dex2jar VS comm

App tampering and repackaging can be performed by using reverse engineering or tampering tools, such as Apktool, dex2jar, etc.

A week ago I purchased a bluetooth device that takes some measurements. You require an Android or iOS application. The first thing the iOS app did was request permission for your location. Immediate fired up MITMproxy [1] running in transparent `--mode wireguard` and installed it's certificate in the iOS trust store. It was sending a whole bunch of data to China and HK. Since I don't have a jailbroken iPhone, it's off to Android.

For BLE scanning, Android does require permissions for location, but this application is using a Chinese branded tracking SDK and sending encrypted (within already encrypted TLS). So it's time to start reversing and instrumenting the runtime.

Well - not so easy, they used a commercial packer that encrypts their compiled bytecode and decrypts and runs it within a C++ library. I managed to bull the Dalvik out of memory using Frida[2], covert it to java bytecode with dex2jar[3] then into decompiled java with jadx [3].

Since the developer relied on the packer to hide/obfuscate their software, it's quite easy to follow. The libraries that do the location tracking on the otherhand are obfuscated so now I'm at the stage of identifying where to hook before the encrypted blobs are sent to servers in China.

I've sunk about 8 hours into this so far. The message here is that to understand what some applications on your phone does you need to really invest time and effort. The developers increase the cost to the consumer to know what their application is doing by obfuscation, encryption and packing. It's asymmetric.

[1] https://mitmproxy.org/posts/wireguard-mode/

[2] https://frida.re/docs/android/

[3] https://github.com/skylot/jadx

[3] https://github.com/pxb1988/dex2jar

I think they forgot to google translate the disadvantages of JEB Decompiler

I haven't used JEB to comment, but I've gotten a lot of mileage out of https://github.com/pxb1988/dex2jar#readme and then feed the normal Java jars it produces into https://github.com/mstrobel/procyon#readme and (of course) one shouldn't overlook picking your favorite tool for dealing with AndroidManifest.xml which often has fun things hiding in it

While digging up those links, I was reminded that some folks enjoy https://github.com/Konloch/bytecode-viewer#is-there-a-demo because it can be easier to "try out" a few of the decompilation engines, but I don't use it because it's hard to do batch things with it, versus dex2jar into procyon is automation friendly

Take a look at apktool: https://ibotpeaches.github.io/Apktool/ and dex2jar: https://github.com/pxb1988/dex2jar

We're basically working on this at Comm: https://github.com/CommE2E/comm

Comm | engineering | NYC onsite | https://comm.app

We're a startup based in NYC working on a Web3 Discord alternative. Comm is crypto-native messaging based on federated keyservers.

Our long-term goal is to build an alternative to the centralized application server model that dominates Web2. Sophisticated apps (like Discord) need a backend, but today those backends are controlled by corporations. Our solution is to replace the cloud with a network of personal, private application servers (we call them “keyservers”).

I starting working on Comm earlier this year. I’m a programmer by trade and previously worked at FB for 5 years. We currently have 10 devs actively contributing to our codebase, including 5 here in NYC.

We raised a small pre-seed round in February from some big names and are looking to raise our seed in the next 4 months.

Job descriptions available here: https://comm.careers

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