bookmark_borderUpcoming blockchain CTF games

Cryptocurrency is the hype and we are ready with our partners to publish the next CryptoCurrency is The Flag games. For the 5th and 6th time, paricitpants will have the chance to win real Ethereum and HODLbag tokens while learning about cryptocurrency, information security and hacking.

The upcoming events will be organized online and the participation is open for everyone local or remote. Like last year, a new guide for beginners will be provided, so don’t worry if you are just getting started! You are here to improve yourself!

If you want to have a look at past challenges, you can read the writeups written by previous CCTF participant:

BIPolognese

Don’t be eval

Pwncoin Challenge

Foxy

EVM bytes


CCTF Vol5 is focusing more on beginners, so don’t miss the chance. If you join the conference you will have extended rewards!

Live during BDAY 4.0 || 2021.May.18.

https://blockchainbudapest.hu/


Feel like a 1337 haxor? Prove yourself with the hardest crypto* challenges during BsidesBud and win the local prizes too!

CCTF Vol6 during BsidesBUD 2021 || April 27.

https://2021.bsidesbud.com/


Get ready and HODL yourself!

bookmark_borderCCTF4 Hacktivity Writeups 3. (Final)

Foxy challenge

The data given was printable ASCII, which implied a fair chance that it was encrypted (or obfuscated) with a cipher that always outputs such characters. Two obvious suggestions come to mind: base64 and rot13. However, the ciphertext didn’t exactly look like any mainstream base64 output, nor the rot13 of anything reasonable; what other similar ciphers (or encodings) are there?

The key giveaway was the hint of “!47 -> 42”. The main part of the solution was to take the rot42 of the reverse rot47 of the data. This produced what looked like base64 output. It was the base64 of the flag.

Author: Mr. SI

Ethereum VM bytecode challenge

The task was to uncover the flag from a thing that looked like 0x6080… To those familiar with Ethereum smart contract programming, this thing is obviously Ethereum VM bytecode; for others, as a starting point: the problem said “sometimes the code is 404”.

Decompiling the bytecode using an Ethereum VM decompiler <https://ethervm.io/decompile>, we could discover the following:

1. The constructor is uninteresting, it just sets up the contract’s long-term code.
2. The reverse engineered long-term code contains:
function getflag() {
storage[0x01] = 0x0b47326dc54f49d6f674;
}
which looks like a giveaway.

But actually “0x0b47326dc54f49d6f674” wasn’t accepted as the flag. Unfortunately, the bytes therein don’t appear to encode anything sensible either. However:

3. The rest of the long-term code is also not really interesting: it has methods to plainly store and retrieve data.
4. There was no inclination about any deployments of this smart contract.

So one ought to have been baffled: the flag must really be somehow in that outstanding constant. It turns out that the number 0x0b47326dc54f49d6f674 was indeed the flag, but the system accepted it only in decimal format.

Author: Mr. SI