I'm struggling with coming up with a decent security strategy to bootstrap
some keys in android (api >= 16). I am storing some reasonably sensitive
data locally, and don't want to just write the data to a DB or a file
locally without any sort of encryption.
That means I need an encryption key (AES 256) somewhere to encrypt, say,
entries in SharedPreferences, or a file, or whatever. So far so good.
But how do I protect that AES key? I've tried using the KeyStore
("AndroidKeyStore") to generate an RSA keypair, which I use to wrap/unwrap
the AES key (apparently generating an AES key in the keystore isn't
supported in lower api levels, and even this strategy doesn't work < api
18). The hope is that that keypair is at least semi-securely stored
somewhere, preferably with hardware support on phones that have it.
This seems to mostly work on Android 4 and 5, but on Android 6, as soon as
the app restarts, and I retrieve the keypair from the KeyStore, the key is
unusable and I get "InvalidKeyException: Need RSA private or public key".
Is there a known (but unknown to me) bug in Android 6? Some new security
quirk that won't let me get the keypair back?
Anyone dealt with this sort of security-bootstrapping in android? Is there
some well-known and workable strategy people have used?
Regards,
jan
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