Rail Fence Cipher

Encryption Algorithm

The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded. It was already used by the ancient Greeks.

In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag). Finally the message is then read off in rows.

For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Then reads off:

WECRLTEERDSOEEFEAOCAIVDEN

To decrypt a message we take the zig-zag shape and fill the ciphertext along the rows.

? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

The first row has seven spots that can be filled with "WECRLTE".

W . . . E . . . C . . . R . . . L . . . T . . . E
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Now the 2nd row takes "ERDSOEEFEAOC".

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Leaving "AIVDEN" for the last row.

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Taken from https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher

How to run

dependencies:

• PHP 7.2+
• composer

Both PhpUnit and PhpSpec are included in composer.json:

$ composer install

Usage

There are two scripts (encode.php and decode.php) to be executed in the root directory:

  php encode.php textToBeEncoded numberOfRails
  php decode.php textToBeDecoded numberOfRails

with two input parameters for each script:

  php encode.php string int

The output will be displayed in the command line after the script was executed.

Example

   php encode.php Hello 2
    The encoded string is: Hloel
    
   php decode.php Hloel 2
    The decoded string is: Hello 

How to test

Dependencies:

• phpspec 6.3
• phpunit 9.4

Run test suites in in root directory

$ vendor/bin/phpspec run
$ vendor/bin/phpunit tests --testdox --colors

Run phpunit code coverage

$ vendor/bin/phpunit tests --coverage-text

Notes on design

I implemented the functionality of this task into several classes in order to practice unit testing with phpspec. I don't consider an object oriented abstraction necessarily useful in this case for the following reasons:

There is no state or property that the methods need to manipulate or maintain. Consequently, all methods could be declared static. Besides, this implementation does not make use of either inheritance or polymorphism.