public static XECPrivateKey fromRawX25519PrivateKey(byte[] rawScalarBytes) {
        try {
            NamedParameterSpec paramSpec = new NamedParameterSpec("X25519");
            KeyFactory keyFactory        = KeyFactory.getInstance("XDH");
            return (XECPrivateKey) keyFactory.generatePrivate(new XECPrivateKeySpec(paramSpec, rawScalarBytes));
        } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
            throw new RuntimeException(e);
        }
    }
Returns the bytes representing the BigInteger of the X25519 public key EC point U coordinate
security-utils/src/main/java/com/yahoo/security/KeyUtils.java