[Vinavil]

— Community CTF #03 —
Find the Flag

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FIRST 3 WINNERS WILL BE ANNOUNCED IN THIS THREAD

🏆 HerrWinston @loadman@

Only 2 winners for this challenge.

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This is a reverse engineering challenge created and managed by @Vinavil@. Download the binary below and find the hidden flag.

Flag format: vibe{...}

Difficulty: Medium

Official Hint: The key isn't what it seems. Look closer.

Rules: Do NOT share hints, clues, or the answer in this thread. PM Vinavil directly with the correct flag. First 3 correct submissions win.

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Download: crackme_03

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Official Walkthrough

1. Open Ghidra, create a new project and import the binary. Let auto-analysis run with default settings. Since the binary is stripped, navigate manually through Functions in the Symbol Tree and find the one calling printf and fgets.

2. In the decompiler you will find a validation loop. The input bytes are being XOR'd against a value before being compared to an encoded array stored in the binary. Your first instinct will be to grab that value and use it as the key that is the wrong move.

3. Look closer. The XOR value is not a constant. It comes from a separate function that gets called on every iteration with the current index as argument. Click into that function and read it carefully. It performs a small arithmetic operation using three values combined together, one of which changes based on the index. This means every byte of the flag is encrypted with a different key.

4. Once you fully understand the key derivation logic, locate the encoded byte array in the Listing view. It is 30 bytes long.

5. Replicate the key derivation logic yourself in Python, CyberChef, or even by hand. Apply it to each byte of the encoded array. The output is the flag.

6. Submit to Vinavil via PM.

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This is a step up from CTF #02. More challenges are coming. Good luck.
Also a special thanks to 7 for making this possible!

[Hackers]

Get the submissions in, you don't want to be #4+ !

[InspectorGadget]

Sadly I'm at work for another 3 hours so people are probably going to beat me :(

To accommodate timezones, perhaps instead of 3 'fastest first ' winners, you could allow all winners within 24 hours and rank them in order of flag submission.

Should provide an incentive for people who can't check for the thread at the same time on most days. ❤️

[Money-]

I want more web challenges!

[7]

I want more web challenges!

Your wish is Vibe's command!

More web challenges Vibe!

[Vinavil]

Your wish is Vibe's command!

More web challenges Vibe!

On the way!

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Sadly I'm at work for another 3 hours so people are probably going to beat me :(

To accommodate timezones, perhaps instead of 3 'fastest first ' winners, you could allow all winners within 24 hours and rank them in order of flag submission.

Should provide an incentive for people who can't check for the thread at the same time on most days. ❤️

I'll work on a public sharable dashboard, that sound cool right? LOL

[Au Jus Demon]

On the way!

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I'll work on a public sharable dashboard, that sound cool right? LOL

Could end up making something like the H4CK3R$ have with their missions.

[HerrWinston]

I have sent you the full explanation and final answer, hopefully everything is right and understandable, great night and awesome CTF !!

[Pοιsonlεaf]

I like these but as I told Hackers yesterday, I doubt I'm doing these as intended xD
idk I'm more about doing it reverse engi on the source code with python.

[InspectorGadget]

I like these but as I told Hackers yesterday, I doubt I'm doing these as intended xD
idk I'm more about doing it reverse engi on the source code with python.

The way I see it, if you get the flag, then it doesn't matter if it's as intended.

[Vinavil]

Still haven't got 3 answers keep trying guys

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In 24 Hours i will be posting the only winner of this Challenge!

[loadman]

link is dead ?

[Biggy_Asian]

This content is no longer available.

[Hackers]

link is dead ?

This content is no longer available.

You can DL challenge 3 here:
https://mega.nz/file/pillBJqA#C6shF2xmdL...s8cqy9N1Nc

[Vinavil]

Link expired due to challenge closing. In the next hour ill be announcing the winners.