This Python script implements the Railfence cipher, which is a form of transposition cipher that encrypts a plaintext message by writing it diagonally across a set of "rails" and then reading the ciphertext off in rows.

The script provides two main functions:

1. encrypt_railfence(plaintext, num_rails): Converts the plaintext string into a set of "rails". The encrypted message, along with the number of rails, is saved in encrypted_message.json.

2. decrypt_railfence(ciphertext, num_rails): Decrypts the encrypted message stored in encrypted_message.json using the provided number of rails. The decrypted text is saved in decrypted_message.txt.

Below is a sample encryption of the plaintext:

WEAREDISCOVEREDFLEEATONCE

To create the cipher, write the message diagonally on a "railfence" with 3 rails...

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 . .

The ciphertext is obtained by reading the rails from left to right, top to bottom:

WECRLTEERDSOEEFEAOCAIVDEN

Usage: railfence.py [OPTIONS] [PLAINTEXT]

  This utility either encrypts or decrypts a message using railfence
  encryption.

Options:
  -r, --rails INTEGER  Number of rails.  [default: 3]
  -e, --encrypt        Encrypt the plaintext.
  -d, --decrypt        Decrypt the encrypted text.
  --version            Show the version and exit.
  --help               Show this message and exit.

To encrypt a plaintext message:
$ python railfence.py -r 3 -e "WEAREDISCOVEREDFLEEATONCE"

This will create a file encrypted_message.json containing the encrypted message and the number of rails used. Note that this example is equivalent to:

$ python railfence.py "WEAREDISCOVEREDFLEEATONCE"

since a message by itself (no options) will automatically encrypt over 3 rails.

To decrypt an encrypted message:
$ python railfence.py -d or python railfence.py

Either one will read encrypted_message.json and decrypt the message using the stored number of rails, creating a decrypted_message.json file with the original plaintext.

If only a message is provided on the command line, it will be encrypted. If there is no message and no options, encrypted_message.txt will be decrypted.

Programmatically, the three "rails" are three lists within a single list. In this case the list looks like this:

[
    ['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']
]

The list if flattened and then, using join(), a single string is generated:

WECRLTEERDSOEEFEAOCAIVDEN

1. This program requires knowing the number of rails used for encryption in order to decrypt the message correctly. If the number of rails is unknown, decryption is not possible except by brute force (not terribly hard!).
   
   
2. The number of rails used for encryption is stored with the encrypted message, violating every known law of cybersecurity. Considering that decryption requires knowing the number of rails, that number must be preserved. For the time being it is preserved with the encrypted message. Changing that strategy to improve security would be trivial while saving the number of rails depends on human memory which is known to be unreliable.

• click (for command-line interface)