6.189 Homework 1 Exercise OPT.2 – Secret Messages

# 6.189 Homework 1 Exercise OPT.2 – Secret Messages
# A simple shift cypher
# Done in Python 3.2.2

mode = -1
shift = 0
message_in = ''
message_out = ''

while mode:
    try:
        mode = int(input("0 to Quit\n1 to Encode\n2 to Decode\n :"))
        if mode < -1 or mode > 2:
            print ("Not a valid selection.\n")
            mode = -1
        else: break
    except:
        print ("Not a valid selection.\n")

if mode==0:
    print("Bye Bye!")
    exit(0)
else:
    while shift==0:
        try:
            shift = int(input("Enter a shift value from 1 to 50: "))
            if shift < 1 or mode > 50:
                print ("Not a valid selection.\n")
                shift = 0
            else: break
        except:
            print ("Not a valid selection.\n")


message_in = input("What is the message to encode? :")

print("You typed:", message_in)

for letter in message_in:
    temp = ord(letter)
    if mode == 1:
        if temp >= shift:
            message_out=message_out+chr(temp-shift)
        else:
            message_out=message_out+chr(temp-shift+126)
    else:
        if temp+shift < 127:
            message_out=message_out+chr(temp+shift)
        else:
            message_out=message_out+chr(temp+shift-126)

print("En/De-codes to:", message_out)