to_der

static VALUE
ossl_asn1data_to_der(VALUE self)
{
    VALUE value, der;
    int tag, tag_class, is_cons = 0;
    long length;
    unsigned char *p;

    value = ossl_asn1_get_value(self);
    if(rb_obj_is_kind_of(value, rb_cArray)){
        is_cons = 1;
        value = join_der(value);
    }
    StringValue(value);

    tag = ossl_asn1_tag(self);
    tag_class = ossl_asn1_tag_class(self);
    if((length = ASN1_object_size(1, RSTRING_LEN(value), tag)) <= 0)
        ossl_raise(eASN1Error, NULL);
    der = rb_str_new(0, length);
    p = (unsigned char *)RSTRING_PTR(der);
    ASN1_put_object(&p, is_cons, RSTRING_LEN(value), tag, tag_class);
    memcpy(p, RSTRING_PTR(value), RSTRING_LEN(value));
    p += RSTRING_LEN(value);
    ossl_str_adjust(der, p);

    return der;
}