IssueTracker

This problem started with the latest stock Google Android update (14 UQ1A.240205.002.A1), testing on a Pixel 7 and other devices. This problem did not exist prior to the more recent Android update.

Problem: If a MasterKey is deleted (deleteEntry) from the AndroidKeyStore, that alias is corrupt. Deleting the app and reinstall seems to work, but if you kill the app and restart the MasterKey at that alias fails with exceptions.

How to reproduce: I have attached a minimal android studio example. Below describes the problem in code.

Using Android Security Jetpack: implementation("androidx.security:security-crypto-ktx:1.1.0-alpha06").

In the android manifest, you can set android:allowBackup="false" to be sure that is not interfering.

Create some encrypted preferences

        private val spec = KeyGenParameterSpec.Builder(
            MasterKey.DEFAULT_MASTER_KEY_ALIAS,
            KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT
        )
            .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
            .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
            .setKeySize(MasterKey.DEFAULT_AES_GCM_MASTER_KEY_SIZE)
            .build()
        val masterKey = MasterKey.Builder(context)
                    .setKeyGenParameterSpec(spec)
                    .build()
        val encryptedPrefs = EncryptedSharedPreferences.create(
                        context,
                        encryptedFileName,
                        masterKey,
                        EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,
                        EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM
                    )

• Next store a few preferences.
• Then delete the masterKey. The app will NEVER fully recover from this point (even uninstall/reinstall will not work).

            val keyStore = KeyStore.getInstance("AndroidKeyStore")
            keyStore.load(null)
            try {
                keyStore.deleteEntry(MasterKey.DEFAULT_MASTER_KEY_ALIAS)
            } catch (_ : Exception) { }

• if you will the app and restart it, it will go sideways and throw exceptions.
• Now uninstall the app and then reinstall it.
• Save a few encryptedPrefs and retrieve them. It will appear to work.
• Kill the app and restart. This is where things go sideways.
• When attempting to read the previously saved encryptedPrefs, the following exception will be thrown: javax.crypto.AEADBadTagException

It is as though the MasterKey is forever corrupted even surviving deleting the app and installing a new version. This occurs even if using a custom master key alias.

Expected Behavior: The expected behavior is that if the MasterKey alias is deleted, then a new one should be created that is fully functional and not throw exceptions. It should definitely not result in corruption in a re-installed app after the prior was deleted.

• Is this a security related issue? Yes/No - hmm. I suppose it depends on what an app does when the MasterKey no longer works.