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155 lines
3.9 KiB
ReStructuredText
155 lines
3.9 KiB
ReStructuredText
Monoalphabetic Cipher and Inverse Written in Python
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####################################################
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:date: 2012-02-13 22:39
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:author: Russell Ballestrini
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:tags: Code, Security, Python
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:slug: monoalphabetic-cipher-and-inverse-written-in-python
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:status: published
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introduction and background
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===============================
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A monoalphabetic cipher uses fixed substitution over the entire message.
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You can build a monoalphabetic cipher using a Python dictionary, like so:
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.. code-block:: python
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monoalpha_cipher = {
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'a': 'm',
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'b': 'n',
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'c': 'b',
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'd': 'v',
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'e': 'c',
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'f': 'x',
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'g': 'z',
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'h': 'a',
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'i': 's',
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'j': 'd',
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'k': 'f',
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'l': 'g',
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'm': 'h',
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'n': 'j',
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'o': 'k',
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'p': 'l',
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'q': 'p',
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'r': 'o',
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's': 'i',
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't': 'u',
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'u': 'y',
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'v': 't',
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'w': 'r',
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'x': 'e',
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'y': 'w',
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'z': 'q',
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' ': ' ',
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}
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We can create an inverse of this cipher dictionary by switching the key and value places:
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.. code-block:: python
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inverse_monoalpha_cipher = {}
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for key, value in monoalpha_cipher.iteritems():
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inverse_monoalpha_cipher[value] = key
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Now that we have both the cipher and the inverse_cipher, we may encrypt a message.
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**Encryption example:**
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.. code-block:: python
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message = "This is an easy problem"
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encrypted_message = []
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for letter in message:
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encrypted_message.append(monoalpha_cipher.get(letter, letter))
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print(''.join(encrypted_message))
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Result:
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``Tasi si mj cmiw lokngch``
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Using the inverse_cipher, We may *decrypt* a message.
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**Decryption example:**
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.. code-block:: python
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encrypted_message = "rmij'u uamu xyj?"
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decrypted_message = []
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for letter in encrypted_message:
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decrypted_message.append( inverse_monoalpha_cipher.get(letter, letter))
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print(''.join( decrypted_message ))
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Result:
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``wasn't that fun?``
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monoalphabetic_cipher.py
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=============================
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Here is a toy library I wrote to make the process repeatable -
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``monoalphabetic_cipher.py``:
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.. code-block:: python
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from string import letters, digits
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from random import shuffle
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def random_monoalpha_cipher(pool=None):
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"""Generate a Monoalphabetic Cipher"""
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if pool is None:
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pool = letters + digits
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original_pool = list(pool)
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shuffled_pool = list(pool)
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shuffle(shuffled_pool)
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return dict(zip(original_pool, shuffled_pool))
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def inverse_monoalpha_cipher(monoalpha_cipher):
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"""Given a Monoalphabetic Cipher (dictionary) return the inverse."""
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inverse_monoalpha = {}
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for key, value in monoalpha_cipher.iteritems():
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inverse_monoalpha[value] = key
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return inverse_monoalpha
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def encrypt_with_monoalpha(message, monoalpha_cipher):
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encrypted_message = []
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for letter in message:
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encrypted_message.append(monoalpha_cipher.get(letter, letter))
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return ''.join(encrypted_message)
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def decrypt_with_monoalpha(encrypted_message, monoalpha_cipher):
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return encrypt_with_monoalpha(
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encrypted_message,
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inverse_monoalpha_cipher(monoalpha_cipher)
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)
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monoalphabetic_cipher.py example usage
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==========================================
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Here I show how to use the library:
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.. code-block:: python
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>>> # load the module / library as 'mc'.
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>>> import monoalphabetic_cipher as mc
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>>> # generate a random cipher (only if needed).
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>>> cipher = mc.random_monoalpha_cipher()
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>>> # output the cipher (store for safe keeping).
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>>> print(cipher)
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>>> # encrypt a message with the cipher.
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>>> mc.encrypt_with_monoalpha('Hello all you hackers out there!', cipher)
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'sXGGt SGG Nt0 HSrLXFC t0U UHXFX!'
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>>> # decrypt a message with the cipher.
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>>> mc.decrypt_with_monoalpha('sXGGt SGG Nt0 HSrLXFC t0U UHXFX!', cipher)
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'Hello all you hackers out there!'
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.. contents::
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