Module szyfrow.railfence

Railfence transposition cipher.

Works by splitting the text into sections, then reading across them to generate the rows in the cipher. The rows are then combined to form the ciphertext.

Example: the plaintext "hellotherefriends", with a height of four, written out in the railfence as h h i etere* lorfns l e d (with the * showing the one character to finish the last section). Each 'section' is two columns, but unfolded. In the example, the first section is 'hellot'.

"""[Railfence transposition cipher](https://en.wikipedia.org/wiki/Rail_fence_cipher).

Works by splitting the text into sections, then reading across them to
generate the rows in the cipher. The rows are then combined to form the
ciphertext.

Example: the plaintext "hellotherefriends", with a height of four, written 
out in the railfence as 
   h h i
   etere*
   lorfns
   l e d
(with the * showing the one character to finish the last section). 
Each 'section' is two columns, but unfolded. In the example, the first
section is 'hellot'.
"""
import math
from enum import Enum
from itertools import starmap, zip_longest
from szyfrow.support.utilities import *
from szyfrow.support.language_models import *


def railfence_encipher(message, height, fillvalue=''):
    """Railfence cipher.
    Works by splitting the text into sections, then reading across them to
    generate the rows in the cipher. The rows are then combined to form the
    ciphertext.

    Example: the plaintext "hellotherefriends", with a height of four, written 
    out in the railfence as 
       h h i
       etere*
       lorfns
       l e d
    (with the * showing the one character to finish the last section). 
    Each 'section' is two columns, but unfolded. In the example, the first
    section is 'hellot'.

    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 2, fillvalue='!')
    'hlohraateerishsslnpeefetotsigaleccpeselteevsmhatetiiaogicotxfretnrifneihr!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 3, fillvalue='!')
    'horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihr!!lhateihsnefttiaece!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 5, fillvalue='!')
    'hresleogcseeemhetaocofrnrner!!lhateihsnefttiaece!!ltvsatiigitxetifih!!oarspeslp!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 10, fillvalue='!')
    'hepisehagitnr!!lernesge!!lmtocerh!!otiletap!!tseaorii!!hassfolc!!evtitffe!!rahsetec!!eixn!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 3)
    'horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihrlhateihsnefttiaece'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 5)
    'hresleogcseeemhetaocofrnrnerlhateihsnefttiaeceltvsatiigitxetifihoarspeslp'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 7)
    'haspolsevsetgifrifrlatihnettaeelemtiocxernhorersleesgcptehaiaottneihesfic'
    """
    sections = chunks(message, (height - 1) * 2, fillvalue=fillvalue)
    n_sections = len(sections)
    # Add the top row
    rows = [cat([s[0] for s in sections])]
    # process the middle rows of the grid
    for r in range(1, height-1):
        rows += [cat([s[r:r+1] + s[height*2-r-2:height*2-r-1] for s in sections])]
    # process the bottom row
    rows += [cat([s[height - 1:height] for s in sections])]
    # rows += [wcat([s[height - 1] for s in sections])]
    return cat(rows)

def railfence_decipher(message, height, fillvalue=''):
    """Railfence decipher. 
    Works by reconstructing the grid used to generate the ciphertext, then
    unfolding the sections so the text can be concatenated together.

    Example: given the ciphertext 'hhieterelorfnsled' and a height of 4, first
    work out that the second row has a character missing, find the rows of the
    grid, then split the section into its two columns.

    'hhieterelorfnsled' is split into
        h h i
        etere
        lorfns
        l e d
    (spaces added for clarity), which is stored in 'rows'. This is then split
    into 'down_rows' and 'up_rows':

    down_rows:
       hhi
       eee
       lrn
       led

    up_rows:
       tr
       ofs

    These are then zipped together (after the up_rows are reversed) to recover 
    the plaintext.

    Most of the procedure is about finding the correct lengths for each row then
    splitting the ciphertext into those rows.

    >>> railfence_decipher('hlohraateerishsslnpeefetotsigaleccpeselteevsmhatetiiaogicotxfretnrifneihr!', 2).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihr!!lhateihsnefttiaece!', 3).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('hresleogcseeemhetaocofrnrner!!lhateihsnefttiaece!!ltvsatiigitxetifih!!oarspeslp!', 5).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('hepisehagitnr!!lernesge!!lmtocerh!!otiletap!!tseaorii!!hassfolc!!evtitffe!!rahsetec!!eixn!', 10).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihrlhateihsnefttiaece', 3)
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('hresleogcseeemhetaocofrnrnerlhateihsnefttiaeceltvsatiigitxetifihoarspeslp', 5)
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('haspolsevsetgifrifrlatihnettaeelemtiocxernhorersleesgcptehaiaottneihesfic', 7)
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    """
    # find the number and size of the sections, including how many characters
    #   are missing for a full grid
    n_sections = math.ceil(len(message) / ((height - 1) * 2))
    padding_to_add = n_sections * (height - 1) * 2 - len(message)
    # row_lengths are for the both up rows and down rows
    row_lengths = [n_sections] * (height - 1) * 2
    for i in range((height - 1) * 2 - 1, (height - 1) * 2 - (padding_to_add + 1), -1):
        row_lengths[i] -= 1
    # folded_rows are the combined row lengths in the middle of the railfence
    folded_row_lengths = [row_lengths[0]]
    for i in range(1, height-1):
        folded_row_lengths += [row_lengths[i] + row_lengths[-i]]
    folded_row_lengths += [row_lengths[height - 1]]
    # find the rows that form the railfence grid
    rows = []
    row_start = 0
    for i in folded_row_lengths:
        rows += [message[row_start:row_start + i]]
        row_start += i
    # split the rows into the 'down_rows' (those that form the first column of
    #   a section) and the 'up_rows' (those that ofrm the second column of a 
    #   section).
    down_rows = [rows[0]]
    up_rows = []
    for i in range(1, height-1):
        down_rows += [cat([c for n, c in enumerate(rows[i]) if n % 2 == 0])]
        up_rows += [cat([c for n, c in enumerate(rows[i]) if n % 2 == 1])]
    down_rows += [rows[-1]]
    up_rows.reverse()
    return cat(c for r in zip_longest(*(down_rows + up_rows), fillvalue='') for c in r)


def railfence_break(message, max_key_length=20,
                     fitness=Pbigrams, chunksize=500):
    """Breaks a railfence cipher using a range of lengths and
    n-gram frequency analysis

    >>> railfence_break(railfence_encipher(sanitise( \
            "It is a truth universally acknowledged, that a single man in \
             possession of a good fortune, must be in want of a wife. However \
             little known the feelings or views of such a man may be on his \
             first entering a neighbourhood, this truth is so well fixed in \
             the minds of the surrounding families, that he is considered the \
             rightful property of some one or other of their daughters."), \
        7)) # doctest: +ELLIPSIS
    (7, -709.46467226...)
    >>> railfence_break(railfence_encipher(sanitise( \
            "It is a truth universally acknowledged, that a single man in \
             possession of a good fortune, must be in want of a wife. However \
             little known the feelings or views of such a man may be on his \
             first entering a neighbourhood, this truth is so well fixed in \
             the minds of the surrounding families, that he is considered the \
             rightful property of some one or other of their daughters."), \
        7), \
        fitness=Ptrigrams) # doctest: +ELLIPSIS
    (7, -997.0129085...)
    """
    def worker(message, height, fitness):
        plaintext = railfence_decipher(message, height)
        fit = fitness(plaintext)
        return height, fit

    sanitised_message = sanitise(message)
    results = starmap(worker, [(sanitised_message, i, fitness)
                               for i in range(2, max_key_length+1)])
    return max(results, key=lambda k: k[1])

Functions

def cat(iterable, /)

Concatenate any number of strings.

The string whose method is called is inserted in between each given string. The result is returned as a new string.

Example: '.'.join(['ab', 'pq', 'rs']) -> 'ab.pq.rs'

def lcat(iterable, /)

Concatenate any number of strings.

The string whose method is called is inserted in between each given string. The result is returned as a new string.

Example: '.'.join(['ab', 'pq', 'rs']) -> 'ab.pq.rs'

def railfence_break(message, max_key_length=20, fitness=<function Pbigrams>, chunksize=500)

Breaks a railfence cipher using a range of lengths and n-gram frequency analysis

>>> railfence_break(railfence_encipher(sanitise(             "It is a truth universally acknowledged, that a single man in              possession of a good fortune, must be in want of a wife. However              little known the feelings or views of such a man may be on his              first entering a neighbourhood, this truth is so well fixed in              the minds of the surrounding families, that he is considered the              rightful property of some one or other of their daughters."),         7)) # doctest: +ELLIPSIS
(7, -709.46467226...)
>>> railfence_break(railfence_encipher(sanitise(             "It is a truth universally acknowledged, that a single man in              possession of a good fortune, must be in want of a wife. However              little known the feelings or views of such a man may be on his              first entering a neighbourhood, this truth is so well fixed in              the minds of the surrounding families, that he is considered the              rightful property of some one or other of their daughters."),         7),         fitness=Ptrigrams) # doctest: +ELLIPSIS
(7, -997.0129085...)

Expand source code

def railfence_break(message, max_key_length=20,
                     fitness=Pbigrams, chunksize=500):
    """Breaks a railfence cipher using a range of lengths and
    n-gram frequency analysis

    >>> railfence_break(railfence_encipher(sanitise( \
            "It is a truth universally acknowledged, that a single man in \
             possession of a good fortune, must be in want of a wife. However \
             little known the feelings or views of such a man may be on his \
             first entering a neighbourhood, this truth is so well fixed in \
             the minds of the surrounding families, that he is considered the \
             rightful property of some one or other of their daughters."), \
        7)) # doctest: +ELLIPSIS
    (7, -709.46467226...)
    >>> railfence_break(railfence_encipher(sanitise( \
            "It is a truth universally acknowledged, that a single man in \
             possession of a good fortune, must be in want of a wife. However \
             little known the feelings or views of such a man may be on his \
             first entering a neighbourhood, this truth is so well fixed in \
             the minds of the surrounding families, that he is considered the \
             rightful property of some one or other of their daughters."), \
        7), \
        fitness=Ptrigrams) # doctest: +ELLIPSIS
    (7, -997.0129085...)
    """
    def worker(message, height, fitness):
        plaintext = railfence_decipher(message, height)
        fit = fitness(plaintext)
        return height, fit

    sanitised_message = sanitise(message)
    results = starmap(worker, [(sanitised_message, i, fitness)
                               for i in range(2, max_key_length+1)])
    return max(results, key=lambda k: k[1])

def railfence_decipher(message, height, fillvalue='')

Railfence decipher. Works by reconstructing the grid used to generate the ciphertext, then unfolding the sections so the text can be concatenated together.

Example: given the ciphertext 'hhieterelorfnsled' and a height of 4, first work out that the second row has a character missing, find the rows of the grid, then split the section into its two columns.

'hhieterelorfnsled' is split into h h i etere lorfns l e d (spaces added for clarity), which is stored in 'rows'. This is then split into 'down_rows' and 'up_rows':

down_rows: hhi eee lrn led

up_rows: tr ofs

These are then zipped together (after the up_rows are reversed) to recover the plaintext.

Most of the procedure is about finding the correct lengths for each row then splitting the ciphertext into those rows.

>>> railfence_decipher('hlohraateerishsslnpeefetotsigaleccpeselteevsmhatetiiaogicotxfretnrifneihr!', 2).strip('!')
'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
>>> railfence_decipher('horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihr!!lhateihsnefttiaece!', 3).strip('!')
'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
>>> railfence_decipher('hresleogcseeemhetaocofrnrner!!lhateihsnefttiaece!!ltvsatiigitxetifih!!oarspeslp!', 5).strip('!')
'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
>>> railfence_decipher('hepisehagitnr!!lernesge!!lmtocerh!!otiletap!!tseaorii!!hassfolc!!evtitffe!!rahsetec!!eixn!', 10).strip('!')
'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
>>> railfence_decipher('horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihrlhateihsnefttiaece', 3)
'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
>>> railfence_decipher('hresleogcseeemhetaocofrnrnerlhateihsnefttiaeceltvsatiigitxetifihoarspeslp', 5)
'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
>>> railfence_decipher('haspolsevsetgifrifrlatihnettaeelemtiocxernhorersleesgcptehaiaottneihesfic', 7)
'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'

Expand source code

def railfence_decipher(message, height, fillvalue=''):
    """Railfence decipher. 
    Works by reconstructing the grid used to generate the ciphertext, then
    unfolding the sections so the text can be concatenated together.

    Example: given the ciphertext 'hhieterelorfnsled' and a height of 4, first
    work out that the second row has a character missing, find the rows of the
    grid, then split the section into its two columns.

    'hhieterelorfnsled' is split into
        h h i
        etere
        lorfns
        l e d
    (spaces added for clarity), which is stored in 'rows'. This is then split
    into 'down_rows' and 'up_rows':

    down_rows:
       hhi
       eee
       lrn
       led

    up_rows:
       tr
       ofs

    These are then zipped together (after the up_rows are reversed) to recover 
    the plaintext.

    Most of the procedure is about finding the correct lengths for each row then
    splitting the ciphertext into those rows.

    >>> railfence_decipher('hlohraateerishsslnpeefetotsigaleccpeselteevsmhatetiiaogicotxfretnrifneihr!', 2).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihr!!lhateihsnefttiaece!', 3).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('hresleogcseeemhetaocofrnrner!!lhateihsnefttiaece!!ltvsatiigitxetifih!!oarspeslp!', 5).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('hepisehagitnr!!lernesge!!lmtocerh!!otiletap!!tseaorii!!hassfolc!!evtitffe!!rahsetec!!eixn!', 10).strip('!')
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihrlhateihsnefttiaece', 3)
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('hresleogcseeemhetaocofrnrnerlhateihsnefttiaeceltvsatiigitxetifihoarspeslp', 5)
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    >>> railfence_decipher('haspolsevsetgifrifrlatihnettaeelemtiocxernhorersleesgcptehaiaottneihesfic', 7)
    'hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers'
    """
    # find the number and size of the sections, including how many characters
    #   are missing for a full grid
    n_sections = math.ceil(len(message) / ((height - 1) * 2))
    padding_to_add = n_sections * (height - 1) * 2 - len(message)
    # row_lengths are for the both up rows and down rows
    row_lengths = [n_sections] * (height - 1) * 2
    for i in range((height - 1) * 2 - 1, (height - 1) * 2 - (padding_to_add + 1), -1):
        row_lengths[i] -= 1
    # folded_rows are the combined row lengths in the middle of the railfence
    folded_row_lengths = [row_lengths[0]]
    for i in range(1, height-1):
        folded_row_lengths += [row_lengths[i] + row_lengths[-i]]
    folded_row_lengths += [row_lengths[height - 1]]
    # find the rows that form the railfence grid
    rows = []
    row_start = 0
    for i in folded_row_lengths:
        rows += [message[row_start:row_start + i]]
        row_start += i
    # split the rows into the 'down_rows' (those that form the first column of
    #   a section) and the 'up_rows' (those that ofrm the second column of a 
    #   section).
    down_rows = [rows[0]]
    up_rows = []
    for i in range(1, height-1):
        down_rows += [cat([c for n, c in enumerate(rows[i]) if n % 2 == 0])]
        up_rows += [cat([c for n, c in enumerate(rows[i]) if n % 2 == 1])]
    down_rows += [rows[-1]]
    up_rows.reverse()
    return cat(c for r in zip_longest(*(down_rows + up_rows), fillvalue='') for c in r)

def railfence_encipher(message, height, fillvalue='')

Railfence cipher. Works by splitting the text into sections, then reading across them to generate the rows in the cipher. The rows are then combined to form the ciphertext.

Example: the plaintext "hellotherefriends", with a height of four, written out in the railfence as h h i etere* lorfns l e d (with the * showing the one character to finish the last section). Each 'section' is two columns, but unfolded. In the example, the first section is 'hellot'.

>>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 2, fillvalue='!')
'hlohraateerishsslnpeefetotsigaleccpeselteevsmhatetiiaogicotxfretnrifneihr!'
>>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 3, fillvalue='!')
'horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihr!!lhateihsnefttiaece!'
>>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 5, fillvalue='!')
'hresleogcseeemhetaocofrnrner!!lhateihsnefttiaece!!ltvsatiigitxetifih!!oarspeslp!'
>>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 10, fillvalue='!')
'hepisehagitnr!!lernesge!!lmtocerh!!otiletap!!tseaorii!!hassfolc!!evtitffe!!rahsetec!!eixn!'
>>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 3)
'horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihrlhateihsnefttiaece'
>>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 5)
'hresleogcseeemhetaocofrnrnerlhateihsnefttiaeceltvsatiigitxetifihoarspeslp'
>>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 7)
'haspolsevsetgifrifrlatihnettaeelemtiocxernhorersleesgcptehaiaottneihesfic'

Expand source code

def railfence_encipher(message, height, fillvalue=''):
    """Railfence cipher.
    Works by splitting the text into sections, then reading across them to
    generate the rows in the cipher. The rows are then combined to form the
    ciphertext.

    Example: the plaintext "hellotherefriends", with a height of four, written 
    out in the railfence as 
       h h i
       etere*
       lorfns
       l e d
    (with the * showing the one character to finish the last section). 
    Each 'section' is two columns, but unfolded. In the example, the first
    section is 'hellot'.

    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 2, fillvalue='!')
    'hlohraateerishsslnpeefetotsigaleccpeselteevsmhatetiiaogicotxfretnrifneihr!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 3, fillvalue='!')
    'horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihr!!lhateihsnefttiaece!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 5, fillvalue='!')
    'hresleogcseeemhetaocofrnrner!!lhateihsnefttiaece!!ltvsatiigitxetifih!!oarspeslp!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 10, fillvalue='!')
    'hepisehagitnr!!lernesge!!lmtocerh!!otiletap!!tseaorii!!hassfolc!!evtitffe!!rahsetec!!eixn!'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 3)
    'horaersslpeeosglcpselteevsmhatetiiaogicotxfretnrifneihrlhateihsnefttiaece'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 5)
    'hresleogcseeemhetaocofrnrnerlhateihsnefttiaeceltvsatiigitxetifihoarspeslp'
    >>> railfence_encipher('hellothereavastmeheartiesthisisalongpieceoftextfortestingrailfenceciphers', 7)
    'haspolsevsetgifrifrlatihnettaeelemtiocxernhorersleesgcptehaiaottneihesfic'
    """
    sections = chunks(message, (height - 1) * 2, fillvalue=fillvalue)
    n_sections = len(sections)
    # Add the top row
    rows = [cat([s[0] for s in sections])]
    # process the middle rows of the grid
    for r in range(1, height-1):
        rows += [cat([s[r:r+1] + s[height*2-r-2:height*2-r-1] for s in sections])]
    # process the bottom row
    rows += [cat([s[height - 1:height] for s in sections])]
    # rows += [wcat([s[height - 1] for s in sections])]
    return cat(rows)

def wcat(iterable, /)

Concatenate any number of strings.

The string whose method is called is inserted in between each given string. The result is returned as a new string.

Example: '.'.join(['ab', 'pq', 'rs']) -> 'ab.pq.rs'