TrendMicro CTF 2018 Writeup

$ cat oracle.py
#!/usr/bin/python -u
#-*- coding:utf-8 -*-

# Let's exploit easy and quick!
# 1) apt install valgrind
# 2) use callgrind to find instruction count

flag = 'TMCTF{'
n = 0

import os
import sys

# format given by admin
charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789{}"
while True:
    n += 1
    total_call_count = {}
    for i in charset:
        cmd = "valgrind --tool=callgrind --dump-instr=yes --callgrind-out-file=temp/call_count ./oracle '" + flag + i + "A'  2>&1"
        # print(cmd)
        res =  os.popen(cmd).read()

        call_count = res.split("Collected : ")[1].split()[0]
        call_count = int(call_count)
        # total_call_count { 'call_count': [occured_count, occured_by], ... }
        if not total_call_count.get(call_count):
                total_call_count[call_count] = [1, [i]]
        else:
                total_call_count[call_count][0] += 1
                total_call_count[call_count][1].append(i)
        print(n, i, call_count)
    ## get lowest/highest idx,
    idx_call_count = total_call_count.keys()
    print(idx_call_count)
    idx_call_count.sort()
    highest_count_idx = idx_call_count[-1]
    lowest_count_idx = idx_call_count[0]
    # get highest idx
    flag_char = total_call_count[highest_count_idx][1][0]
    flag += flag_char
    print(n, total_call_count, highest_count_idx, flag)

import struct, os, time, threading, urllib, requests, ctypes, base64
from Cryptodome.Cipher import AES, ARC4
from Cryptodome.Hash import SHA

infile = 'flag'
encfile = 'orig.CRYPTED'
keyfile = 'keyfile'
sz = 1024
bs = 16

def decrypt_request():
    pcap_req = ("35998fdb7fe3b7940b9375a68a654ff949c58dcb9b1aebb048d6aa74d905b7b0c6e04b404eb61129f92" +
      "ad912703850201582ce39e77bfe739fec528741b202f8923a9f8d6303617d8e6e35a0d644115e238522c6d0cacd1afdae23050452c998e39a")
    _hash_chksum = pcap_req[:40]
    _hash_content = pcap_req[40:]
    dec = ARC4.new(_hash_chksum.decode('hex'))
    return dec.decrypt(_hash_content.decode('hex'))
    # 'id=d1&key=2f87011fadc6c2f7376117867621b606&iv=95bc0ed56ab0e730b64cce91c9fe9390'

def generate_keyfile():
    # n = hex(ord(id) + bs)
    n = hex(ord('d1'.decode('hex')) + 16)
    iv = "95bc0ed56ab0e730b64cce91c9fe9390".decode('hex')
    key = "2f87011fadc6c2f7376117867621b606".decode('hex')

    key = ''.join((chr(ord(x) ^ int(n, 16)) for x in key))
    iv = ''.join((chr(ord(y) ^ int(n, 16)) for y in iv))
    keyfile = open("keyfile", "wb")
    keyfile.write(key + iv)
    keyfile.close()
    print(n, iv, key)
    return True

def decrypt():
    global keyfile
    key = ''
    iv = ''
    if not os.path.exists(encfile):
        exit(0)
    while True:
        time.sleep(10)
        if os.path.exists(keyfile):
            keyin = open(keyfile, 'rb')
            key = keyin.read(bs)
            iv = keyin.read(bs)
            if len(key) != 0 and len(iv) != 0:
                aes = AES.new(key, AES.MODE_CBC, iv)
                fin = open(encfile, 'r')
                fsz = struct.unpack('<H', fin.read(struct.calcsize('<H')))[0]
                fout = open(infile, 'w')
                fin.seek(2, 0)
                while True:
                    data = fin.read(sz)
                    n = len(data)
                    if n == 0:
                        break
                    decrypted = aes.decrypt(data)
                    n = len(decrypted)
                    if fsz > n:
                        fout.write(decrypted)
                    else:
                        fout.write(decrypted[:fsz])
                    fsz -= n

                fin.close()
                os.remove(encfile)
                break

print(decrypt_request())
generate_keyfile()
decrypt()

# ----Trend Micro CTF 2018. Flag for this challenge is: TMCTF{MJB1200}

$ cat OceanOfSockets.py

...
def request():
    try:
        connection = httplib.HTTPConnection(sys.argv[1], sys.argv[2])
        connection.request('GET', '/tmctf.html')
        resTMCF = connection.getresponse()
        readData = resTMCF.read()
        if 'OceanOfSockets' in readData:
            headers = {'User-Agent': 'Mozilla Firefox, Edge/12',
             'Content-type': 'text/html',
             'Cookie': '%|r%uL5bbA0F?5bC0E9b0_4b2?N'}
            connection.request('GET', '/index.html', '', headers)
        else:
            sys.exit(0)
    except:
        pass
...

>>> [chr((ord(i) + 47)) for i in '%|r%uL5bbA0F?5bC0E9b0_4b2?N']
['T', '\xab', '\xa1', 'T', '\xa4', '{', 'd', '\x91', '\x91', 'p', '_', 'u', 'n', 'd', '\x91', 'r', '_', 't', 'h', '\x91', '_', '\x8e', 'c', '\x91', 'a', 'n', '}']

>>> [chr((ord(i) + 47) % 0x5e) for i in '%|r%uL5bbA0F?5bC0E9b0_4b2?N']
['T', 'M', 'C', 'T', 'F', '\x1d', '\x06', '3', '3', '\x12', '\x01', '\x17', '\x10', '\x06', '3', '\x14', '\x01', '\x16', '\n', '3', '\x01', '0', '\x05', '3', '\x03', '\x10', '\x1f']