encryption program

import string
what_to_do=raw_input("what do you want to do \"encrypt\" or \"decrypt\": ")
Low_letter=string.lowercase
Up_letter=string.uppercase
def encrypt(in_txt):
    encr_txt=[]
    for letter in in_txt:
        if letter in Low_letter:
            index=Low_letter.index(letter)
            invert_L_letter=index+1
            encr_txt.append(Low_letter[-invert_L_letter])
        elif letter in Up_letter:
            index=Up_letter.index(letter)
            invert_U_letter=index+1
            encr_txt.append(Up_letter[-invert_U_letter])
        else:
            encr_txt.append(letter)
    txt_out="".join(encr_txt)
    print (txt_out)
def decrypt(in_txt):
    decr_txt=[]
    for letter in in_txt:
        if letter in Low_letter:
            index=Low_letter.index(letter)
            invert_L_letter=index+1
            decr_txt.append(Low_letter[-invert_L_letter])
        elif letter in Up_letter:
            index=Up_letter.index(letter)
            invert_U_letter=index+1
            decr_txt.append(Up_letter[-invert_U_letter])
        else:
            decr_txt.append(letter)
    txt_out="".join(decr_txt)
    print (txt_out)
if what_to_do=="encrypt":
    text=raw_input("write your text to encrypt it: ")
    encrypt(text)
elif what_to_do=="decrypt":
    text=raw_input("write your encrypted text to decrypt it: ")
    decrypt(text)
else:
    print ("you don't say what to do!")