After the grille was rotated 2 more times, there should be no empty spaces on the paper; if the message is not finished, the algorithm is repeated on the next sheet.

For example, here's what encrypting the line Meet me at twelve PM would look like (we'll use a grille of size 4x4):

After the encryption, the ciphertext reads as tmmveeewepeatlmt .

In this task you are given a message and a key (4x4 square grille); length of the message is divisible by 16. You need to encrypt a message with the rotating grille cipher. There is a problem, though: some of the keys are defective. A "correct" key is made in such a way that, when laid over a grid and rotated around, every cell is shown exactly once. So with defective keys, either some cells are shown more than once, or after applying the key in all four directions there are still unfilled spaces on the sheet. See examples of bad keys below:

If the key is correct, return encrypted message; if the key is defective - return None.

Input: plaintext: str, grille: list of str (positions of holes are marked by "Х" )

Output: ciphertext: str or None

Example:

```grille_encrypt('cardangrilletest', ['.X..', '.X..', '...X', 'X...']) == 'actilangeslrdret'
grille_encrypt('quickbrownfoxjumpsoverthelazydog',
['X...', '...X', '..X.', '.X..']) == 'qxwkbnjufriumcoopyeerldsatoogvhz'
grille_encrypt('quickbrownfoxjumpsoverthelazydog',
['.XX.', '.XX.', '..X.', 'X...']) == None
grille_encrypt('cardangrilletest', ['...X', '....', '....', '....']) == None
```

Preconditions: len(key) == 4
all(len(l) == 4 for l in key)
len(plaintext) % 16 == 0
all(all(cell == "X" or cell == "." for cell in line) for line in key)

PS: If you are interested in decryption of a Rotating Grille Cipher, check out mission Cipher Map .

## Become Awesome

• No Limits
• More Content
20
Settings
Code:
Other:
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`

`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.