int tls13_derive_key(SSL *s, const EVP_MD *md, const unsigned char *secret,
                     unsigned char *key, size_t keylen)
{
    static const unsigned char keylabel[] = "key";

    return tls13_hkdf_expand(s, md, secret, keylabel, sizeof(keylabel) - 1,
                             NULL, 0, key, keylen, 1);
}