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- # 6.00 Problem Set 4
- #
- # Caesar Cipher Skeleton
- #
- import string
- import random
- WORDLIST_FILENAME = "words.txt"
- # -----------------------------------
- # Helper code
- # (you don't need to understand this helper code)
- def load_words():
- """
- 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: file
- inFile = open(WORDLIST_FILENAME, 'r', 0)
- # line: string
- line = inFile.readline()
- # wordlist: list of strings
- wordlist = line.split()
- print " ", len(wordlist), "words loaded."
- return wordlist
- wordlist = load_words()
- def is_word(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:
- >>> is_word(wordlist, 'bat') returns
- True
- >>> is_word(wordlist, 'asdf') returns
- False
- """
- word = word.lower()
- word = word.strip(" !@#$%^&*()-_+={}[]|\:;'<>?,./\"")
- return word in wordlist
- def random_word(wordlist):
- """
- Returns a random word.
- wordlist: list of words
- returns: a word from wordlist at random
- """
- return random.choice(wordlist)
- def random_string(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([random_word(wordlist) for _ in range(n)])
- def random_scrambled(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 apply_shifts!
- """
- s = random_string(wordlist, n) + " "
- shifts = [(i, random.randint(0, 26)) for i in range(len(s)) if s[i-1] == ' ']
- return apply_shifts(s, shifts)[:-1]
- def get_fable_string():
- """
- Returns a fable in encrypted text.
- """
- f = open("fable.txt", "r")
- fable = str(f.read())
- f.close()
- return fable
- # (end of helper code)
- # -----------------------------------
- #
- # Problem 1: Encryption
- #
- def build_coder(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 and numbers.
- shift: -27 < int < 27
- returns: dict
- Example:
- >>> build_coder(3)
- {' ': 'c', 'A': 'D', 'C': 'F', 'B': 'E', 'E': 'H', 'D': 'G', 'G': 'J',
- 'F': 'I', 'I': 'L', 'H': 'K', 'K': 'N', 'J': 'M', 'M': 'P', 'L': 'O',
- 'O': 'R', 'N': 'Q', 'Q': 'T', 'P': 'S', 'S': 'V', 'R': 'U', 'U': 'X',
- 'T': 'W', 'W': 'Z', 'V': 'Y', 'Y': 'A', 'X': ' ', 'Z': 'B', 'a': 'd',
- 'c': 'f', 'b': 'e', 'e': 'h', 'd': 'g', 'g': 'j', 'f': 'i', 'i': 'l',
- 'h': 'k', 'k': 'n', 'j': 'm', 'm': 'p', 'l': 'o', 'o': 'r', 'n': 'q',
- 'q': 't', 'p': 's', 's': 'v', 'r': 'u', 'u': 'x', 't': 'w', 'w': 'z',
- 'v': 'y', 'y': 'a', 'x': ' ', 'z': 'b'}
- (The order of the key-value pairs may be different.)
- """
- ### TODO.
- assert shift >= 0 and shift < 27, 'shift %s is not between 0 and 27' % shift
- # #numbers.Integral used in case of long integers
- assert isinstance(shift, int), 'shift is not an integer'
- coder = {}
- lowercase_and_space = string.ascii_lowercase + ' '
- uppercase_and_space = string.ascii_uppercase + ' '
- # Shift letters over shift places
- shifted_lowercase_and_space = lowercase_and_space[shift:] + lowercase_and_space[:shift]
- shifted_uppercase_and_space = uppercase_and_space[shift:] + uppercase_and_space[:shift]
- # Construct Caesar cipher dictionary
- # Add uppercase letters first so ' ' will be overwritten to point to lowercase letter
- for i in range(len(uppercase_and_space)):
- coder[uppercase_and_space[i]] = shifted_uppercase_and_space[i]
- for i in range(len(lowercase_and_space)):
- coder[lowercase_and_space[i]] = shifted_lowercase_and_space[i]
- return coder
- print build_coder(3)
- def build_encoder(shift):
- """
- Returns a dict that can be used to encode a plain text. For example, you
- could encrypt the plain text by calling the following commands
- >>>encoder = build_encoder(shift)
- >>>encrypted_text = apply_coder(plain_text, encoder)
- The cipher is defined by the shift value. Ignores non-letter characters
- like punctuation and numbers.
- shift: 0 <= int < 27
- returns: dict
- Example:
- >>> build_encoder(3)
- {' ': 'c', 'A': 'D', 'C': 'F', 'B': 'E', 'E': 'H', 'D': 'G', 'G': 'J',
- 'F': 'I', 'I': 'L', 'H': 'K', 'K': 'N', 'J': 'M', 'M': 'P', 'L': 'O',
- 'O': 'R', 'N': 'Q', 'Q': 'T', 'P': 'S', 'S': 'V', 'R': 'U', 'U': 'X',
- 'T': 'W', 'W': 'Z', 'V': 'Y', 'Y': 'A', 'X': ' ', 'Z': 'B', 'a': 'd',
- 'c': 'f', 'b': 'e', 'e': 'h', 'd': 'g', 'g': 'j', 'f': 'i', 'i': 'l',
- 'h': 'k', 'k': 'n', 'j': 'm', 'm': 'p', 'l': 'o', 'o': 'r', 'n': 'q',
- 'q': 't', 'p': 's', 's': 'v', 'r': 'u', 'u': 'x', 't': 'w', 'w': 'z',
- 'v': 'y', 'y': 'a', 'x': ' ', 'z': 'b'}
- (The order of the key-value pairs may be different.)
- HINT : Use build_coder.
- """
- ### TODO.
- def build_decoder(shift):
- """
- Returns a dict that can be used to decode an encrypted text. For example, you
- could decrypt an encrypted text by calling the following commands
- >>>encoder = build_encoder(shift)
- >>>encrypted_text = apply_coder(plain_text, encoder)
- >>>decrypted_text = apply_coder(plain_text, decoder)
- The cipher is defined by the shift value. Ignores non-letter characters
- like punctuation and numbers.
- shift: 0 <= int < 27
- returns: dict
- Example:
- >>> build_decoder(3)
- {' ': 'x', 'A': 'Y', 'C': ' ', 'B': 'Z', 'E': 'B', 'D': 'A', 'G': 'D',
- 'F': 'C', 'I': 'F', 'H': 'E', 'K': 'H', 'J': 'G', 'M': 'J', 'L': 'I',
- 'O': 'L', 'N': 'K', 'Q': 'N', 'P': 'M', 'S': 'P', 'R': 'O', 'U': 'R',
- 'T': 'Q', 'W': 'T', 'V': 'S', 'Y': 'V', 'X': 'U', 'Z': 'W', 'a': 'y',
- 'c': ' ', 'b': 'z', 'e': 'b', 'd': 'a', 'g': 'd', 'f': 'c', 'i': 'f',
- 'h': 'e', 'k': 'h', 'j': 'g', 'm': 'j', 'l': 'i', 'o': 'l', 'n': 'k',
- 'q': 'n', 'p': 'm', 's': 'p', 'r': 'o', 'u': 'r', 't': 'q', 'w': 't',
- 'v': 's', 'y': 'v', 'x': 'u', 'z': 'w'}
- (The order of the key-value pairs may be different.)
- HINT : Use build_coder.
- """
- ### TODO.
- def apply_coder(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
- Example:
- >>> apply_coder("Hello, world!", build_encoder(3))
- 'Khoor,czruog!'
- >>> apply_coder("Khoor,czruog!", build_decoder(3))
- 'Hello, world!'
- """
- encoded_word = ''
- for i in range (0, len(text)):
- if text[i] not in coder:
- encoded_word = encoded_word + text[i]
- else:
- encoded_word = encoded_word + coder[text[i]]
- return encoded_word
- print apply_coder('Hello, world!', build_coder(3))
- ### TODO.
- def apply_shift(text, shift):
- """
- Given a text, returns a new text Caesar shifted by the given shift
- offset. The empty space counts as the 27th letter of the alphabet,
- so spaces should be replaced by a lowercase letter as appropriate.
- Otherwise, 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
- returns: text after being shifted by specified amount.
- Example:
- >>> apply_shift('This is a test.', 8)
- 'Apq hq hiham a.'
- """
- ### TODO.
- return apply_coder(text, build_coder(shift))
- print apply_shift('This is a test.', 8)
- #
- # Problem 2: Codebreaking.
- #
- def find_best_shift(wordlist, text):
- """
- Decrypts the encoded text and returns the plaintext.
- text: string
- returns: 0 <= int 27
- Example:
- >>> s = apply_coder('Hello, world!', build_encoder(8))
- >>> s
- 'Pmttw,hdwztl!'
- >>> find_best_shift(wordlist, s) returns
- 8
- >>> apply_coder(s, build_decoder(8)) returns
- 'Hello, world!'
- """
- ### TODO
- max_num_valid_words = 0
- best_shift = 0
- for shift in range(27):
- shifted_words = apply_shift(text, shift)
- potential_words = shifted_words.split()
- num_valid_words = 0
- for word in potential_words:
- if is_word(wordlist, word):
- num_valid_words += 1
- if num_valid_words > max_num_valid_words:
- max_num_valid_words = num_valid_words
- best_shift = shift
- return best_shift
- print find_best_shift(wordlist, "Pmttw,hdwztl!")
- #
- # Problem 3: Multi-level encryption.
- #
- def apply_shifts(text, shifts):
- """
- Applies a sequence of shifts to an input text.
- text: A string to apply the Ceasar shifts to
- shifts: A list of tuples containing the location each shift should
- begin and the shift offset. Each tuple is of the form (location,
- shift) The shifts are layered: each one is applied from its
- starting position all the way through the end of the string.
- returns: text after applying the shifts to the appropriate
- positions
- Example:
- >>> apply_shifts("Do Androids Dream of Electric Sheep?", [(0,6), (3, 18), (12, 16)])
- 'JufYkaolfapxQdrnzmasmRyrpfdvpmEurrb?'
- """
- ### TODO.
- encoded_word = ""
- a = dict(shifts)
- for key in a:
- if key == 0:
- initial_shift = apply_shift(text, a[key])
- encoded_word = encoded_word + initial_shift
- print encoded_word
- else:
- new_shift = apply_shift(encoded_word[key:], a[key])
- print new_shift
- encoded_word = encoded_word[0:key] + new_shift
- print encoded_word
- return encoded_word
- # print apply_shifts("JufYkaolfapxQdrnzmasmRyrpfdvpmEurrb?", [(0, 21), (3, 9), (12, 11), (18, 0), (21, 0), (30, 0)])
- #
- # Problem 4: Multi-level decryption.
- #
- def find_best_shifts_rec(wordlist, text, start):
- """
- Given a scrambled string and a starting position from which
- to decode, returns a shift key that will decode the text to
- words in wordlist, or None if there is no such key.
- Hint: You will find this function much easier to implement
- if you use recursion.
- wordlist: list of words
- text: scambled text to try to find the words for
- start: where to start looking at shifts
- returns: list of tuples. each tuple is (position in text, amount of shift)
- """
- ### TODO.
- new_string = text
- for shift in xrange(0, 27):
- new_string = text[0:start] + apply_shift(text[start:len(text)], shift)
- for i in xrange(start, len(new_string)+1):
- if is_word(wordlist, new_string[start:i]):
- if i == len(new_string):
- return [(start, shift)]
- elif new_string[i] == ' ':
- recShifts = find_best_shifts_rec(wordlist, new_string, i+1)
- if recShifts != None:
- return [(start, shift)] + recShifts
- print find_best_shifts_rec(wordlist, "JufYkaolfapxQdrnzmasmRyrpfdvpmEurrb?", 0)
- def decrypt_fable():
- """
- Using the methods you created in this problem set,
- decrypt the fable given by the function get_fable_string().
- Once you decrypt the message, be sure to include as a comment
- at the end of this problem set how the fable relates to your
- education at MIT.
- returns: string - fable in plain text
- """
- fable = get_fable_string()
- shifts = find_best_shifts_rec(wordlist, fable, 0)
- decoded_fable = apply_shifts(fable, shifts)
- return decoded_fable
- print decrypt_fable()
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