to_der

static VALUE
ossl_rsa_to_der(VALUE self)
{
    RSA *rsa;
    int (*i2d_func)(const RSA *, unsigned char **);
    unsigned char *p;
    long len;
    VALUE str;

    GetRSA(self, rsa);
    if (RSA_HAS_PRIVATE(rsa))
        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);
    p = (unsigned char *)RSTRING_PTR(str);
    if(i2d_func(rsa, &p) < 0)
        ossl_raise(eRSAError, NULL);
    ossl_str_adjust(str, p);

    return str;
}