Firebase auth was working fine, the debug build suddenly started failing without any change of code, logging the folloing message
D/PhoneAuthActivity( 7392): signInWithCredential:failure:com.google.firebase.auth.FirebaseAuthException: This app is not authorized to use Firebase Authentication. Please verifythat the correct package name and SHA-1 are configured in the Firebase Console. [ App validation failed ].
The only notable action inbetween a successful login and failure is android studio upgrade.
When you updated Android Studio, the debug key which is used to sign debug apps might have changed. Just add the new SHA-1 checksum of your debug key in the firebase console.
Easiest way would be to connect/sync to firebase from
Tools>Firebase>Authentication>Connect>Sync
If you want to do it manually or for release keystore, first generate SHA-1 checksum using following command:
keytool -list -v -keystore KEYSTORE_PATH -alias ALIAS_NAME
Then copy the SHA-1 checksum and go to:
Firebase Console > Your project > Settings of the app > Add Fingerprint
For more info, read this guide: Authenticating Your Client | Google APIs for Android