public static ECPrivateKey getEcPrivateKey(final byte[] pkcs8PrivateKey)
      throws GeneralSecurityException {
    KeyFactory kf = EngineFactory.KEY_FACTORY.getInstance("EC");
    return (ECPrivateKey) kf.generatePrivate(new PKCS8EncodedKeySpec(pkcs8PrivateKey));
  }
Returns an {@code ECPrivateKey} from {@code pkcs8PrivateKey} which is an encoding of a private
key, encoded according to the ASN.1 type SubjectPublicKeyInfo.

TODO(b/68672497): test that in Java one can always get this representation by using
{@link ECPrivateKey#getEncoded), regardless of the provider.
java_src/src/main/java/com/google/crypto/tink/subtle/EllipticCurves.java