MITx 6.00.1x Python Notes: Caesar Cipher

Problem

The basic idea of the Caesar cipher is that you pick an integer for a key, and shift every letter of your message by the key. For example, if your message was "happy" and your key was 3, "h" becomes "k", "a" becomes "d", and so on, because we are shifting every letter three spots to the right. Here is what the whole alphabet looks like shifted three spots to the right:

Original:  a b c d e f g h i j k l m n o p q r s t u v w x y z
 3-shift:  d e f g h i j k l m n o p q r s t u v w x y z a b c

Using the above key, we can quickly translate the message "happy" to "kdssb" (note how the 3-shifted alphabet wraps around at the end, so x -> a, y -> b, and z -> c).

This project has two phases:

• Encryption. That is, we will encode a message based on a given key.
• Decryption. That is, we will decode a text based on random keys and find out the exact answer.

The required p6words.txt and p6story.txt

Python Code:

	# 6.00x Problem Set 6
	import string
	import random
	WORDLIST_FILENAME = "p6words.txt"
	# Helper code
	#
	def loadWords():
	"""
	Returns a list of valid words. Words are strings of lowercase letters.
	Depending on the size of the word list, this function may
	take a while to finish.
	"""
	print "Loading word list from file..."
	inFile = open(WORDLIST_FILENAME, 'r')
	wordList = inFile.read().split()
	print " ", len(wordList), "words loaded."
	return wordList
	def isWord(wordList, word):
	"""
	Determines if word is a valid word.
	wordList: list of words in the dictionary.
	word: a possible word.
	returns True if word is in wordList.
	Example:
	>>> isWord(wordList, 'bat') returns
	True
	>>> isWord(wordList, 'asdf') returns
	False
	"""
	word = word.lower()
	word = word.strip(" !@#$%^&*()-_+={}[]|\\:;'<>?,./\"")
	return word in wordList
	def randomWord(wordList):
	"""
	Returns a random word.
	wordList: list of words
	returns: a word from wordList at random
	"""
	return random.choice(wordList)
	def randomString(wordList, n):
	"""
	Returns a string containing n random words from wordList
	wordList: list of words
	returns: a string of random words separated by spaces.
	"""
	return " ".join([randomWord(wordList) for _ in range(n)])
	def randomScrambled(wordList, n):
	"""
	Generates a test string by generating an n-word random string
	and encrypting it with a sequence of random shifts.
	wordList: list of words
	n: number of random words to generate and scamble
	returns: a scrambled string of n random words
	NOTE:
	This function will ONLY work once you have completed your
	implementation of applyShifts!
	"""
	s = randomString(wordList, n) + " "
	shifts = [(i, random.randint(0, 25)) for i in range(len(s)) if s[i-1] == ' ']
	return applyShifts(s, shifts)[:-1]
	def getStoryString():
	"""
	Returns a story in encrypted text.
	"""
	return open("p6story.txt", "r").read()
	# End of helper code
	#
	# Problem 1: Encryption
	#
	def buildCoder(shift):
	"""
	Returns a dict that can apply a Caesar cipher to a letter.
	The cipher is defined by the shift value. Ignores non-letter characters
	like punctuation, numbers and spaces.
	shift: 0 <= int < 26
	returns: dict
	"""
	result = {}
	upper = string.ascii_uppercase
	lower = string.ascii_lowercase
	for letter in upper:
	result[letter] = upper[(upper.index(letter) + shift) % 26]
	for letter in lower:
	result[letter] = lower[(lower.index(letter) + shift) % 26]
	return result
	## Result
	# buildCoder(11)
	# {'A': 'L', 'C': 'N', 'B': 'M', 'E': 'P', 'D': 'O', 'G': 'R', 'F': 'Q',
	# 'I': 'T', 'H': 'S', 'K': 'V', 'J': 'U', 'M': 'X', 'L': 'W', 'O': 'Z',
	# 'N': 'Y', 'Q': 'B', 'P': 'A', 'S': 'D', 'R': 'C', 'U': 'F', 'T': 'E',
	# 'W': 'H', 'V': 'G', 'Y': 'J', 'X': 'I', 'Z': 'K', 'a': 'l', 'c': 'n',
	# 'b': 'm', 'e': 'p', 'd': 'o', 'g': 'r', 'f': 'q', 'i': 't', 'h': 's',
	# 'k': 'v', 'j': 'u', 'm': 'x', 'l': 'w', 'o': 'z', 'n': 'y', 'q': 'b',
	# 'p': 'a', 's': 'd', 'r': 'c', 'u': 'f', 't': 'e', 'w': 'h', 'v': 'g',
	# 'y': 'j', 'x': 'i', 'z': 'k'}
	def applyCoder(text, coder):
	"""
	Applies the coder to the text. Returns the encoded text.
	text: string
	coder: dict with mappings of characters to shifted characters
	returns: text after mapping coder chars to original text
	"""
	cipher = str()
	for i in text:
	if i not in string.ascii_uppercase and i not in string.ascii_lowercase:
	cipher += i
	else:
	cipher += coder[i]
	return cipher
	## Result
	# applyCoder('Hello, world!', buildCoder(11))
	# 'Spwwz, hzcwo!'
	def applyShift(text, shift):
	"""
	Given a text, returns a new text Caesar shifted by the given shift
	offset. Lower case letters should remain lower case, upper case
	letters should remain upper case, and all other punctuation should
	stay as it is.
	text: string to apply the shift to
	shift: amount to shift the text (0 <= int < 26)
	returns: text after being shifted by specified amount.
	"""
	return applyCoder(text, buildCoder(shift))
	## Result
	# applyShift('mebgqfkwsjpvuy', 11)
	# 'xpmrbqvhduagfj'
	#
	# Problem 2: Decryption
	#
	wordList = loadWords()
	def findBestShift(wordList, text):
	"""
	Finds a shift key that can decrypt the encoded text.
	text: string
	returns: 0 <= int < 26
	"""
	realWords = 0
	bestShift = 0
	for p in string.punctuation:
	text = text.replace(p, '')
	for shift in range(0, 26):
	textList = applyShift(text, shift).split()
	validWords = 0
	for word in textList:
	if word in wordList:
	validWords += 1
	if validWords > realWords:
	realWords = validWords
	bestShift = shift
	return bestShift
	## Result
	# findBestShift(wordList, 'Vw, jcb bPMzm qa i BI viumL Itdqv!')
	# 18
	def decryptStory():
	"""
	Using the methods you created in this problem set,
	decrypt the story given by the function getStoryString().
	Use the functions getStoryString and loadWords to get the
	raw data you need.
	returns: string - story in plain text
	"""
	story = getStoryString()
	wordList = loadWords()
	bestShift = findBestShift(wordList, story)
	return applyShift(story, bestShift)
	## Result
	# 'Jack Florey is a mythical character created on the spur of a moment to help cover
	# an insufficiently planned hack. He has been registered for classes at MIT twice before,
	# but has reportedly never passed a class. It has been the tradition of the residents of
	# East Campus to become Jack Florey for a few nights each year to educate incoming students
	# in the ways, means, and ethics of hacking.\n'

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