OpenSSL::PKey::RSA#to_der

static VALUE
ossl_rsa_to_der(VALUE self)
{
    RSA *rsa;
    const BIGNUM *n, *e, *d, *p, *q, *dmp1, *dmq1, *iqmp;
    int (*i2d_func)(const RSA *, unsigned char **);
    unsigned char *ptr;
    long len;
    VALUE str;

    GetRSA(self, rsa);
    RSA_get0_key(rsa, &n, &e, &d);
    RSA_get0_factors(rsa, &p, &q);
    RSA_get0_crt_params(rsa, &dmp1, &dmq1, &iqmp);
    if (n && e && d && p && q && dmp1 && dmq1 && iqmp)
        i2d_func = i2d_RSAPrivateKey;
    else
        i2d_func = (int (*)(const RSA *, unsigned char **))i2d_RSA_PUBKEY;
    if((len = i2d_func(rsa, NULL)) <= 0)
        ossl_raise(eRSAError, NULL);
    str = rb_str_new(0, len);
    ptr = (unsigned char *)RSTRING_PTR(str);
    if(i2d_func(rsa, &ptr) < 0)
        ossl_raise(eRSAError, NULL);
    ossl_str_adjust(str, ptr);

    return str;
}