Unsigning and resigning apk inside apks makes an app don't work in some features

Question

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Let's say I have an app called X, this is the older version of the apk that I got from google. I have important data in the app, thus, if I uninstall it, I'll lose them. However, I want to update it, since google play store doesn't provide the app anymore (only available on the official website of the app), I need to download it manually. So, somehow (I watch a tutorial from YT) I need to sign the latest version of the app. Silly of me, I made a mistake that I unsigned the new app and then signed the app afterwards without knowledge of which appropriate sign type I need to apply.

Problem:

So I have unsigned (Removing signature verification) an app using NP Manager (the property says the app has signature status of V2, thus I killed the signature verification v2) , and then I signed the app and do the same i.e. signing the app using the signature scheme V2 (NP Manager has 3 options: v1+v2, v1 and v2 [Android 7.0+]).

At first, it worked successfully, i.e I could install it perfectly, some features worked and nothing seemed to be wrong, alas, there was one (Idk what it's called) bar/menu/section when I opened it, the app stopped immediately (forced close). I had tried to do trial-and-error by investigating what's wrong and the problem seems to come from the unisigning and signing.

My question:

Is it either unsigning or signing the app can harm the app itself which causes some features don't work though the app might be able to be installed successfully? If that's so, how to prevent this problem? Is it because the original signature and rhe new signature I give differs and crash somehow?

P.S. I'm only interested to know as to why the unsigning and signing makes the app error and how to solve it. I hope you would explain the error that happens in my app and why signature matters.

Thanks in advance!

Answer 1

Android OS checks the signature only at installation time. The verification information is saved and can be retrieved by the app at run-time (but the number of apps that do so is very low).

In general apps work no matter how and using what key/certificate they are signed. Only a few apps implement measure against app repackaging/resigning so they check their app signature at runtime and if it does not match the quit or show an error message or something like that.

That an app starts working an then suddenly crashes sound more like a bug of the app. Most likely the app has been developed for an older Android version or a different phone model (there are sometimes slight differences between different models).

Also it does not make any difference if you first remove a signature for an APK and later sign it. By default a new signature totally replaces the old signature.