zh3r0 CTF

Tic Tac Toe [472 pts]

Challenge Description

Challenge Description:

The given file is image

Running xxd image | tail gives:

xxd image | tail
00003480: ff30 2225 372a 3322 321d 1c13 241c 212b  .0"%7*3"2...$.!+
00003490: 1819 1e1a 2019 1416 1318 1412 140f 0f14  .... ...........
000034a0: 0f12 160e 1212 160f 1410 1517 130d 0b0a  ................
000034b0: 0c10 0e0b 0b0b 0a0b 090b 0b08 0a08 0a09  ................
000034c0: 0900 4300 dbff 3d6f 414e 5156 5862 3363  ..C...=oANQVXb3c
000034d0: 6b52 764e 4862 3130 6a64 2f67 3259 3046  kRvNHb10jd/g2Y0F
000034e0: 3264 7630 3262 6a35 535a 6956 4864 3139  2dv02bj5SZiVHd19
000034f0: 5765 7563 3364 3339 794c 364d 4863 3052  Weuc3d39yL6MHc0R
00003500: 4861 3e00 feff 0000 0100 0100 0001 0100  Ha>.............
00003510: 4649 464a 1000 e0ff d8ff                 FIFJ......

The last few bytes when reversed would give ffd8 ffe0 which is the magic number of JPEG files. This suggests that the bytes of the file are reversed. The bytes are then reversed using the following python script to get image.jpeg:

file = open('image', 'rb')
bytearr = list(file.read())
open('image.jpeg', 'wb').write(bytes(reversed(bytearr)))

The content of image.jpeg is

image

Running exiftool on the image reveals a base64 encoded comment aHR0cHM6Ly93d3cueW91dHViZS5jb20vd2F0Y2g/dj01bHNvRkc3bXVQNAo=

image

On decoding the base64 encoded comment, a link to a youtube video titled “Kapela - Rock My Way”. This suggests the use of RockYou.txt. Running stegcracker on the file reveals the password spongebob

image

The cracked file contains:

In [35]: n,e,ct,p+q
Out[35]: 
(156935655500198733255923805969370297538115753312746380213875723177744608509780722798549730106834861986575848272630355804840179947615966722051370804273521733290376009020885919941338141950993008276537987193794648055241515380150115338397065198086893695560540379329063476893211153270247222670504019722793971516489,
 65537,
102778142076243116117419062640171713879684005471846556860689446479305435562766590357152362175278713093609670819423506015563433111872029023117856369287465874159889936283732420732086482645886112577942492103417960605158427793203017078930148395937563028135853490687072326149444788825363901282252753328289332801180,
25089219254058723086004960979954103479984362695038160907003438818016936688465630366701002710571334149929206994096775851785636272938202242921638312612784566)`

This is classic RSA.

\[\begin{align} \phi &= (p - 1) \cdot (q - 1) \\ &= pq - (p + q) + 1 \\ &= n - (p + q) + 1 \\ \end{align}\]

Now we have n, e and phi. So, the cipher-text can be deciphered even without knowing the individual values of p and q.

Decoding the cipher-text gives the flag zh3r0{W0ah_Y0u_W0n_k33p_1t_uP}