private static void publicFromPrivateHelper(int privateKeyLen) {
    byte[] privateKey = new byte[privateKeyLen];
    try {
      X25519.publicFromPrivate(privateKey);
      fail("Expected InvalidKeyException");
    } catch (InvalidKeyException expected) {
    }
  }
Tests against the test vectors in Section 6.1 of RFC 7748. https://tools.ietf.org/html/rfc7748
java_src/src/test/java/com/google/crypto/tink/subtle/X25519Test.java