Monoalphabetic_substitution-Caesar

import string

cipher_text=b"""tm bcsv qolfp
f'dmvd xuhm exl tgak
hlrkiv sydg hxm
qiswzzwf qrf oqdueqe
dpae resd wndo
liva bu vgtokx sjzk
hmb rqch fqwbg
fmmft seront sntsdr pmsecq""".replace(b'\'',b' ')

plain_text = b''
flag_first,flag_last = b'',b''
alphabet = string.printable[10:-64].encode()
shift = 1
for ci in cipher_text.split(b'\n'):
    tmp = ci.split(b' ')
    line_plain_text = b''
    for word in tmp:
        line_plain_text+=bytes([ alphabet[(alphabet.index(c)+shift)%26] if c in alphabet else c for c in word])
        shift+=1
        line_plain_text+=b' '
    flag_first+=bytes([line_plain_text[0]])
    flag_last+=bytes([line_plain_text[-2]])
    plain_text+=line_plain_text+b'\n'
print(plain_text.decode())
print("[+]FLAG:",(flag_first+flag_last).decode())
## ujqcsddessxsffes