AAB files, what are they and what do they work for?

AAB files

The APK format (Android App Package) is no longer the standard for Android devices: since August 2021 the Google Play Store has confirmed the obligation to use the AAB (Android App Bundle) file format for new applications.

This is undoubtedly an important change in the world of Android, with long-term consequences that could completely change the way we relate to this operating system, which however has not even been noticed by most end users.

Android App Bundles: what are they?

AAB (Android App Bundle) is a modular packaging format for Android applications introduced by Google in 2018. Like APK, an AAB package contains the code of the Android applications as well as the resources that are part of it, such as images, audio and other multimedia contents but also translations in different languages.




APK files: what they are and how to install them on your Android If therefore from the end user side there is apparently no difference, why the transition to the AAB standard is so relevant in the Android world and why it plays a role crucial in reducing the size of applications downloaded from the Play Store?

AAB vs APK: all the differences

As already mentioned, the AAB and APK formats have two main differences, respectively in the distribution of resources and in the generation of signature keys. To better understand them, let's take as an example an application that contains, within the package resources, the following elements:

Video in FHD and in 4K Italian and Chinese language Let's suppose that the end user who wants download the app is in possession of a smartphone that only supports FHD resolution and has set Italian as the default language. By downloading a "classic" APK, the user will receive all four resources mentioned in the package, including those not supported by his device.


By downloading an application uploaded to the Play Store in AAB format, Google will return only the necessary resources, creating an APK package tailored to the needs of each user called s plit APK. Returning to our example, there will be no need to increase the weight of the app with 4K content or files containing Asian languages.

Obviously, in case our hypothetical user decides to learn Chinese and set as default language on your device, Google Play will take care of updating the split APK of the application by adding the required functionality.

The second big difference between AAB and APK concerns the signing keys, which in the case of Android App Bundles are managed directly from the Play Store. The signature keys are for security purposes and are used to verify the possible compromise or tampering of a package by malicious people.



In the case of the AAB it will therefore be Google itself, and no longer the developer, to sign the application package downloaded by users. The security of these signature keys is guaranteed by Google's Key Management System, a secure and functional in-cloud structure.

To configure the app signature by Google Play, developers must:

Create an upload key and sign the app package; Prepare the release and change the signing key in the app with the key generated by Google; Register the Google Play app signing key with the API provider, if the application provides for it.

Advantages and disadvantages of AAB files

The most obvious benefit to end users of using the AAB format is the reduction in app package size, although there does not seem to be a consensus estimate on the actual impact of this new format.

In 2018 Dom Elliott, Product Manager at Google Play, estimated in a Medium article a 35% reduction in the size of app packages thanks to the AAB format, specifically designed to solve the problem of excessive weight of the apps, which has a negative impact on the download and usage statistics by users.



In a post from 2021, Google lowers this estimate to 15%. In any case, the transition to a dynamic and modular system for downloading resources linked to an app represents an advantage in favor of AAB files, even if more content than initially expected.

This new format also guarantees developer updates applications faster, since specific features can be targeted to different categories of devices, thus also reducing the risk of internal errors and incompatibilities. To further facilitate this process, Google has recently introduced the performance class system for Android devices.

Android performance classes, what they are and what they are for

Instead of uploading to Play Store Pro and Lite versions of the same product, or separate applications based on target language, developers will be able to upload a single AAB file containing all assets via Android Studio or the command line. The Play Console will allow the upload of packages up to 150MB, with the possibility of extending up to several GB via asset packs for games.

However, there are some disadvantages in using AAB files, mainly due to nature same as Android as a system open to the installation of content from third parties. The changes introduced in the signature keys make apps from the Play Store and updates from other stores, signed by the developer and not by Google, incompatible with each other.



The fact that Google now holds App signing keys also raises some security concerns. The transparency code of the Mountain View company is sufficient not to raise alarms, reports XDA Developers, but in any case there is a potential, albeit very low, risk of data breach or tampering with permissions:

In the traditional model App distribution via APK, Google cannot modify apps without changing the signature. […] With App Bundles and app signing, Google could silently inject its own code into apps before distributing them. The signature would not change because Google would always have the signature key.

It should be specified that the latter possibility is extremely remote: as the recent choices made by the company in Russia show, it is more likely that Google will withdraws completely from a market where the conditions of transparency are not respected rather than authorizing this type of tampering.

The future of Android: the end of sideloading?

As already mentioned, the The use of AAB files complicates the sideloading procedures in the case of updates or beta versions of Play Store apps. Thinking about the long-term prospects of Android, however, this progressive centralization could endanger the activity of sideloading in its entirety.



If on the one hand, Google does not seem willing to put an end to sideloading in the same way as Apple, which hinders this practice in every possible way, on the other hand the progressive replacement of the APK format with AAB undoubtedly makes it more difficult to find and download app packages in this new format outside the Play Store.

However, there are third-party sources such as APKMirror and APKPure that have found a way around this problem by creating alternative packaging formats such as APKM and XAPK, which also contain basic APK files and extra resource packages, which can be installed through applications such as Split APKs Installer.

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