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

Answer 1

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");