Question

I would like to build a cypher with python which decoding text by repeatedly travel through the numbers of the shifter from left to right and then from right to left and shift the letter in the alphabet by the corresponding number.

Example:

• shifter: 123
• text: i like python
• process: i+1=j, space, l+2=n, i+3=l, k+3=n, e+2=g, space, p+1=q, y+1=z, t+2=v, h+3=j, o+3=q, n+2=p
• result: j nlng qzvjqp

The code so far:

import string

numbers = ""

x = 3

while x < 10000:
    numbers = numbers + str(x)
    x += 1

shift = 221

#string.ascii_lowercase
letters = string.ascii_letters + string.punctuation + numbers
text = (raw_input("Please enter text"))

encoded = ''
for letter in text:
    if letter == ' ':
        encoded = encoded + ' '
    else:
        x = letters.index(letter) + shift
        encoded = encoded + letters[x]

print encoded

It uses so far a basic coding method. I would love to know how is it possible to implement the above described encoding system in the code.

Answer 1

Here is your version optimized (and working):

import string

shift = 221

letters = string.ascii_letters + string.punctuation + string.digits
text = raw_input("Please enter text\n")

e = ''.join(letters[letters.index(l) + shift % len(letters)]
            if letters.index(l) is not None else ' ' for l in text)
print e

Edit The part with numbers was really wrong, because it added duplicate letters to your encoding sequence. This means that an encoded text cannot be decoded uniquely.

Edit2: If you want secure en/decryption, you need a modern algorithm like AES. If you want it implement on your own or understand the code, RC4 is a simple (but still secure if used correctly) alternative.

Edit3: Here is Vigenere in an optimized version:

from itertools import cycle, izip
from operator import add, sub
from string import printable as alphabet

def vigenere(text, key, mode='encode'):
    alen = len(alphabet)
    key = cycle(key + key[-2:0:-1])
    op = add if mode == 'encode' else sub
    return ''.join(alphabet[op(alphabet.index(c),
                               alphabet.index(k)) % alen]
                   for c, k in izip(text, key))

c = vigenere('attack at dawn', 'secret', mode='encode')
print c  # CHFBqN8BF8FoIO
print vigenere(c, 'secret', mode='decode')