Read PKCS#8 RSA key

ReadPKCS8.java

public class ReadPKCS8 {
    /**
       Read PKCS#8 RSA key, with or without passphrase. It requires Bouncycastle library
    */
    public static void main(String[] args) throws IOException, PKCSException, OperatorCreationException {
        var file = "a_pkcs8_rsa_key.p8";
        var password = "passphrase";

        Security.addProvider(new BouncyCastleProvider());

        PEMParser pemParser = new PEMParser(new FileReader(file));
        Object object = pemParser.readObject();
        JcaPEMKeyConverter converter = new JcaPEMKeyConverter().setProvider("BC");
        PrivateKey privateKey;
        
        if (object instanceof PEMEncryptedKeyPair) {
            // Encrypted key - we will use provided password
            PEMEncryptedKeyPair ckp = (PEMEncryptedKeyPair) object;
            PEMDecryptorProvider decProv = new JcePEMDecryptorProviderBuilder().build(password.toCharArray());
            KeyPair kp = converter.getKeyPair(ckp.decryptKeyPair(decProv));
            
            privateKey = kp.getPrivate();
        } else if (object instanceof org.bouncycastle.pkcs.PKCS8EncryptedPrivateKeyInfo) {
            PKCS8EncryptedPrivateKeyInfo keyInfo = (PKCS8EncryptedPrivateKeyInfo) object;
            InputDecryptorProvider pkcs8Prov = new JceOpenSSLPKCS8DecryptorProviderBuilder().build(password.toCharArray());
            PrivateKeyInfo privateKeyInfo = keyInfo.decryptPrivateKeyInfo(pkcs8Prov);
            
            privateKey = converter.getPrivateKey(privateKeyInfo);
        } else {
            PEMKeyPair ukp = (PEMKeyPair) object;
            KeyPair kp = converter.getKeyPair(ukp);
            
            privateKey = kp.getPrivate();
        }
        System.out.println(privateKey.getAlgorithm())
    }
}