Re: asn2wrs documentation?

[Kukosa Tomáš <Tomas.Kukosa () ixperta com>]

Hi Peter,

> > Another possibility would be to define own dissector function for the
> > RSAPublicKey fields instead of calling default dissect_ber_integer().
> > E.g. something like this:
> >
> > #.FN_BODY  RSAPublicKey/modulus
> >     gint8 ber_class;
> >     gboolean pc, ind;
> >     gint32 tag;
> >     guint32 len;
> >
> >     offset = dissect_ber_identifier(actx->pinfo, tree, tvb, offset,
> > &ber_class, &pc, &tag);
> >     offset = dissect_ber_length(actx->pinfo, tree, tvb, offset, &len, &ind);
> >    /* integer octets are at the offset */
> >     offset += len;
> > #.END
>
> Yes, this is something I will use, thanks! For integers I think that the
> "ind" part is not needed (can be NULL) as integers are not using the
> indefinite length encoding?

Yes, as the INTEGER can be neither composite not indefinite the 'pc' and 
'ind' variables are not necesary.

> > There is not better asn2wrs document available. The best documentation
> > are examples in the asn1 directory.
>
> Those examples unfortunately have no explanation either. For example,
> there is a FIELD_ATTR member that is used in some places, how does it
> differ from TYPE_ATTR? Any idea what the IMPORT_TAG is used for (it is
> only used in one place)?

The #.TYPE_ATTR changes attributes for the type, i.e. for all fields of 
that type and the #.FIELD_ATTR changes attributes just for one field.

The #.IMPORT_TAG directive is primarily used when some type exported 
from one ASN.1 module (protoA) is imported in another one (protoB).
Those directives are generated into protoA-exp.cnf and then included 
into protoB.cnf using the #.INCLUDE directive.
The #.IMPORT_TAG directive contains information about ASN.1 BER tag 
which is necessary to distinguish the exported type in the module 
importing it.


> Thanks,
> Peter
>
> > Best regards,
> >     Tomas
> >
> >
> > On 15.9.2015 13:46, Peter Wu wrote:
> > > Hi,
> > >
> > > I am working on improving dissection support of the subjectPublicKey
> > > field in X.509 Certificates[1]. Right now these opaque BIT STRING types
> > > are shown as a sequence of bytes, but I would like to dissect the other
> > > fields (like modulus and exponent for RSA and public key y for DSA).
> > > (This work is a prerequisite for a new method of specifying RSA private
> > > key files in the SSL preferences without having to list address+port.)
> > >
> > > These numbers (RSA modulus, DSA y, DSS-Params p, q, g) are larger than
> > > 64-bit and therefore are forced to be displayed as FT_BYTES. The problem
> > > that now occurs is that the original field is lost
> > > (ber.64bit_uint_as_bytes is used instead).
> > >
> > > To tackle that problem, I started using TYPE_ATTR, but since the fields
> > > are still dissected as ber_integer, it does not help. I think I can use
> > > "IMPORT_TAG", but it is not documented on the wiki[2].
> > >
> > > Those who are familiar with the asn2wrs script, is it possible to update
> > > the wiki? Are there other documentation resources available?
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