Challenges for October 2018 SecTalks London

Last month I ran another round of London SecTalks CTF.

There were 10 challenges, and the winner got 9/10 of them during the event (and last one on the following weekend), so difficulty level was about right.

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, May 2018, and July 2018 CTFs.

Archive (5 points)

A small variant of the 16-level 16-way nested archive, to test for basic Unix scripting skills. This time using RAR.

MonoRSA (10 points)

It's RSA-encrypted message, but it uses only one prime, not two. This is extremely insecure, and can be trivially broken, but you still need to do some math.

BCRYPT (15 points)

Each letter of the flag was encrypted with bcrypt. It's easy to break, as long as you know how bcrypt works, which isn't quite the same as plain hashes, so it was causing a bit of confusion.

RSA RNG (20 points)

It's Debian weak RSA key attack all over. We have target's public key and encrypted message. Also a lot of other people's public keys, all generated using same bad RNG. If two keys share a prime it's easy to break them, even if direct factoring isn't viable.

Python (25 points)

Small bit of reverse engineering - small Python password validator obfuscated with one of online tools for it.

MultiRSA (30 points)

A little known fact about RSA is that it works just fine with more than two primes. So this challenge uses 16 - which is fine, except key size was not adjusted appropriately, so each of those primes is small enough to break it.

Binary (35 points)

Binary password validator, provided in two versions (Linux and OSX) for convenience. It was compiled with -O3 which made its encrypting loops unroll, and what was very trivial code turned into hard to understand vectorized mess. Then again, actually running the program might reveal something interesting.

SVG XOR (40 points)

The flag is written in SVG flag, which was encrypted with a XOR cipherer. This turned out to be quite easy, as SVG files have a lot of structure which can be used to attack this.

FPGA (45 points)

Probably the most original challenge this time. A netlist of NAND gates which can validate the flag and tiny emulator were provided. Can you figure out the inputs necessary to get the validator to accept?

This was the only challenge without anyone solving it during the event, but there were some solutions afterwards.

Tweets (50 points)

A flag was hidden in collection of Donald Trump's tweets, encrypted with monoalphabetic cipher. Great exercise for frequency analysis.