How to make shift('w','f') return 'b'?

Question

Using this program to take out spaces, punctuation, and make letters lower case...

def pre_process(s): #Enter: "Jim's secret password."

    s= s.replace("'","")
    s= s.replace('.','')
    s= s.lower()
    s= s.replace(" ","")
    return s

How can I encrypt a message so that the letters each shift by an amount equal to the corresponding letter in the alphabet? For example m shifted 5 times becomes r, but w shifted 5 times becomes b. This is my current code:

def shift(ch,k):    
    return chr(ord('a')+(ord(ch)-ord('a')+k) % 26)

Answer 1

def shift(ch, k):
    return chr(ord('a') + ((ord(ch) - ord('a')) + 
                           (ord(k) - ord('a'))) % 26)

Sort of an explanation:

def shift(ch, k):
    #
    # k_delta
    # ────>
    #
    # ch_delta                 k_delta
    # ────────────────────────>────>
    # a....f.........m....r....w..zab
    # ──────────────>────>         ┊
    # ch_delta       k_delta       ┊
    #                              ┊
    #                             %26

    ch_delta = ord(ch) - ord('a')
    k_delta = ord(k) - ord('a')
    return chr(ord('a') + (ch_delta + k_delta) % 26)

Unless k varies, you can use str.translate to speed up encryption:

import string
message = 'mw'
key = 'f'
enc_table = string.maketrans(
    string.ascii_lowercase,
    ''.join(shift(c, key) for c in string.ascii_lowercase)
)
message.translate(enc_table) # -> 'rb'

I'd also suggest replacing the magic number 26 with len(string.ascii_lowercase) for example.

Decryption can be done using the same function, but with a different key. The relation between them is that enc_delta + dec_delta = 0 modulo 26. From this results that dec_delta = -enc_delta % 26. Therefore:

dec_k = chr(ord('a') + ((ord(enc_k) - ord('a'))) % 26)