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30Apr/12Off

PlaidCTF 2012 – Practical Packets 250 – 80s Thinking Writeup

In this challenge we get a sound wave file named 80s.

Listening to it will immediately bring back some memories if you are in your thirties: DTMF dialing, short modem handshake, data.

The handshake is very short so I guestimated the speed to be 9600 bauds also the overall sound of it was screaming in my brain: "OMG it's a fax!!!1!!"

So it's very simple, it's a recorded fax transfer and you need to decode it to get the fax pages.

This challenge depends very much of your Google-fu. There is many commercial softwares that can decode fax recordings, most for lawful interception purposes. But hopefully, there is a known opensource alternative which is popular in the Asterisk community as the core fax library: SpanDSP

It's a huge library that does many things beside fax decoding so you will need some time to find what you need: a unit test-case for fax decoding!

From there, it's trival, you massage the sound file a bit to 8000Hz and the test-case output a nice TIF file:

The key is: BlastFromThePast^_^

PS: output from fax_decode: fax_decode

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