int tls13_derive_iv(SSL *s, const EVP_MD *md, const unsigned char *secret,
                    unsigned char *iv, size_t ivlen)
{
    static const unsigned char ivlabel[] = "iv";

    return tls13_hkdf_expand(s, md, secret, ivlabel, sizeof(ivlabel) - 1,
                             NULL, 0, iv, ivlen, 1);
}