Using the Android hardware-backed KeyStore

Question

I wish to use the Android hardware-backed KeyStore, but I'm concerned about security and usability. From what I've read here, KeyStore gets wiped when the user changes the device lock, unless setEncryptionRequired() is omitted. For usability sake, it seems this needs to be done, otherwise all hardware-backed keys would get wiped once the device lock is modified.

However, I've also read here that hardware-backed keys are not actually stored in the TEE, but rather, stored as key files in /data/misc/keystore/user_0/, encrypted by a device specific key that is stored within the TEE. Since a change in device lock wipes the KeyStore, it seems that the device specific key is derived from the device lock.

For security reasons, it makes sense to encrypt the key file, otherwise any root user would be able to read the key files and extract the private key, since they'd presumably be in they clear.

So I'm kind of in a dilemma. For usability sake, I should omit setEncryptionRequired(), but for security sake, I should set setEncryptionRequired().

Lastly, is it possible import a private key into the hardware-backed KeyStore using setKeyEntry()? I'm able to do so with no errors but I'm not sure if it's hardware-backed.

Is my understanding correct?

Answer 1

setEncryptionRequired() was deprecated in Android 6.0 (Marshmallow), and never really accomplished very much. The security of Android KeyStore depends on the TEE, not the password.

The blog post you linked to is out of date, at least on devices running Android 6.0 or later. On those devices, you should not use setEncryptionRequired(), and your keys will not be deleted until your app is uninstalled (or a factory reset is done, or your app deletes them). Your keys will be securely wrapped by secret keys that never leave the TEE. In fact, your keys will never leave the TEE in plaintext. When you use your keys, the data is passed into the TEE along with the encrypted key. The TEE unwraps the key then processes and returns the encrypted/signed/whatever data.

Yes, you can import private keys using setKeyEntry(). If you want to be sure that your key is hardware-backed, use KeyInfo.isInsideSecureHardware(). For example (this is from the documentation):

PrivateKey key = ...; // Android KeyStore key

KeyFactory factory = KeyFactory.getInstance(key.getAlgorithm(), "AndroidKeyStore");
KeyInfo keyInfo;
boolean isHardwareBacked = false;
try {
    keyInfo = factory.getKeySpec(key, KeyInfo.class);
    isHardwareBacked = keyInfo.isInsideSecureHardware();
} catch (InvalidKeySpecException e) {
    // Not an Android KeyStore key.
}