Ph0wn 2018 - Healing the Toothbrush 1

Category: Reverse

Challenge description

This challenge was based on the famous smart toothbrush of @cryptax / Axelle Apvrille!

Ph0wn aliens have abducted my smart toothbrush. My toothbrush has hidden this horrible episode far in its subconscious mind, but I know encrypted memories of the event are still there… My psychiatrist tells me I need to get my toothbrush talk, that it will help it heal.

To do so, the psychiatrist advises, as a first step, to find the decryption key to those events. This key is hidden within the official Android application Beam_v1.3.3_apkpure.com.apk (sha256: df8956a138a05230fb26be27a22dc767775b55b1d2250be25aa899c8bbee53b9).

My psychiatrist provides the following information:

• The toothbrush uses Bluetooth Low Energy to communicate.
• It is useless to understand the entire application. You should concentrate on what handles toothbrush events. The class for those events is called BrushEvent.
• Is my toothbrush really smart?

Important :

• In this stage, you do not need the smart toothbrush.
• Do not try to install the Android application on your smartphone: it is not malicious but it requires a client login (which you don’t have) to operate, so it will be useless to run it…
• Please do not connect to the toothbrush via Bluetooth, it may cause service disruptions for stage 2. And it won’t help for stage 1. Actually, if you don’t need Bluetooth on your smartphone or laptop, we recommend you disable it.
• For this stage, you need to flag ph0wn{hexstring of encryption key}. The encryption key is required to complete stage 2.

Author: cryptax

Challenge resolution

We used the usual unzip, dex2jar and JD-GUI/Procyon tools to reverse the Java code of the app.

We were told to look for the BrushEvent class. There was a false lead with hexadecimal values that looked promising but were actually client_id and client_secret values for OAuth.

We then decided to simply look for “crypt” in all the files and indeed it was a good idea

As a bonus we found the exact encryption algorithm: AES, in ECB mode, without padding. We kept this in mind for the second stage.

Author: Clément Notin | @cnotin

Post date: 2018-12-16