2020: rgbctf: lo-fi

2020/rgbctf/lofi/README.md

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+# Lo-Fi
+
+writeup by [haskal](https://awoo.systems) for [BLÅHAJ](https://blahaj.awoo.systems)
+
+**Forensics/OSINT**
+**499 points**
+**7 solves**
+**(BLÅHAJ got 1st solve 🦈📈)**
+
+>I made a "inspirational" lo-fi track! ft. Alec's very smooth aussie accent
+>
+>You know, "Just do it", "Don't let your dreams be ones and zeroes", I'm pretty sure he says
+>something along those lines... all inspirational sayings mean the same thing anyway
+
+provided file: <lofi.wav>
+
+## writeup
+
+listening to the provided file, it's just a short lo-fi hip hop beat to study/chill/pwn to but at
+the very end you can notice the sound of encoding something in the spectrogram (it's a very
+distinctive sound). visualize the spectrogram to see this, here i used audacity.
+
+![an audio spectrogram displaying the encoded word "tinyurl"](spectrogram.png)
+
+this is all we get here, no full url yet. it's not going to be that easy :P. however from the flavor
+text we can deduce that the rest of the url might be encoded in this song as ones and zeroes. here i
+listened to the song again and noticed that in the second part, the synth notes are really off-beat,
+which is strange even for lo-fi. they don't seem to follow any sort of pattern. this must be where
+the encoded data is.
+
+here i go really jank low tech and take a gigantic w i d e screenshot of audacity and load it into
+glimpse to try to extract these notes. i overlaid a grid and tweaked it until it matched the pattern
+of synth notes in the waveform. it's important here to realize the synth notes are the ones that
+rise going from quiet to loud. the loud to quiet parts are the bass drum and the snare that make up
+the beat. therefore, look for triangles that point left, and ignore the ones that point right. if
+you listen to the song while doing this it's also more obvious where the notes are exactly. we find
+that the synth notes are triplets, with 6 intervals (6 bits) per measure.
+
+![the image as described, now the encoded bits are slightly clearer](grid.png)
+
+write down the bits, 1 if there's a note and 0 if there's no note, and then decode as 8-bit ascii.
+append this to `https://tinyurl.com/<data goes here>` and the resulting url contains the flag.
+
+## addendum
+
+i'm really surprised by how few solves this got. i guess it's kinda hard to notice what's going on
+unless you listen carefully and read the flavor text.