ØxOPOSɆC Summer Challenge 2020 | Misc Challenges Write-up

Inês Martins -

1 . My Paste | 100

Oh no! I forgot where I left my paste. 🗑
Perhaps you can help me find it?

The source code has a little hidden clue:

<span class="challenge-desc"><br>
Oh no! I forgot where I left my paste.  🗑
<p>Perhaps you can help me find it?</p>
<p style="color: white">2WvYM5Qx</p></span>

Since 2WvYM5Qx is not the correct flag, it was a matter of thinking outside the box to get to the final answer:  https://pastebin.com/2WvYM5Qx.

Good job!
flag{f1n0_5Up3r_b0ck_fr3squ1nho}

2. Slowly But Surely | 200

I think aliens may be trying to communicate.
What are they trying to say?

Let's take a look at some basic info about the trasmission.wav file using mediainfo:

~ mediainfo transmission.wav

General
Complete name                            : transmission.wav
Format                                   : Wave
File size                                : 10.6 MiB
Duration                                 : 1 min 56 s
Overall bit rate mode                    : Constant
Overall bit rate                         : 768 kb/s

Audio
Format                                   : PCM
Format settings                          : Little / Signed
Codec ID                                 : 1
Duration                                 : 1 min 56 s
Bit rate mode                            : Constant
Bit rate                                 : 768 kb/s
Channel(s)                               : 1 channel
Sampling rate                            : 48.0 kHz
Bit depth                                : 16 bits
Stream size                              : 10.6 MiB (100%)

After discarding the usual audio steganography I decided to take a look at the audio's spectrogram using Sonic Visualiser.

Because I'm not an expert on audio signals, I decided to Google Image search for signals that looked similar to the one shown on the spectrogram. It didn't take long until I came across this write-up with an eerily similar signal, that corresponds to a SSTV (Slow Scan TeleVision) transmission.

Slow Scan television (SSTV) is a picture transmission method used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or color.

This plus the hint let me know I was on the right track (however I later found out I should really be using sigid to identify the signal).

After spending a lot of time trying to make some Linux tools work for this task I had to use Windows RX-SSTV to finally decode the message:

3. Run (300)

This was the only challenge I wasn't able to finish. It turns out this image is actually a program "written" in the the Piet esolang.

Piet is a stack-basedesoteric programming language in which programs look like abstract paintings. It uses 20 colors, of which 18 are related cyclically through a lightness cycle and a hue cycle. [Source]

There's a handy utility called npiet that we could use to get the flag from the file (I had to convert it to PPM first).

Helpful Sources