Problem set 4 is to implement a Caesar Cipher including encryption and decrpytion of a text string. For example, encrypting a text by 1 and decrypting it:
>>> import ps4
Loading word list from file…
55909 words loaded.
>>> s = ps4.apply_shift(“The files are *in* the computer”, 1)
>>> s
‘Uifagjmftabsfa*jo*auifadpnqvufs’
>>> ps4.find_best_shift(ps4.wordlist, s)
1
>>> ps4.apply_coder(s, ps4.build_decoder(1))
‘The files are *in* the computer‘
On a multiple-shift decryption, my control flow only allows the function to decrypt text from left to right. That way we keep track of our position in the text as we shift our way across it, while also counting the number of shifts to find each valid word according to the dictionary. Until I implemented this, my valid_word checker might find a word farther down the string, move the ‘cursor’ and get completely derailed. This is obvious around line 475.
Another ‘ah-ha’ moment was trying to keep track of the counter through a recursive function. To do so, I turned the ‘start’ variable in the same function as above into an array instead of an int. start[0] functions as the cursor, start[1] as the number of shifts, and further tuples track where shifts started and how many occurred. I am not entirely sure this is allowed per the instructions, but the means are currently justifying the end result.
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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(-27 < shift < 27) alphabet = [' '] i = 97 # build array of alphabet chars beginning with ' ' while i < 123: alphabet.append(chr(i)) i += 1 # create dict where key is letter and value is letter + shift coder = {} i = 0 while i < len(alphabet): coder[alphabet[i]] = alphabet[(i + shift) % len(alphabet)] i += 1 i = 1 # repeat for uppercase while i < len(alphabet): coder[alphabet[i].upper()] = alphabet[(i + shift) % len(alphabet)].upper() i += 1 return coder 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. assert( 0 <= shift < 27) encoder = build_coder(shift) return encoder # watch it run # print build_encoder(3) 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. ### bug KeyError on decoder['C'] ### something to do with the do uppercase and i starting at 1? decoder = {} # get a copy of the encoding dict temp = build_coder(shift) # reverse values -> keys for i in temp: decoder[temp[i]] = i return decoder # watch it run # print build_decoder(3) 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!' """ ### TODO. ### 4/19/15 ### currently KeyError on punctuation encoded_char = [] for i in text: # ignore any letter/symbols not present in the coder dict (punctuation) if i in coder: encoded_char.append(coder[i]) else: encoded_char.append(i) encoded_text = ''.join(encoded_char) return encoded_text # watch it run # print apply_coder("Hello, world!", build_encoder(3)) 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. encoded_text = apply_coder(text, build_encoder(shift)) return encoded_text # watch it run # 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 ### returns the shift int answer split_string = list(text) alpha_string = [] for i in split_string: if i.isalpha(): alpha_string.append(i) else: continue # print alpha_string i = 0 while i < 27: potential_decode = apply_shift(''.join(alpha_string), i) # print potential_decode # split result potential_decode = potential_decode.split() # all words in dictionary? winner = True for word in potential_decode: if word.lower() in wordlist: continue else: winner = False if winner == True: return abs(i-27) # all good things must come to an end i += 1 return None # watch it run # s = apply_coder('Hello, world!', build_encoder(8)) # print find_best_shift(wordlist, s) # print apply_coder(s, build_decoder(8)) # # 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. # for each pair in shifts (text, shifts) for i in shifts: # get text to shift from [text:] text_to_shift = text[i[0]:] # get prefix to conserve from [:text] text = text[:i[0]] # shift the text by shifts shifted_text = apply_shift(text_to_shift, i[1]) # recombine text = text + shifted_text return text # watch it run # print apply_shifts("Do Androids Dream of Electric Sheep?", [(0,6), (3, 18), (12, 16)]) # # Problem 4: Multi-level decryption. # def find_best_shifts(wordlist, text): """ Given a scrambled string, returns a shift key that will decode the text to words in wordlist, or None if there is no such key. Hint: Make use of the recursive function find_best_shifts_rec(wordlist, text, start) wordlist: list of words text: scambled text to try to find the words for returns: list of tuples. each tuple is (position in text, amount of shift) Examples: >>> s = random_scrambled(wordlist, 3) >>> s 'eqorqukvqtbmultiform wyy ion' >>> shifts = find_best_shifts(wordlist, s) >>> shifts [(0, 25), (11, 2), (21, 5)] >>> apply_shifts(s, shifts) 'compositor multiform accents' >>> s = apply_shifts("Do Androids Dream of Electric Sheep?", [(0,6), (3, 18), (12, 16)]) >>> s 'JufYkaolfapxQdrnzmasmRyrpfdvpmEurrb?' >>> shifts = find_best_shifts(wordlist, s) >>> print apply_shifts(s, shifts) Do Androids Dream of Electric Sheep? """ 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. ########## # my logic assumes that words will be decrypted from left to right in order # This seems inefficient # I think I am abusing the start variable to pass counting values ########## # best case scenario, start at end AKA no shifts, check if words in dict # track the number of shifts in position start[1] if len(start) < 2: start.append(0) print text # is every item in result in dictionary? text_given = text.split() done_shifting = True # best case scenario for i in text_given: if is_word(wordlist, i): continue else: done_shifting = False # best case didn't happen if done_shifting == True: print "decipher complete" print text return start[2:] # save any valid words before start valid_words = text[:start[0]] # shift text after start and count it text_to_shift = text[start[0]:] shifted_text = apply_shift(text_to_shift, 1) start[1] += 1 # split on ' ' and check results for valid words split_shifted_text = shifted_text.split(' ') # recombine into a string to later resend to recursive function unsplit_shifted_text = ' '.join(split_shifted_text) recombined_text = valid_words + unsplit_shifted_text # check for valid words and move start for each start_began = start[0] for i in split_shifted_text: if is_word(wordlist, i): # watch it find words print i, " is a word" a = raw_input(">") start[0] += len(i)+1 else: break # assures decrypt from left to right if start[0] != start_began: start.append((start_began, abs(start[1] - 27))) start[1] = 0 # reset shifting counter # will always run one extra cycle to check correctness at the top return find_best_shifts_rec(wordlist, recombined_text, start) # watch it run # s = random_scrambled(wordlist, 3) # or # s = apply_shifts("Do Androids Dream of Electric Sheep?", [(0,25), (3, 1), (12, 1)]) # print find_best_shifts_rec(wordlist, s, [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 """ ### TODO. s = get_fable_string() print find_best_shifts_rec(wordlist, s, [0]) # fails on 30th word # goes into infinite loop # potentially a problem with a double-space and my recombined strings # decrypt_fable() |