@Test
  public void testX25519ThrowsIllegalArgExceptionWhenPrivateKeySizeIsGreaterThan32Bytes()
      throws Exception {
    x25519Helper(33, 32);
  }
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