This time we’re going to look at quite interesting challenge from 34C3 Junior CTF called spi, with a little bit of backstory.

• Category: misc
• Difficulty: easy
• Points: 70

We are provided only with a audio file called spi_zahlensender.

Cryptic message

Looking at the file firstly does not seem to tell us anything interesting. Just repeating numbers told by a woman. But in order to understand this task more, I decided to make quick DuckDuckGo search looking for something connected to the file name zahlensender.

And there it is, a Wiki article about Number Stations. We can quickly learn that such stations transmited messages just like in our challenge. Another great article to read about Number Stations can be read on Crypto Museum.

As I read more about this topic, I decided it’s essential to write down those numbers. But who would want to manually write down numbers for more than 2 minutes. That’s why I decided to use this tool, which made most of the work for me.

Now it was time to correct some mistakes and convert these numbers. Here’s the final form.

Most of the numbers were in ASCII range, so I decided to decode each of them.

Base64! Let’s run another round of decoding.

Now we have Morse Code. But no online service could not read this, stating errors in the message. That’s were I started to look for mistakes in writing down numbers from audio, ASCII conversion. But after a while, on Wiki page of Morse Code, I found out that dots and dashes are swapped.

As we know the flag starts with 3, so I looked for Morse Code representation of this number, which is ...--. Now we can swap these characters.

Which after decoding shows the flag 34C4.5PY.V5.5PY.

Keep learning and stay safe! ~ W3ndige