Misc 2017 CTF | 343C Writeup

In this challenge we downloaded a .ogg file. If you opened this OGG file with Audacity, a lady saying apparently random numbers will give you a long string.

I wrote down the string (and graduated, married, had kids and retired in the meantime) and this is what I got left:

76 83 48 116 76 105 52 103 76 83 48 116 76 83 52 103 76 105 48 117 76 83 65 116 76 83 48 116 76 105 65 116 76 105 48 117 76 83 52 103 76 83 48 116 76 83 48 103 76 83 52 117 76 83 65 117 76 83 52 117 73 67 48 117 76 83 52 116 76 105 65 116 76 83 48 117 73 67 48 116 76 83 48 116 73 67 48 117 76 83 52 116 76 105 65 116 76 83 48 116 76 83 65 116 76 105 52 116 73 67 52 116 76 105 52 78 67 103 61 61

So at first it was confusing, and due to some difficulties with the audio, I couldn't understand some numbers. Specially the three, so my awesome teammate helped me decode it.

Now you see, at the end of this string you can see '61 61'. In the ASCII table 61 in decimal is =, so 61 61 would be ==. After changing every number to its decimal counterpart, we got left with this:


LS0tLi4gLS0tLS4gLi0uLSAtLS0tLiAtLi0uLS4gLS0tLS0gLS4uLSAuLS4uIC0uLS4tLiAtLS0uIC0tLS0tIC0uLS4tLiAtLS0tLSAtLi4tIC4tLi4NCg==

Hmm, where have I seen that before? Right! A base64 encoding.

Weird string right? Well, yes. But once again, my teammate said 'hey, that looks like morse code!'. Yes it does!

After decoding the base64, the string in morse code looked like this:

---.. ----. .-.- ----. -.-.-. ----- -..- .-.. -.-.-. ---. ----- -.-.-. ----- -..- .-..

But that morse string gave the flag 89<AA>9#0XL##0#0XL, which was not right.

Strange.
Once again, what if the dots got turned to hyphens and the hyphens to dots?

...-- ....- -.-. ....- .-.-.- ..... .--. -.-- .-.-.- ...- ..... .-.-.- ..... .--. -.--

That gave us 34C4.5PY.V5.5PY


Yay!