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beep64

Warmup, DTMF, T9, base64

Challange starts with .wav that are around 30 min long.

After playing it it sounds like old analog phones that were in used in early 2000s Something like that:

They used DTMF encoding for each key in keyboard usually 0-9 * #

To confirm that Signal spectrum can be analized to read what frequencies are used.

That can be done in Audacity by using Plot Spectrum that results in following graph:

Plot spectrum of sine.wav

After analyzing the graph peak frequencies they are

  • 697Hz

  • 770Hz

  • 852Hz

  • 941Hz

  • 1209Hz

  • 1336Hz

  • 1477Hz

That are DTMF frequencies!

DTMF stands for Dual-tone multi-frequency signaling for example pressing key 1 resulted in wavefrom something like that

The best part of it that frequencies are in hearing range because they are in 697-1633Hz range.

More info can be found in wikipedia:

LogoDTMF signalingWikipediachevron-right

To decode what key are being pressend I used following dtmf decoder from github:

LogoGitHub - ribt/dtmf-decoder: Extract phone numbers from an audio recording of the dial tones.GitHubchevron-right

That outputs log string of digits separeted with * sign

Numbers sometimes repeat, but no more than 4times. This looks like T9 dictionary

T9 is old-school way of inputing characters to mobile phones before they transformed to smartphones

nokia 3310 with t9 dict

So signle 8 is letter t

44 transorm to h

33 to e

333 to f

First four characters are the f

For making it less cumbersome to work with it I removed * characters

Next used online decoder from T9 to text

LogoMr. Matt .Netmrmatt.netchevron-right

Which resulted in following string:

Which clearly is encoded in binary.

Replacing space with ZERO with 0 and ONE with 1 yelds

This decodes to base64

decoding from base64

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