Finding Goku

This is a writeup of the Crypto challenge Finding Goku by apoorvCTF

Points: 372

Premise

Multiple Gokus appear, but only one is real. Spot subtle differences in power levels, speech, or battle knowledge to find the true one. Beware of trickster illusions and deceptive clones trying to mislead you. Use logic, observation, and special hints to uncover the real Saiyan warrior!

nc chals1.apoorvctf.xyz 5002

Challenge files:

challenge.py

Observations

Looking at the challenge file, we see a program that runs through a few checks with two hex strings. These strings need to meet the following criteria to give us the flag:

• They cant be identical
• They have to be valid hex data
• They need to have the characters GOKU at the start of them
• Their md5 signatures must match

If we can achieve all these criteria, we’ll have a flag.

To do this, we can use a tool like hashclashes md5_fastcoll, which generates md5 collisions for us, and do so with some hex data that starts with the desired stirng, GOKU

The two hexes I generated with this method were the following: Hex 1:

474f4b55000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000f25c50622e32b4a66ed61fa6c2635ad3876b36a0ae4e4ca632e27a2d081ec0eb65e503ec383f8b52e7b96c3b3f184e253c22fd51aaa3852d919ab55eb68ddf5aa6a743114a8a6ca3dddf9a199b4fd5995e275d3b07fe401ad91bf3cc587259ae8b21dee129496386c5459d0f88fa5666ec7bd4b3c2c8dbf3ac13007ce7295b7b

Hex 2:

474f4b55000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000f25c50622e32b4a66ed61fa6c2635ad3876b3620ae4e4ca632e27a2d081ec0eb65e503ec383f8b52e7b96c3b3f984e253c22fd51aaa3852d919ab5deb68ddf5aa6a743114a8a6ca3dddf9a199b4fd5995e275dbb07fe401ad91bf3cc587259ae8b21dee129496386c5459d0f887a5666ec7bd4b3c2c8dbf3ac1300fce7295b7b

Solution

Sending these two hexes to the challenge instance gives us the flag

apoorvctf{y0u_f0und_7h3_r34l_g0ku_hurr4yyy}

Tools and sources used:

• hashclashes
• md5sum