AndroidKeyStore getEntry is consistently failing after certain point

Question

i am using the AndroidKeyStore to generate a RSA key pair, which are used to encrypt/decrypt the internal data.

The code which does that is as follows - it tries to retrieve the existing RSA key pair (via an alias ). If none exists then it tries to generate a new one. the code is as -

private void initializePublicPrivateKeys(){
    try
    {
        KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
        keyStore.load(null);
        KeyStore.PrivateKeyEntry entry = (KeyStore.PrivateKeyEntry) keyStore.getEntry(APP_RSA_KEY_PAIR_SECRET_ALIAS, null);
        _app_privateRSAKey = entry.getPrivateKey();
        _app_publicRSAKey = entry.getCertificate().getPublicKey();
    }
    catch(Exception e){

    }
}

private void initializeRSAKeyPairs() {

    initializePublicPrivateKeys();
    boolean isKeyNotGenerated = _app_privateRSAKey == null || _app_publicRSAKey == null;

    if(isKeyNotGenerated)
    {
        //Check here, if we already stored some data with previous RSA key pair - if a entry is present in SharedPreference then that would mean we had previously generated a RSA key pair and the entry is in-turn encrypted by this key pair.

        generateAppRSAPublicPrivateKeys();
        initializePublicPrivateKeys();// initialize it again , since we have new keys generated.
    }
}

@TargetApi(18)
private void generateAppRSAPublicPrivateKeys(){
    Calendar cal = Calendar.getInstance();
    Date now = cal.getTime();
    // the certificate created would be valid for 25 years. This is just a random value.
    cal.add(Calendar.YEAR, 25);
    Date end = cal.getTime();

    try{
        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA", "AndroidKeyStore");
        Context appContext = getApplicationContext();


        KeyPairGeneratorSpec.Builder keyPairGeneratorBuilder =
                new KeyPairGeneratorSpec.Builder(appContext)
                        .setAlias("myrsaalias")
                        .setStartDate(now)
                        .setEndDate(end)
                        .setSerialNumber(BigInteger.valueOf(1))
                        .setSubject(new X500Principal(String.format("CN=%s, OU=%s", "myrsaalias",
                                appContext.getApplicationInfo().packageName)));

        if(Build.VERSION.SDK_INT >= 19){
            keyPairGeneratorBuilder.setKeySize(2048); 
        }

        generator.initialize(keyPairGeneratorBuilder.build());
        generator.generateKeyPair();
    }
    catch(Exception e){
        e.printStackTrace();
        throw new IllegalArgumentException("Failed to generate RSA Public Private Key pair");
    }
}

This code works fine. Once the key pair is generated, I use them to encrypt/decrypt data ( store this data in shared preference ) BUT after certain point of time ( after some app relaunch ) the initializePublicPrivateKeys function fails to retrieve the key pairs.(after this point it fails consistently ) and so what ever data that is stored in the shared preference in encrypted form is lost since i dont have the corresponding public key to decrypt that at all. ( if i generate a new one then, i guess, this would be different and would return incorrect results when i decrypt the data)

I was wondering in what cases this initializePublicPrivateKeys function can fail ?

PS: for now, I can't capture the exception details because the problem is occurring on some customer device which i don't have access to and I need to figure out the source of the problem myself. Also, device password or PIN is not changed during this period ( i confirmed this with my customer )

thanks in advance,

Answer 1

Is it possible the user changed their screen lock type to an insecure method such as None or Swipe thus triggering all the keys in the AndroidKeyStore to be deleted? It is a known issue discussed in this blog article: https://doridori.github.io/android-security-the-forgetful-keystore/

Also mentioned here: https://code.google.com/p/android/issues/detail?id=61989

If you do not have strict security requirements and want your app to work even if the user has no screen lock then perhaps just use a keypair stored in a keystore file in your data area and skip the AndroidKeyStore.