python, encryption of any size code with a smaller key

Question

So I'm trying to work to create a program which can take two inputs such as

encrypt('12345','12')

and it will return

'33557'

where the code ('12345') and been incremented by the key ('12'), working from right to left.

I have already created one which will work for when the code and key are both 8 long, but I cannot work out how to do this should the code be allowed to be any size, possibly with nested for statments?

Here is the one i did early so you can see better what i am trying to do

def code_block(char,charBlock):
    if len(char) == 8 and len(charBlock) == 8:    #Check to make sure both are 8 didgets.
        c = char
        cB = charBlock
        line = ""
        for i in range(0,8):
            getDidget = code_char2(front(c),front(cB))
            c = last(c)
            cB = str(last(cB))
            line =line + getDidget
        print(line)
    else:
        print("Make sure two inputs are 8 didgets long")

def front(word):
    return word[:+1]

def last(word):
    return word[+1:]

Answer 1

Some code tested on Python 3.2:

from decimal import Decimal
import itertools

def encrypt(numbers_as_text, code):
    key = itertools.cycle(code[::-1])

    num = Decimal(numbers_as_text)

    power = 1
    for _ in numbers_as_text:
        num += power * int(next(key))
        power *= Decimal(10)

    return num



if __name__ == "__main__":
    print(encrypt('12345','12'))

Some explanation:

• code[::-1] is a cool way to reverse a string. Stolen from here
• itertools.cycle endlessly repeats your key. So the variable key now contains a generator which yields 2, 1, 2, 1, 2, 1, etc
• Decimal is a datatype which can handle arbitrary precision numbers. Actually Python 3's integer numbers would be sufficient because they can handle integer numbers with arbitrary number of digits. Calling the type name as a function Decimal(), calls the constructor of the type and as such creates a new object of that type. The Decimal() constructor can handle one argument which is then converted into a Decimal object. In the example, the numbers_as_text string and the integer 10 are both converted into the type Decimal with its constructor.
• power is a variable that starts with 1 is multiplied by 10 for every digit that we have worked on (counting from the right). It's basically a pointer to where we need to modify num in the current loop iteration
• The for loop header ensures we're doing one iteration for each digit in the given input text. We could also use something like for index in range(len(numbers_as_text)) but that's unnecessarily complex

Of course if you want to encode text, this approach does not work. But since that wasn't in your question's spec, this is a function focused on dealing with integers.