CTF_NAHAMCON_Rotten

README.md

Written quickly to solve a puzzle in a CTF.

Another competitor came up with a much nicer way to display the output as it was revealed - it wasn't immediately obvious to me how long the flag was going to be, but it looked like it'd be 30-40 characters, so I figured that collecting 200 characters should get me what I needed without grinding on the server unnecessarily. It worked, in a not very elegant sort of way.

Note that I ended up not using the rot() function at all, rot_alpha() ended up doing all of the work. I don't remember why I started going one direction then went the other, it just turned out that way.

In retrospect I'm not sure why I thought it was better to create all 26 possible rot() variations then search them, instead of generating and checking one at a time. Another "it made sense at the time" thing.

rotten.py

import socket
import sys
import time
import re
import pprint

BUFFER = 4096
SLEEP = 0.5


def rot(s, n):
    chars = "abcdefghijklmnopqrstuvwxyz"
    trans = chars[n:] + chars[:n]
    rot_char = lambda c: trans[chars.find(c)] if chars.find(c) > -1 else c
    return "".join(rot_char(c) for c in s)


def rot_alpha(n):
    from string import ascii_lowercase as lc, ascii_uppercase as uc

    lookup = str.maketrans(lc + uc, lc[n:] + lc[:n] + uc[n:] + uc[:n])
    return lambda s: s.translate(lookup)


def decoder(text):
    ciphertext = text
    shift = 0
    ciphertext = ciphertext.decode("utf-8")
    ciphertext = ciphertext.strip()
    decodes = []
    for i in range(27):
        decodes.append(rot_alpha(i)(ciphertext))

    for i in decodes:
        if "send back" in i:
            # print(i)
            return i
    print("ROT failed!")
    for i in decodes:
        print("[{}] {}".format(i, decodes[i]))

    sys.exit(0)


key = {}
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_address = ("jh2i.com", 50034)
print("connecting to {}".format(server_address))
sock.connect(server_address)
sock.settimeout(60)
line = sock.recv(BUFFER)
print("Received first line: {}".format(line))
sock.sendall(line)
print("Sent it back, entering big loop.")
iterations = 0
while True:

    line = sock.recv(BUFFER)
    # print("Received line: {}".format(line))
    solution = decoder(line)
    helpful = re.search("character", solution)
    if helpful:
        iterations += 1
        position = solution[helpful.end() : -3]
        # print("Position is: {}".format(position))
        words = position.split(" ")
        # print("Words is: {}".format(words))
        finalposition = str(words[1].strip())
        # print("Finalposition is: {}".format(finalposition))
        key[finalposition] = solution[-2:-1]
        # pprint.pprint(key)
        if iterations % 10 == 0:
            print(iterations)
    sock.sendall(solution.encode("utf-8"))
    if iterations == 200:
        for i in range(len(key)):
            print("{}".format(key[str(i)]), end="")
        print()
        sys.exit(0)