Android KeyStore private exponent cannot be extracted

Question

I want to generate a RSA keypair in the Android Keystore. Since Android 4.3 is should be possible to generate RSA keys in the Android system Keystore. I generate my RSA key by (works fine) ``` Calendar notBefore = Calendar.getInstance(); Calendar notAfter = Calendar.getInstance(); notAfter.add(1, Calendar.YEAR); KeyPairGeneratorSpec spec = new KeyPairGeneratorSpec.Builder(ctx) .setAlias("key") .setSubject( new X500Principal(String.format("CN=%s, OU=%s", "key", ctx.getPackageName()))) .setSerialNumber(BigInteger.ONE) .setStartDate(notBefore.getTime()) .setEndDate(notAfter.getTime()).build(); KeyPairGenerator kpg; kpg = KeyPairGenerator.getInstance("RSA", "AndroidKeyStore"); kpg.initialize(spec); KeyPair kp = kpg.genKeyPair(); publicKey = kp.getPublic(); privateKey = kp.getPrivate(); ``` my RSA encryption looks like (works also): ``` public static byte[] RSAEncrypt(final byte[] plain) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException…

Accepted Answer

According to the code, I think that the OpenSSL provider prevents the private exponent to be exported when the key has been generated into the device.

```
@Override
public final BigInteger getPrivateExponent() {
    if (key.isEngineBased()) {
        throw new UnsupportedOperationException("private exponent cannot be extracted");
    }

ensureReadParams();
    return privateExponent;
}
```

Thus, you probably need to specify that you want to use the same crypto provider when retrieving the cipher instance. This provider supports these RSA ciphers:

- RSA/ECB/NoPadding

- RSA/ECB/PKCS1Padding

You should create an instance of the cipher this way:

```
Cipher cipher1 = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidOpenSSL");
```

Android KeyStore private exponent cannot be extracted

Solution

Problem