BSides Leeds Challenge Flag Guide

One of the many many awesome things @LargeCardinal and co did for BSidesLeeds this year was a challenge to collect as many hidden flags as possible. Sadly nobody reported collecting any flags on the day (probably because there were so many other things for people to do and see) so they decided of offer an additional week for people to find them.

I found out at the closing ceremony that not only had I found a flag, because I didn’t recognise it as such I’d displayed it all lunchtime to the entire dining room and also unwittingly tweeted it out. I decided as I’d stumbled across 1 flag, I’d spend my weekend looking for the remaining ones.

Here’s where I found them.

Flag 1 – “The Programme Polyglot one”

The day before the event, the BSides Leeds website was updated with a link to the event programme, When viewed as a PDF the file works as expected, but if you download it and run strings against it, it reveals some interesting secrets

Hum, an interesting couple of filenames found after the %%EOF marker that normally indicates the end of a PDF.

Those people at the BSides Leeds 2018 closing ceremony with exceptional memories may remember that the challenge prize was POC || GTFO , which I was lucky enough to already own. However, I remembered one of the articles in there was was on a ZIP/PDF polyglot.

Unlike last time I came across this puzzle, where I wrote a lot of code to extract the zip file from the pdf, I now knew the true power of a polyglot file format and just unzipped it and viewed the contents to get the first flag!

Flag 2 – The Bandersnatch One

So, whilst the obvious intention was to find it via the programme, LargeCardinal had obviously realised nobody was looking for Flag 2, so tweeted out the link.

The website was obviously riffing on the excellent Netflix Black Mirror “Choose Your Own Adventure” retro gaming episode Bandersnatch, but seemed to have no hidden features, so instead I started examining the linked .wav file.

Now, whilst the BBC Master and classic Microvitec CUB monitor I’d spotted downstairs and the fact Tom Hargreaves was talking about Acorn Hacking, should have been an obvious clue, I genuinely recognised the wav file as probably a BBC B tape image from it’s sound alone (whilst most 8 bit micros had similar tape formats the all had distinctive “noises”).

Over lunch I returned to the BBC Master (the enhanced version of the BBC C) disconnected the mp3 player that was connected to the tape port and hooked up the headphone socket on the MacBook.

Despite it being nearly 3 decades since I’d last used a real “Beeb”, muscle memory took over and all it took was

Ctrk+break (hard reset the machine*)
*TAPE (disable the default floppy disk system and instead use the tape input)
*LOAD “” (load from tape, but do not run)

Once I’d actually loaded the software, I realised that CHAIN “” (load and run) would have been more efficient, but it was job done, regardless.

The software then loaded this amazing image and some wonderful 8-bit tunes from the tape.

Now, what I hadn’t realise when I tweeted this picture and left in on display to everyone over lunch, was the the Herman Houser quote above the owl was in { braces } and there actually a flag!

You can see and here the full thing in all it’s glory at

*The speed at which is rebooted had my colleagues believing it was only a BBC Master case, but had much more modern hardware inside. The didn’t realise that in the 8-bit world sub-second boot times were the norm

Flag 3 – “The first BLE One”

This years conference badge was designed to house an ESP32 LoRa dev board. As SBG were doing a badge flashing station, I’d seen an early version of the software, but it seems LargeCardinal added a bunch of hidden features and most importantly FLAGS in the version we didn’t get until the very last minute.

The firmware was available from https://github.com/unprovable/BSidesLeeds2019
and from the filenames and sizes it looked like bsides-bade.ino.bin was likely to be the most interesting, so as normal I started with strings and immediately hit some interesting stuff.

So, by just hooking the badge up to a terminal by doing screen /dev/ttyUSB0 115200 and using the chat functions of the software, a number of the strings we found were triggered, but as few more stood out as NOT things we’d seen on screen including

  • deadbeef-1337-h44x-f1a9-b51d3sb1ef1g
  • HiddenFlagIsNotAFlag
  • Bluebird
  • beb5483e-36e1-4688-b7f5-ea07361b26a8
  • SNEK
  • SNAAAAKE!
  • LargeCardinalFeelsBlue
  • … and something referring to Bluebird/Blue stuff.

It also showed the output of the help command

Supported Commands:
? – this message…
n – change Tx nickname…
d – print Tx nickname…
c – [TODO – put what c does here…]

The first two I discounted as intentional red herrings, the fourth one a little googling told me was a common string used in Bluetooth low energy (BLE). So lots of Blue references and 2 Snakes.

It didn’t take long to realise that typing either SNEK or SNAAAKE! caused the badges of everyone in radio range to turn into a game of snake! Very very cool (and quite annoying I’m sure) but ultimately, no flag.

So, one of the features of the board is it’s a BLE transmitter and we have a load of Blue reference, so l decided try and find something to start it.

/c (as hinted at in the help) doesn’t seem to do much, but some experimenting showed /b outputted a different error message to other commands and a little more experimentation led me to trying

/b LargeCardinalFeelsBlue

which worked and started a BLE server

Now, as the only BLE stuff I had handy was a BlueFruit board that wasn’t soldered up, I took a bit of a short cut and purchased an iPhone app the scanned for BLE. It was money well spent as I got the following

I have to admit, it took me far too long to realise that Blue DA BA D33 DA BA DA 1 was a reference to the Eiffel 65 tune

Flag 3 – “The Second BLE one”

I also have to admit that whilst I was one click away from the next flag, I missed it for quite some time. If once you discovered the BLE Service, you attempted to connect to it an enumerated the services, another key would jump out at you.

Flag 4 – “The Easy One”

A couple of people had mentioned on twitter that they found “the easy flag”, but do far I’d not found a flag that stood out as particularly easy.

It took me far far too long to notice there was one command on the badge I’d missed.

Simply entering /flag (or even just /f) rewarded you with {Easiest-flag-ever}

Flag 5 – “The One that nearly sent me blind”

On the day a few people noticed some tiny black on black writing on the badge and text on the back that hinted towards an XORing against a hex key

The text was incredibly hard to read by a old guy like me, but after trying dozens of light sources, magnifying glasses, etchings, high res cameras, play-doh and pestering family members to have a look, I eventually caught the light at just the right angle to give me a hex string, when when XORed against 0x5ca1ab1e give the final flag!

Summary

I can’t finish this without a massive Thank You to LargeCardinal and all his helpers, the challenge was very very fun and was very much the cherry on top of an awesome con.

Additional

LargeCardinal has told me I missed a flag, not just me, but EVERYONE did. Apparently the Black Badges worn by Mark and award to Chloe and Emlyn for their help over the last 2 years, actually had a different challenge on them to the white badges, but nobody thought to check them!

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