# Ideas about Python Quick DER

> *It should be reasonably straightforward to map Quick DER to Python.*

We may create an object that invokes the parser, and returns the output as a
list, whose length matches the number of `DER_PACK_STORE` instructions.  This
is a single run of the parser; we may run it again for parts of what we find.

The delivered list is mirrorred in Python instructions for packing it into a
dictionary format.  This includes labels from the ASN.1 syntax for both the
`DER_PACK_STORE` and `DER_PACK_ENTER` stages, although `None` may at some
points be used to suggest skipping the action (notable not entering a new
dictionary level).  This list can be derived along with the parser bytecodes.

Some of the entries need further parsing.  The `SEQUENCE OF` and `SET OF` types
should be mapped to a list and set, respectively.  Things typed as `ANY` remain
as they are, namely unparsed DER content (that may however cause a user program
to invoke the parser again).

Building DER is the reverse process, and it can follow the same process.

It may be interesting to create a class for the parser, so as to enable it
to `pack()` and `unpack()` based on contained instructions.  These classes,
and certainly their descriptions, could be automatically generated by an
ASN.1 compiler (such as `asn2quickder` which would be modified for generation
of Python code) and the methods would be defined in an abstract class
imported into the generated classes.

What we'd end up with is a concept where we generate a class like
`quick-der.rfc4120-Ticket` and can use it as follows:

    import quick_der.rfc4120_Ticket as Ticket

    def show_ticket (der):
            """Access individual parts of the Ticket, and print them.
               Then compose the owner's name from its constituent parts.
            """
            tkt = Ticket (der)
            print 'Ticket for Realm', tkt.realm
            print '       has name-type', tkt.sname.name_type
            for nm in tkt.sname.name_string:
                    print '       has name-string component', nm
            owner = '/'.join (tkt.sname.name_string) + '@' + tkt.realm
            print 'In short, it is for', owner

    def rebase_ticket (der, newrealm):
            """This violates RFC 4120, but is still a nice demo of modifying
               DER data in Python.  The violation is caused by the mismatch
               of the realm with the encrypted copy in tkt.enc_part
            """
            tkt = Ticket (der)
            tkt.realm = newrealm
            return tkt.pack ()