Rot and polyalphabetic ciphers in Python 2.7

Question

I'm writing a "crypto library" and gradually adding algortihms. Now it's just ancient cryptography, but I'm going to expand the library. It doesn't depend on any external libraries.

class Crypto():
 def __init__(self, rotInt=3, letters=list("abcdefghijklmnopqrstuvwxyz")):
 self.letters = letters
 self.length = len(self.letters)
 self.rotInt = rotInt
 def rot(self, string, rotInt, letters, length): # To decrypt, use a negative number
 result = []
 for letter in string:
 if letter == " ": # Skip spaces
 result.append(" ")
 else:
 letterIndex = letters.index(letter)
 resultIndex = letterIndex + rotInt
 resultIndex = resultIndex % length-1
 result.append(letters[resultIndex])
 return "".join(result) # Return result as a string
 def getRotArr(self, string, negative=False): # Negative is for decryption
 result = []
 for char in string:
 if negative:
 result.append(0-(self.letters.index(char)+1))
 else:
 result.append(self.letters.index(char)+1)
 return result
 def polyalphabetic(self, string, rotArr, letters, length):
 result = []
 rotArrIndex = 0
 for letter in string:
 if letter == " ": # Skip spaces
 result.append(" ")
 else: # Spaces would mess up rotArrIndex
 if rotArrIndex > len(rotArr)-1: # Repeat when end of rotArr is reached
 rotArrIndex = 0
 rotInt = rotArr[rotArrIndex]
 letterIndex = letters.index(letter)
 resultIndex = letterIndex + rotInt
 resultIndex = resultIndex % length
 result.append(letters[resultIndex])
 rotArrIndex += 1
 return "".join(result) # Return result as a string

• Thomas Ward: I feel like the obligatory "You should not roll your own crypto libraries" warning should be put here, because using your own implementation of preestablished crypto while fun or good for a quick exercise of skills usually results in some 'holes' in the mix. So, obligatory warning about rolling your own crypto.
• Samuel Shifterovich: @ThomasWard This isn't supposed to be a crypto lib to use. Just a personal challenge, "can I code a crypto lib that can .....".
• Thomas Ward: Yes, I know, but that's why I said "while it can be fun or good for a skills exercise" in my previous post. It's just an obligatory warning to ship with everything (so that others who may think "This guy's cool for writing his own crypto library for things" won't go using it.) It's a standard 'obligatory warning' to the world, is all, wasn't implying you're going to be using it in production :)
• Samuel Shifterovich: @ThomasWard Or worse, "Hey, this guy can write his own crypto lib and it looks simple, what if I wrote my own for this website I was assigned, too" :P

Answer 1

Handling exceptions

If you create library user should be aware what is going wrong if he uses it in the wrong way.

def rot(self, string, rotInt, letters, length): # To decrypt, use a negative number
 result = []
 for letter in string:
 if letter == " ": # Skip spaces
 result.append(" ")
 else:
 letterIndex = letters.index(letter)

Here, you will receive IndexError which is not very informative, probably you would like to make your exceptions for such situations.

Performance of string concatenations

As you can see from this post, most efficient methods are using list, cStringIO and list comprehension with str.join. I recommend you to rewrite your rot method to use list comprehension and make all required checks in the beginning of the method.

Sanity defaults

def rot(self, string, rotInt, letters, length)

length parameter is just a length of the letters, so it can be computed in the method.

Because rot method is not the static method, you already have initialized class instance so you can use letters array received in __init__ method in case if the user does not provide letters to the rot method itself.

For an example, you can write something like:

def rot(self, string, rotation_index, letters=None):
 letters = letters or self.letters
 length = len(letters)

Data structures for characters set

Storing characters set in the form of the list in order to use index method every time you what to figure out a position of the element is not the best choice in case of performance. You should probably use additional dict for excample:

index_of = dict([(e, i) for i, e in enumerate(letters)])

You will get dict with keys of your letters and values - their positions in the letters iterable. So you can find out position and find is character found in your characters set with

<character> in index_of
index_of[<character>]

• Samuel Shifterovich: Exceptions: Good point Performance: I'll take a look on that, thanks Defaults: I tried def rot(self, string, rotInt, letters=self.letters, length) and it didn't work (obviously, I just tried it) and forgot to tweak the args. index_of: Cool trick. Thanks a lot.
• outoftime: @SamuelShifterovich it does not require to pass defaults direct into method declaration string. You can just leave them with None by default and make all the checks in the method body.
• Samuel Shifterovich: Yeah, what I was going to do. , letters=None, .....): letters=self.letters if letters == None else letters
• outoftime: @SamuelShifterovich probably you would like to use or statement instead.
• Graipher: Dictionary comprehensions are a thing, so there is no need for an intermediate list. You can either use dict((e, i) for i, e in enumerate(letters)) or {e: i for i, e in enumerate(letters)}.

Answer 2

rotInt, letters, length are defined in class. Use it.

Instead of for loop use map.

def rot(self, string, letters=None, rotInt=None):
 if letters is None:
 letters = self.letters
 length = len(letters)
 if rotInt is None:
 rotInt = self.rotInt
 return ''.join(map(lambda letter: letter in letters and letters[(letters.index(letter) + rotInt) % length] or letter, string))

and or

It's like bool ? a : b in C / C++. It's equivalent to:

if bool:
 return a
else:
 return b

• Samuel Shifterovich: I don't understand the and and or statements here, could you please explain them?
• Samuel Shifterovich: So in this context it's equivalent to return letters[(letters.index(letter) + self.rotInt) % self.length] if letters[(letters.index(letter) + self.rotInt) % self.length] else letter?
• Tomasz Jakub Rup: Yes, but only if string contains only letters from letters.