Crash casting AndroidKeyStoreRSAPrivateKey to RSAPrivateKey

Question

I'm following this tutorial: How to use the Android Keystore to store passwords and other sensitive information. It (loosely) ties up with the Google Sample app: BasicAndroidKeyStore.

I can encrypt my data using the public key, and I can decrypt on devices running Lollipop. However I have a Nexus 6 running marshmallow and this crashes giving the error:

java.lang.RuntimeException: Unable to create application com.android.test: java.lang.ClassCastException: android.security.keystore.AndroidKeyStoreRSAPrivateKey cannot be cast to java.security.interfaces.RSAPrivateKey

Here is the code it crashes on:

KeyStore.Entry entry;

//Get Android KeyStore
ks = KeyStore.getInstance(KeystoreHelper.KEYSTORE_PROVIDER_ANDROID_KEYSTORE);

// Weird artifact of Java API.  If you don't have an InputStream to load, you still need to call "load", or it'll crash.
ks.load(null);

// Load the key pair from the Android Key Store
entry = ks.getEntry(mAlias, null);

KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry) entry;

//ERROR OCCURS HERE::
RSAPrivateKey rsaPrivateKey = (RSAPrivateKey) privateKeyEntry.getPrivateKey();

Cipher output = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidOpenSSL");

output.init(Cipher.DECRYPT_MODE, rsaPrivateKey);

I'm reluctant to put this down to an Android M oddity as I see no reason why the java crypto libraries would have changed. If the M release comes around and our app immediately crashes on M I'll be in big trouble.

I am doing something wrong? The error very specifically says you can't cast to RSAPrivateKey, so does anyone know a better way to get the RSAPrivateKey from the entry?

Many many thanks.

Answer 1

I managed to get this working by removing the Provider from Cipher.getInstance and not casting to a RSAprivateKey.

KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry) entry;

Cipher output = Cipher.getInstance("RSA/ECB/PKCS1Padding");
output.init(Cipher.DECRYPT_MODE, privateKeyEntry.getPrivateKey());

I'm not 100% but I think the reason for this I believe is the change in marshmallow from OpenSSL to BoringSSL. https://developer.android.com/preview/behavior-changes.html#behavior-apache-http-client

Anyway, the above worked for M and below.

Answer 2

Issue

1. We are trying to parse "java.security.PrivateKey to java.security.interfaces.RSAPrivateKey" & "java.security.PublicKey to java.security.interfaces.RSAPublicKey". That's why we are getting ClassCastException.

Solution

1. We don't need to parse the key, we can directly use the "java.security.PrivateKey" & "java.security.PublicKey" for Encryption & Decryption.

Encryption

KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry)entry; 
PublicKey publicKey = privateKeyEntry.getCertificate().getPublicKey(); // Don't TypeCast to RSAPublicKey

Decryption

KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry)entry;
PrivateKey privateKey = privateKeyEntry.getPrivateKey(); // Don't TypeCast to RSAPrivateKey

Answer 3

I resolved this issue by also following this(apart from @James answer above): On Android 6.0 you should not use "AndroidOpenSSL" for cipher creation, it would fail with "Need RSA private or public key" at cipher init for decryption. Simply use Cipher.getInstance("RSA/ECB/PKCS1Padding") and it will work.