to_der

static VALUE
ossl_asn1cons_to_der(VALUE self)
{
    int tag, tn, tc, explicit;
    long seq_len, length;
    unsigned char *p;
    VALUE value, str;

    tag = ossl_asn1_default_tag(self);
    tn = NUM2INT(ossl_asn1_get_tag(self));
    tc = ossl_asn1_tag_class(self);
    explicit = ossl_asn1_is_explicit(self);
    value = join_der(ossl_asn1_get_value(self));

    seq_len = ASN1_object_size(1, RSTRING_LEN(value), tag);
    length = ASN1_object_size(1, seq_len, tn);
    str = rb_str_new(0, length);
    p = (unsigned char *)RSTRING_PTR(str);
    if(tc == V_ASN1_UNIVERSAL)
        ASN1_put_object(&p, 1, RSTRING_LEN(value), tn, tc);
    else{
        if(explicit){
            ASN1_put_object(&p, 1, seq_len, tn, tc);
            ASN1_put_object(&p, 1, RSTRING_LEN(value), tag, V_ASN1_UNIVERSAL);
        }
        else ASN1_put_object(&p, 1, RSTRING_LEN(value), tn, tc);
    }
    memcpy(p, RSTRING_PTR(value), RSTRING_LEN(value));
    p += RSTRING_LEN(value);
    ossl_str_adjust(str, p);

    return str;
}