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Subject: Playing the "simpleton's advocate": how much complexity can we throw overboard?


Hi,
 
stepping back from the choice of binding for a moment, I would like to
explore a bit, how much the next major version of STIX might differ
from what we have today in terms of simplification.
 
So, as an experiment, below I will go through the 'Indicator'
entity and propose changes. Here I will try to take the side of
"as simple as simple can be", be the "simpleton's" advocate, so
to speak, in the hope of getting some discussion going regarding
what is maybe too simplistic vs what kind of complexity we might
be able to throw over board.
 
Here is the overview of the indicator entity/concept
taken from page 13 of
 
https://github.com/STIXProject/specifications/blob/master/documents/pdf%20versions/STIX_Indicator_Draft.pdf
 
 
Let me propose the following simplifications:
 
- version:
  Keep.
 
- negate:
  Remove -- the use case is mostly academic, but if the
  flag is there, it completely changes the semantics of how
  the indicator is to be handled and is a real pain for implementation,
  even though it is hardly ever used
 
  (Note: as shall be explained below, the 'negate' will not be needed anymore
   for building logical expressions of indicators, because we shall throw those
   out, too).
 
- Title:
  Make the field mandatory, but it may be empty.
 
- Type:
  Remove, if there isn't someone who can come up with a
  convincing case in which the field is really useful and where
  the intended semantics could not be communicated with
  a reference to a COA.
 
- Alternative_ID:
  Unsure, probably keep.
 
- Description:
  - There should be exactly 1 description field (which may be empty).
 
    Yes, this precludes fancy schemes of having different descriptions
    for different audiences/access levels, but those should amount
    to less than 0.1% of all indicator objects ever to be
    produced and thus do not warrant substantial complication of
    the standard.
 
  - There should be exactly one text representation, and that should *NOT*
    be html on account of security issues with rendering HTML in the
    browser. Chose Markdown, RestructuredText or whatever.
 
- Short_Description:
 
  Remove: I am tired of receiving indicator objects in which
  two or three out of three of the fields "title",
  "description", "short_description"
  are empty -- one description field should be plenty.
 
  (Besides: everybody keeps telling me that STIX is for
  machine-2-machine rather than *-2-human communication, so
  why the big fuss about description fields?)
 
 
- Valid_Time_Proposition:
 
  Timing info for indicators is badly needed, but I have yet to encounter usage
  of it. Can we come up with sensible guidelines of how to use the field?
  It should only be kept if we can...
 
- IndicatorExpression
 
  In the present specification, this field can either
  contain what is essentially a logical formula (codified as a tree)
  of indicators or an observable composition.
 
  - Remove logical operations: allow only to reference a list
    of indicators that constitute the present indicator.
 
  - If you want to do complicated and/or/not-reasoning about
    observable stuff: that is essentially a signature,
    so find a signature-language of your choice (including
    and/or/not-combinations of CybOX observables, if that
    is what works best) and formulate the signature
    as a test mechanism.
 
  - For everything else: find a way to encode observable
    patterns as simple key-value pairs and include/reference a list
    of them here. Should work fine for the majority of
    current use-cases. At most, allow a structured list that
    groups basic observable patterns (key-value) pairs with
    respect to a cybox observable to which they belong.   
 
- Indicated_TTP:
 
  Use yet-to-be-designed relationship mechanism.
 
- Kill_Chain_Phases:
 
  Define *ONE* standard STIX kill chain and work with that.
  Being able to include alternative models is nice, but
  complicates tooling quite a bit ... and, remember, this
  is an exchange format, so one common kill-chain model
  understood in the same way by everybody will work best.
 
- Test Mechanisms
 
  Make this a stand-alone entity/object rather than part
  of the Indicator object. As mentioned above: use this also
  to communicate more complicate observable patterns expressed in CybOX.
 
- Likely_Impact.
 
  Remove: I have never encountered it being used and have
  a hard time coming up with a convincing use case.
 
- Suggested_COAs:
 
  Use yet-to-be-designed relationship mechanism.
 
- Handling:
 
  Replace with some new mechanism for marking stuff with
  handling information.
 
- Confidence
 
  Keep: we need a simple way to distinguish high-confidence
  from low-confidence indicators.
 
- Sightings:
 
  Find some other way to do sightings (cf. the discussion on
  the mailing list)
 
- Related_*: Use yet-to-be-designed relationship mechanism.
 
- Producer:
 
  Either simplify drastically or better yet, leave away
  and consider for the next but one major release.
 
Like I said above: this as an experiment of being very simple
indeed (though my mind is probably too convoluted with
all things CTI to come up with the super-simple thing).
I am sure some of it is too simple, but with the current
version of STIX we have already one extreme of complexity --
to map out our design space, we need to get a feeling
for the other extreme as well, I think.
 
Kind regards,
 
Bernd
 
--------
 
Bernd Grobauer, Siemens CERT
 
 
 
 


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