Note: This post is just about the how I reverse engineered the tool, for info on how to use it, you want this post.

While poking around the development and debugging features on Samsung handsets I found the ability to run IMS Debugging directly from the handset.

Alas, the option is only available in the commercial version, it’s just there for carriers, and requires a One Time Password to unlock.

When tapping on the option a challenge is generated with a key.

Interestingly I noticed that the key changes each time and can reject you even in aeroplane mode, suggesting the authentication happens client side.

This left me thinking – If the authentication happens client side, then the App has to know what the valid password for the key shown is…

Some research revealed you can pull APKs off an Android phone, so I downloaded a utility called “APK Extractor” from the Play store, and used it to extract the Samsung Sysdump utility.

So now I was armed with the APK on my local machine, the next step was to see if I could decompile the APK back into source code.

Some Googling found me an online APK decompiler, which I fed the compiled APK file and got back the source code.

I did some poking around inside the source code, and then I found an interesting directory:

Here’s a screenshot of the vanilla code that came out of the app.

I’m not a Java expert, but even I could see the “CheckOTP” function and understand that that’s what validates the One Time Passwords.

The while loop threw me a little – until I read through the rest of the code; the “key” in the popup box is actually a text string representing the current UNIX timestamp down to the minute level. The correct password is an operation done on the “key”, however the CheckOTP function doesn’t know the challenge key, but has the current time, so generates a challenge key for each timestamp back a few minutes and a few minutes into the future.

I modified the code slightly to allow me to enter the presented “key” and get the correct password back. It’s worth noting you need to act quickly, enter the “key” and enter the response within a minute or so.

In the end I’ve posted the code on an online Java compiler,

Generate OTP Response from Key (Challenge)

Replace yy182 with your challenge. I suggest you try the 0 offset and type it in quickly.

I did a write up on how to use the features this unlocks in this post.

If you’re leaning about VoLTE & IMS networks, or building your own, I’d suggest checking out my other posts on the topic.