public static byte[] toRawX25519PublicKeyBytes(XECPublicKey publicKey) {
        // Raw byte representation is in little-endian, while BigInteger representation is
        // big-endian. Basically undoes what we do on the input path in fromRawX25519PublicKey().
        return Arrays.reverse(publicKey.getU().toByteArray());
    }
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