OpenSSL::PKey::EC#private_key

static VALUE ossl_ec_key_get_private_key(VALUE self)
{
    OSSL_3_const EC_KEY *ec;
    const BIGNUM *bn;

    GetEC(self, ec);
    if ((bn = EC_KEY_get0_private_key(ec)) == NULL)
        return Qnil;

    return ossl_bn_new(bn);
}