C3

Description

This is the Custom Cyclical Cipher!Download the ciphertext here. Download the encoder here. Enclose the flag in our wrapper for submission. If the flag was "example" you would submit "picoCTF{example}".

The encoder given was,

import sys
chars = ""
from fileinput import input
for line in input():
  chars += line

lookup1 = "\n \"#()*+/1:=[]abcdefghijklmnopqrstuvwxyz"
lookup2 = "ABCDEFGHIJKLMNOPQRSTabcdefghijklmnopqrst"

out = ""

prev = 0
for char in chars:
  cur = lookup1.index(char)
  out += lookup2[(cur - prev) % 40]
  prev = cur

sys.stdout.write(out)

So after seeing the previous challenge, I turned to ChatGPT, which after a bit of back and forth, I obtained this script:

import sys

ciphertext = "DLSeGAGDgBNJDQJDCFSFnRBIDjgHoDFCFtHDgJpiHtGDmMAQFnRBJKkBAsTMrsPSDDnEFCFtIbEDtDCIbFCFtHTJDKerFldbFObFCFtLBFkBAAAPFnRBJGEkerFlcPgKkImHnIlATJDKbTbFOkdNnsgbnJRMFnRBNAFkBAAAbrcbTKAkOgFpOgFpOpkBAAAAAAAiClFGIPFnRBaKliCgClFGtIBAAAAAAAOgGEkImHnIl"

lookup1 = "\n \"#()*+/1:=[]abcdefghijklmnopqrstuvwxyz"
lookup2 = "ABCDEFGHIJKLMNOPQRSTabcdefghijklmnopqrst"

out = ""

prev = 0
for char in ciphertext:
    cur = lookup2.index(char)
    out += lookup1[(cur + prev) % 40]
    prev = (cur + prev) % 40

sys.stdout.write(out)

When run, you should be able to obtain the following script from the ciphertext:

#asciiorder
#fortychars
#selfinput
#pythontwo

chars = ""
from fileinput import input
for line in input():
    chars += line
b = 1 / 1

for i in range(len(chars)):
    if i == b * b * b:
        print chars[i] #prints
        b += 1 / 1

As we can see, we obtained a Python2 script which if we run,

cat part2.py | python part2.py

We obtain the flag of

picoCTF{adlibs}