Python 3.3 cypher script - python-3.x

Question

So i have this code here in python 3.3, it cyphers text with the ceaser cypher.
What i need to know is how do i make a script that will convert it back from the original so that the person i send it too can read it.
message = input("Input: ")
key = 11
coded_message = ""
for ch in message:
code_val = ord(ch) + key
if ch.isalpha():
if code_val > ord('z'):
code_val -= ord('z') - ord('a')
coded_message = coded_message + chr(code_val)
else:
coded_message = coded_message + ch
# print ("Input: " + message)
print ("Output: " + coded_message)
One more thing, I plan to be putting this is a tkinter message box, with the two entry fields used for the input and output. one field should be used to type what i want to convert and the other should be used to show what the text looks like after it has been crypted. The button should start the encryption. here is the code:
import sys
from tkinter import *
def mHello():
mLabel = Label(mGui,text = input("Hello World"))
mLabel.grid(row=3, column=0,)
mGui = Tk()
ment = StringVar()
mGui.geometry("450x450+250+250")
mGui.title("My TKinter")
# input label
mLabel = Label(mGui,text = "Input",)
mLabel.grid(row=1,column=0,)
# output label
mLabeltwo = Label(mGui,text = "Input",)
mLabeltwo.grid(row=2,column=0,)
# convert button
mButton = Button(text = "Convert",command = mHello)
mButton.grid(row=3,column=0)
# input entry
mEntry = Entry(mGui,textvariable=ment)
mEntry.grid(row=1,column=1)
# output entry
mEntryTwo = Entry(mGui,textvariable=ment)
mEntryTwo.grid(row=2,column=1)
mGui.mainloop()
By the way i am only 15 and this is my 2nd day learning python.
Some credit goes to sources on this forum that have provided me with some code snippets
Thank-you in advance guys!

Answer 1

Before i say anything else you should be aware that minds much greater the mine have advised against writing your own cypher script for anything other then learning
If you want them to be able to decode your code then provide them with a key. so in your case:
s maps to h
t maps to i
f maps to t
I hope this code illustrates my suggestion:
In [1]: from cyro import your_cyrptic_function
In [2]: key = {'s':'h', 't':'i', 'f':'t'}
In [3]: secret_word = your_cyrptic_function('hit')
In [4]: decyrpted_secret_word = ''
In [5]: for letter in secret_word:
decyrpted_secret_word += key[letter]
...:
In [6]: print(decyrpted_secret_word)
hit
For the code above i turned your original code into a function:
def your_cyrptic_function(secret):
message = secret
key = 11
coded_message = ""
for ch in message:
code_val = ord(ch) + key
if ch.isalpha():
if code_val > ord('z'):
code_val -= ord('z') - ord('a')
coded_message = coded_message + chr(code_val)
else:
coded_message = coded_message + ch
# print ("Input: " + message)
return coded_message
there are several great ways to do this in python. If your interested in cyptopgraphy then check out Udacities class cs387 applied cryptography