DeStringCare

What is it?

It is a tool for extracting StringCare obfuscated secrets in Android apk files. Some of these StringCare protected secrets may contain API addresses and API keys.

Warning: It is not recommended to store important API keys on the client-side, especially the keys to third party services. A better approach is to have your own API service, and create unique API keys for each app user. This allows to revoke API keys and banning user if necessary.

Installation

pip install DeStringCare

Usage

usage: destringcare.py [-h] [-r] [-o] apk [replaced]

Extract StringCare secrets from an Android APK.

positional arguments:

• apk - Path to the apk
• replaced - Path to the replaced values

optional arguments:

• -h, --help show this help message and exit
• -r, --resign Resign and save xml file
• -o, --other Include a list of other secrets

Example:

destringcare --resign Appname_v1.0.2494.apk

Replace values

Create mapped.json

{
  "firebase_database_url": "https://some-path.somewhere.io",
  "google_storage_bucket": "another-path.appspot.com"
}

Then run destringcare with the additional argument:

destringcare --resign Appname_v1.0.2494.apk mapped.json

Print other extra properties

Some common property keys may not be encoded by Stringcare. These may be in plaintext and thus included in the output.

destringcare --other Appname_v1.0.2494.apk

How to use it? Where to get apk?

1. First download a Android apk.

   • Use a website like https://apkpure.com/ (beware that the app may be tampered with, and so not recommended).
   • Use GPlayCli / GPlayWeb to download using your own Gmail credentials.
   • Use adb tool which pulls the apk from your Android device or emulator.
     1. Download the app via Google Play store to your Android device or emulator.
     2. List packages and find the app you want.

        adb shell pm list packages
        

     3. Print path to the apk file.

        adb shell pm path
        

     4. Pull the apk file.

        adb pull /full/path/to/the.apk
        

2. Extract the StringCare secrets.

   destringcare Appname_v1.0.2494.apk
   

   You will get an output as JSON file:

   {
       "mixpanel_api_key": "7b23daa71cdbb9e6d07f29a36de960f3"
   }
   

How to resign StringCare secrets?

destringcare --resign Appname_v1.0.2494.apk

It loads the first key from the keystore file ~/.android/debug.keystore.

Then it reencrypts the apk secrets and saves it as an xml file in resigned-strings.xml.

Resigning the StringCare secrets with your own key allows you to repackage the application and use it in your Android device. You would need to replace the original strings.xml with resigned-strings.xml file.

Where the resigned version can be used?

Using apktool it is possible to decode an application and to repackage it later again.

As it is necessary to resign the application in order for it to work in Android, it will be also necessary to update the StringCare protected file with a new signing key.

Typical workflow:

1. Decode the apk using apktool into apk directory.

   apktool d Appname_v1.0.2494.apk -o apk
   

2. Run DeStringCare to extract the strings.xml with secrets and resign it.

   destringcare --resign Appname_v1.0.2494.apk
   

3. Replace StringCare protected xml file at path apk/res/values/strings.xml.

   It may contain a line like the following:

   <string name="mixpanel_api_key">367E864309B5E7E3E6642483AF380497...</string>
   

4. Rebuild the apk.

   apktool b -d apk -o app-unsigned.apk
   

5. Resign the apk.

   jarsigner -verbose -sigalg MD5withRSA -digestalg SHA1 -keystore ~/.android/debug.keystore -storepass android app-unsigned.apk androiddebugkey
   

6. Zipaligning the apk.

   zipalign -v 4 app-unsigned.apk app.apk
   

How to contribute?

If you have questions or enhancement ideas, open an issue.

If you have made improvements to the code, create a merge request.