Module szyfrow.caesar

Enciphering and deciphering using the Caesar cipher. Also attempts to break messages that use a Caesar cipher.

The Caesar cipher operates one letter at a time. It converts each letter to a number, then enciphers that number by adding the key. The result is taken mod 26 and converted back into a letter.

"""Enciphering and deciphering using the [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher). 
Also attempts to break messages that use a Caesar cipher.

The Caesar cipher operates one letter at a time. It converts each letter to a 
number, then enciphers that number by adding the key. The result is taken mod 
26 and converted back into a letter.

"""

from szyfrow.support.utilities import *
from szyfrow.support.language_models import *

def caesar_encipher_letter(accented_letter, shift):
    """Encipher a letter, given a shift amount.

    Accented version of latin letters (such as é and ö) are converted to their
    non-accented versions before encryption.

    >>> caesar_encipher_letter('a', 1)
    'b'
    >>> caesar_encipher_letter('a', 2)
    'c'
    >>> caesar_encipher_letter('b', 2)
    'd'
    >>> caesar_encipher_letter('x', 2)
    'z'
    >>> caesar_encipher_letter('y', 2)
    'a'
    >>> caesar_encipher_letter('z', 2)
    'b'
    >>> caesar_encipher_letter('z', -1)
    'y'
    >>> caesar_encipher_letter('a', -1)
    'z'
    >>> caesar_encipher_letter('A', 1)
    'B'
    >>> caesar_encipher_letter('é', 1)
    'f'
    """
    # letter = unaccent(accented_letter)
    # if letter in string.ascii_letters:
    #     if letter in string.ascii_uppercase:
    #         alphabet_start = ord('A')
    #     else:
    #         alphabet_start = ord('a')
    #     return chr(((ord(letter) - alphabet_start + shift) % 26) + 
    #                alphabet_start)
    # else:
    #     return letter

    letter = unaccent(accented_letter)
    if letter in string.ascii_letters:
        cipherletter = unpos(pos(letter) + shift)
        if letter in string.ascii_uppercase:
            return cipherletter.upper()
        else:
            return cipherletter
    else:
        return letter

def caesar_decipher_letter(letter, shift):
    """Decipher a letter, given a shift amount
    
    >>> caesar_decipher_letter('b', 1)
    'a'
    >>> caesar_decipher_letter('b', 2)
    'z'
    """
    return caesar_encipher_letter(letter, -shift)

def caesar_encipher(message, shift):
    """Encipher a message with the Caesar cipher of given shift
    
    >>> caesar_encipher('abc', 1)
    'bcd'
    >>> caesar_encipher('abc', 2)
    'cde'
    >>> caesar_encipher('abcxyz', 2)
    'cdezab'
    >>> caesar_encipher('ab cx yz', 2)
    'cd ez ab'
    >>> caesar_encipher('Héllo World!', 2)
    'Jgnnq Yqtnf!'
    """
    enciphered = [caesar_encipher_letter(l, shift) for l in message]
    return cat(enciphered)

def caesar_decipher(message, shift):
    """Decipher a message with the Caesar cipher of given shift
    
    >>> caesar_decipher('bcd', 1)
    'abc'
    >>> caesar_decipher('cde', 2)
    'abc'
    >>> caesar_decipher('cd ez ab', 2)
    'ab cx yz'
    >>> caesar_decipher('Jgnnq Yqtnf!', 2)
    'Hello World!'
    """
    return caesar_encipher(message, -shift)


def caesar_break(message, fitness=Pletters):
    """Breaks a Caesar cipher using frequency analysis

    It tries all possible keys, scores the fitness of the text decipherd with 
    each key, and returns the key that produces the most fit deciphered text.

    >>> caesar_break('ibxcsyorsaqcheyklxivoexlevmrimwxsfiqevvmihrsasrxliwyrh' \
          'ecjsppsamrkwleppfmergefifvmhixscsymjcsyqeoixlm') # doctest: +ELLIPSIS
    (4, -130.849989015...)
    >>> caesar_break('wxwmaxdgheetgwuxztgptedbgznitgwwhpguxyhkxbmhvvtlbhgtee' \
          'raxlmhiixweblmxgxwmhmaxybkbgztgwztsxwbgmxgmert') # doctest: +ELLIPSIS
    (19, -128.82410410...)
    >>> caesar_break('yltbbqnqnzvguvaxurorgenafsbezqvagbnornfgsbevpnaabjurer' \
          'svaquvzyvxrnznazlybequrvfohgriraabjtbaruraprur') # doctest: +ELLIPSIS
    (13, -126.25403935...)
    """
    sanitised_message = sanitise(message)
    best_shift = 0
    best_fit = float('-inf')
    for shift in range(26):
        plaintext = caesar_decipher(sanitised_message, shift)
        fit = fitness(plaintext)

        if fit > best_fit:
            best_fit = fit
            best_shift = shift

    return best_shift, best_fit

Functions

def caesar_break(message, fitness=<function Pletters>)

Breaks a Caesar cipher using frequency analysis

It tries all possible keys, scores the fitness of the text decipherd with each key, and returns the key that produces the most fit deciphered text.

>>> caesar_break('ibxcsyorsaqcheyklxivoexlevmrimwxsfiqevvmihrsasrxliwyrh'           'ecjsppsamrkwleppfmergefifvmhixscsymjcsyqeoixlm') # doctest: +ELLIPSIS
(4, -130.849989015...)
>>> caesar_break('wxwmaxdgheetgwuxztgptedbgznitgwwhpguxyhkxbmhvvtlbhgtee'           'raxlmhiixweblmxgxwmhmaxybkbgztgwztsxwbgmxgmert') # doctest: +ELLIPSIS
(19, -128.82410410...)
>>> caesar_break('yltbbqnqnzvguvaxurorgenafsbezqvagbnornfgsbevpnaabjurer'           'svaquvzyvxrnznazlybequrvfohgriraabjtbaruraprur') # doctest: +ELLIPSIS
(13, -126.25403935...)

Expand source code

def caesar_break(message, fitness=Pletters):
    """Breaks a Caesar cipher using frequency analysis

    It tries all possible keys, scores the fitness of the text decipherd with 
    each key, and returns the key that produces the most fit deciphered text.

    >>> caesar_break('ibxcsyorsaqcheyklxivoexlevmrimwxsfiqevvmihrsasrxliwyrh' \
          'ecjsppsamrkwleppfmergefifvmhixscsymjcsyqeoixlm') # doctest: +ELLIPSIS
    (4, -130.849989015...)
    >>> caesar_break('wxwmaxdgheetgwuxztgptedbgznitgwwhpguxyhkxbmhvvtlbhgtee' \
          'raxlmhiixweblmxgxwmhmaxybkbgztgwztsxwbgmxgmert') # doctest: +ELLIPSIS
    (19, -128.82410410...)
    >>> caesar_break('yltbbqnqnzvguvaxurorgenafsbezqvagbnornfgsbevpnaabjurer' \
          'svaquvzyvxrnznazlybequrvfohgriraabjtbaruraprur') # doctest: +ELLIPSIS
    (13, -126.25403935...)
    """
    sanitised_message = sanitise(message)
    best_shift = 0
    best_fit = float('-inf')
    for shift in range(26):
        plaintext = caesar_decipher(sanitised_message, shift)
        fit = fitness(plaintext)

        if fit > best_fit:
            best_fit = fit
            best_shift = shift

    return best_shift, best_fit

def caesar_decipher(message, shift)

Decipher a message with the Caesar cipher of given shift

>>> caesar_decipher('bcd', 1)
'abc'
>>> caesar_decipher('cde', 2)
'abc'
>>> caesar_decipher('cd ez ab', 2)
'ab cx yz'
>>> caesar_decipher('Jgnnq Yqtnf!', 2)
'Hello World!'

Expand source code

def caesar_decipher(message, shift):
    """Decipher a message with the Caesar cipher of given shift
    
    >>> caesar_decipher('bcd', 1)
    'abc'
    >>> caesar_decipher('cde', 2)
    'abc'
    >>> caesar_decipher('cd ez ab', 2)
    'ab cx yz'
    >>> caesar_decipher('Jgnnq Yqtnf!', 2)
    'Hello World!'
    """
    return caesar_encipher(message, -shift)

def caesar_decipher_letter(letter, shift)

Decipher a letter, given a shift amount

>>> caesar_decipher_letter('b', 1)
'a'
>>> caesar_decipher_letter('b', 2)
'z'

Expand source code

def caesar_decipher_letter(letter, shift):
    """Decipher a letter, given a shift amount
    
    >>> caesar_decipher_letter('b', 1)
    'a'
    >>> caesar_decipher_letter('b', 2)
    'z'
    """
    return caesar_encipher_letter(letter, -shift)

def caesar_encipher(message, shift)

Encipher a message with the Caesar cipher of given shift

>>> caesar_encipher('abc', 1)
'bcd'
>>> caesar_encipher('abc', 2)
'cde'
>>> caesar_encipher('abcxyz', 2)
'cdezab'
>>> caesar_encipher('ab cx yz', 2)
'cd ez ab'
>>> caesar_encipher('Héllo World!', 2)
'Jgnnq Yqtnf!'

Expand source code

def caesar_encipher(message, shift):
    """Encipher a message with the Caesar cipher of given shift
    
    >>> caesar_encipher('abc', 1)
    'bcd'
    >>> caesar_encipher('abc', 2)
    'cde'
    >>> caesar_encipher('abcxyz', 2)
    'cdezab'
    >>> caesar_encipher('ab cx yz', 2)
    'cd ez ab'
    >>> caesar_encipher('Héllo World!', 2)
    'Jgnnq Yqtnf!'
    """
    enciphered = [caesar_encipher_letter(l, shift) for l in message]
    return cat(enciphered)

def caesar_encipher_letter(accented_letter, shift)

Encipher a letter, given a shift amount.

Accented version of latin letters (such as é and ö) are converted to their non-accented versions before encryption.

>>> caesar_encipher_letter('a', 1)
'b'
>>> caesar_encipher_letter('a', 2)
'c'
>>> caesar_encipher_letter('b', 2)
'd'
>>> caesar_encipher_letter('x', 2)
'z'
>>> caesar_encipher_letter('y', 2)
'a'
>>> caesar_encipher_letter('z', 2)
'b'
>>> caesar_encipher_letter('z', -1)
'y'
>>> caesar_encipher_letter('a', -1)
'z'
>>> caesar_encipher_letter('A', 1)
'B'
>>> caesar_encipher_letter('é', 1)
'f'

Expand source code

def caesar_encipher_letter(accented_letter, shift):
    """Encipher a letter, given a shift amount.

    Accented version of latin letters (such as é and ö) are converted to their
    non-accented versions before encryption.

    >>> caesar_encipher_letter('a', 1)
    'b'
    >>> caesar_encipher_letter('a', 2)
    'c'
    >>> caesar_encipher_letter('b', 2)
    'd'
    >>> caesar_encipher_letter('x', 2)
    'z'
    >>> caesar_encipher_letter('y', 2)
    'a'
    >>> caesar_encipher_letter('z', 2)
    'b'
    >>> caesar_encipher_letter('z', -1)
    'y'
    >>> caesar_encipher_letter('a', -1)
    'z'
    >>> caesar_encipher_letter('A', 1)
    'B'
    >>> caesar_encipher_letter('é', 1)
    'f'
    """
    # letter = unaccent(accented_letter)
    # if letter in string.ascii_letters:
    #     if letter in string.ascii_uppercase:
    #         alphabet_start = ord('A')
    #     else:
    #         alphabet_start = ord('a')
    #     return chr(((ord(letter) - alphabet_start + shift) % 26) + 
    #                alphabet_start)
    # else:
    #     return letter

    letter = unaccent(accented_letter)
    if letter in string.ascii_letters:
        cipherletter = unpos(pos(letter) + shift)
        if letter in string.ascii_uppercase:
            return cipherletter.upper()
        else:
            return cipherletter
    else:
        return letter

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 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'