Python String Comparisons Using A Word List

Question

Eventually I will be able to post simple questions like this in a chat room, but for now I must post it. I am still struggling with comparison issues in Python. I have a list containing strings that I obtained from a file. I have a function which takes in the word list (previously created from a file) and some 'ciphertext'. I am trying to Brute Force crack the ciphertext using a Shift Cipher. My issue is the same as with comparing integers. Although I can see when trying to debug using print statements, that my ciphertext will be shifted to a word in the word list, it never evaluates to True. I am probably comparing two different variable types or a /n is probably throwing the comparison off. Sorry for all of the posts today, I am doing lots of practice problems today in preparation for an upcoming assignment.

  def shift_encrypt(s, m):

   shiftAmt = s % 26
   msgAsNumList = string2nlist(m)

   shiftedNumList = add_val_mod26(msgAsNumList, shiftAmt)
   print 'Here is the shifted number list: ', shiftedNumList

   # Take the shifted number list and convert it back to a string
   numListtoMsg = nlist2string(shiftedNumList)
   msgString = ''.join(numListtoMsg)

   return msgString

 def add_val_mod26(nlist, value):
   newValue = value % 26
   print 'Value to Add after mod 26: ', newValue
   listLen = len(nlist)
   index = 0
   while index < listLen:
       nlist[index] = (nlist[index] + newValue) % 26
       index = index + 1
   return nlist

 def string2nlist(m):
   characters =    ['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']
   numbers = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]
   newList = []
   msgLen = len(m)         # var msgLen will be an integer of the length

   index = 0               # iterate through message length in while loop
   while index < msgLen:
       letter = m[index]   # iterate through message m
       i = 0
       while i < 26:
           if letter == characters[i]:
               newList.append(numbers[i])
           i = i + 1
       index = index + 1
    return newList

  def nlist2string(nlist):
    characters = ['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']
    numbers = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]
    newList = []
    nListLen = len(nlist)

    index = 0
    while index < nListLen:
        num = nlist[index]
        newNum = num % 26
        i = 0
        while i < 26:
            num1 = newNum
            num2 = numbers[i]
            if (num1 == num2):
            newList.append(characters[i])
        i = i + 1
    index = index + 1
    return newList


  def wordList(filename):

    fileObject = open(filename, "r+")                    
    wordsList = fileObject.readlines()
    return wordsList


  def shift_computePlaintext(wlist, c):

    index = 0
    while index < 26:
        newCipher = shift_encrypt(index, c)
        print 'The new cipher text is: ', newCipher
        wordlistLen = len(wlist)
        i = 0
        while i < wordlistLen:
            print wlist[i]
            if newCipher == wlist[i]:
                return newCipher
            else:
                print 'Word not found.'
            i = i + 1
        index = index + 1    

   print 'Take Ciphertext and Find Plaintext from Wordlist Function: \n'
   list = wordList('test.txt')
   print list
   plainText = shift_computePlaintext(list, 'vium')
   print 'The plaintext was found in the wordlist: ', plainText

When the shift amount = 18, the ciphertext = name which is a word in my wordlist, but it never evaluates to True. Thanks for any help in advance!!

Answer 1

It's hard to be sure with the information we have so far, but here's a guess:

wordsList = fileObject.readlines()

This is going to return you a list of strings with the newlines preserved, like:

['hello\n', 'my\n', 'name\n', 'is\n', 'jesi\n']

So, inside shift_computePlaintext, when you iterate over wlist looking for something that matches the decrypted 'vium', you're looking for a string that matches 'name', and none of them match, including 'name\n'.

In other words, exactly what you suspected.

There are a few ways to fix this, but the most obvious are to use wlist[i].strip() instead of wlist[i], or to strip everything in the first place by using something like wordsList = [line.strip() for line in fileObject] instead of wordsList = fileObject.readlines().

---

A few side notes:

There is almost never a good reason to call readlines(). That returns a list of lines that you can iterate over… but the file object itself was already an iterable of lines that you can iterate over. If you really need to make sure it's a list instead of some other kind of iterable, or make a separate copy for later, or whatever, just call list on it, as you would with any other iterable.

You should almost never write a loop like this:

index = 0
while index < 26:
    # ...
    index = index + 1

Instead, just do this:

for index in range(26):

It's easier to read, harder to get wrong (subtle off-by-one errors are responsible for half the frustrating debugging you will do in your lifetime), etc.

And if you're looping over the length of a collection, don't even do that. Instead of this:

wordlistLen = len(wlist)
i = 0
while i < wordlistLen:
    # ...
    word = wlist[i]
    # ...
    i = i + 1

… just do this:

for word in wlist:

… or, if you need both i and word (which you occasionally do):

for i, word in enumerate(wlist):

Meanwhile, if the only reason you're looping over a collection is to check each of its values, you don't even need that. Instead of this:

wordlistLen = len(wlist)
while i < wordlistLen:
    print wlist[i]
    if newCipher == wlist[i]:
        return newCipher
    else:
        print 'Word not found.'
    i = i + 1

… just do this:

if newCipher in wlist:
    return newCipher
else:
    print 'Word not found.'

Here, you've actually got one of those subtle bugs: you print 'Word not found' over and over, instead of only printing it once at the end if it wasn't found.