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Wednesday, August 01, 2018

Challenges for July 2018 SecTalks London

Cat sitting on a desk by from flickr (PD)

Last month I ran another round of London SecTalks CTF.

I only created 6/9 challenges this time, 3 Android challenges were created by imhotep.

Challenge files and code used to generate them available on github.

There are no answers below, but some serious hints which might make it too easy.

For previous rounds, see posts about September 2017, November 2017 CTFs, and May 2018.

Hidden treasure (5 points)

As per tradition, there was a bonus challenge with a zip bomb. This time at the end of 16-deep archive with 16 branches each there were pictures with a treasure, with EXIF tags containing either the flag or information that you failed.

Android 1 (10 points)

The simplest Android challenge wasn't even all that Android specific - as the flag was hidden directly inside the .apk.

RE1 (15 points)

Easy reverse engineering challenge was just a 32-bit Linux binary, which verified passed flag character by character, in random order.

Disassembling it, you'd get instructions like:

cmpb   $0x74,0x10(%ebx)
jne    80484b7

If you arranged them correctly, you'd see the flag. Or you could just grep for all characters, and use anagram solver - apparently that was an option too.

Android 2 (20 points)

This one required actually running the app, or fairly complex static analysis. The app saved a file with the flag to device storage. If you can find the file, the flag is yours.

RSA (25 points)

This challenge was actually quite realistic.
Bob sent a message we need to decrypt.

Probably due to bad RNG, it looks like Bob and Alice picked same N for their keys, and we managed to steal Alice's private key as well.

Perhaps there's a way to take advantage of this.
And there is a way. Having private key (n, e, d) there's algorithm to factor n into p, q. It's not completely obvious, but it's fairly short to implement. With p, q (shared between both), and Bob's e, you can trivially get Bob's d. Then you can decrypt the message.

RE2 (30 points)

It was a slightly harder version of reverse engineering challenge. The only difference was that flag was encoded into Base64 before checking, which could throw people off, but binary wasn't stripped so b64_encode method was a massive hint.

Android 3 (35 points)

The challenge app is making HTTP request, which needs to be intercepted and modified to get the flag from the server.

Once you get the request it's obvious what to do, and there were hints provided how to setup proxy on Android to get them.

CTR (40 points)

Another somewhat realistic challenge. A collection of Elon Musk's tweets and a flag was encrypted using AES-128 in CTR mode. Reusing same IV for all of them.

This turns CTR mode into XOR with random keystream, and breaking XOR cipher is fairly basic. It turns out nobody remembered mode names, so I had to give people some hints before they tried CBC or CFB attacks on it.

Croatian Monoalphabetic (50 points)

This challenge was worth the most points, but in principle it was very easy, just a bit time-consuming - and I ended up giving people hints how to approach that.

It's just statistical analysis of another Latin script language. So you can start by assuming space is the most common character, then use character frequency tables for Croatian, or lists of most common words, or take advantage of the fact that flag (very long word) is embedded inside.

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