to_der

static VALUE
ossl_asn1prim_to_der(VALUE self)
{
    ASN1_TYPE *asn1;
    int tn, tc, explicit;
    long len, reallen;
    unsigned char *buf, *p;
    VALUE str;

    tn = NUM2INT(ossl_asn1_get_tag(self));
    tc = ossl_asn1_tag_class(self);
    explicit = ossl_asn1_is_explicit(self);
    asn1 = ossl_asn1_get_asn1type(self);

    len = ASN1_object_size(1, ossl_i2d_ASN1_TYPE(asn1, NULL), tn);
    if(!(buf = OPENSSL_malloc(len))){
        ossl_ASN1_TYPE_free(asn1);
        ossl_raise(eASN1Error, "cannot alloc buffer");
    }
    p = buf;
    if (tc == V_ASN1_UNIVERSAL) {
        ossl_i2d_ASN1_TYPE(asn1, &p);
    } else if (explicit) {
        ASN1_put_object(&p, 1, ossl_i2d_ASN1_TYPE(asn1, NULL), tn, tc);
        ossl_i2d_ASN1_TYPE(asn1, &p);
    } else {
        ossl_i2d_ASN1_TYPE(asn1, &p);
        *buf = tc | tn | (*buf & V_ASN1_CONSTRUCTED);
    }
    ossl_ASN1_TYPE_free(asn1);
    reallen = p - buf;
    assert(reallen <= len);
    str = ossl_buf2str((char *)buf, reallen); /* buf will be free in ossl_buf2str */

    return str;
}