misp-website/_posts/2019-09-12-Decaying-Of-Indicators.md

# Problem statement
MISP being a P2P system, various users and organisations are sharing data, sometimes without even knowing each others. While having access to a lot of information is extremelly benificial for all parties, it, however, also induces challenges to deal with.

In this blogpost, we will mainly discuss about information **quality** and **freshness**, other issues like **trust**, **use-cases**, **interests**, etc. are partially taken into account but will not be presented. Nevertheless, these concepts are examined in this [paper](https://arxiv.org/abs/1902.03914) along with a detailed explanation of the solution we've choosen to tackle these issues. 

Our main objective is to provide users a **simple yet customizable system** to automatically (or manually) mark an *Indicator Of Compromise* (or more generic, an *Attribute*) as **expired**.

Before getting started to show how the model presented in the paper is implemented in MISP, we first need to have a look at some concepts needed to better understand how components are working and tied together.

# The (potentially) annoying bits of theory
The solution currently supported in MISP is based on two components: ``base_score`` and ``score``. The idea is to have an initial fixed value called ``base_score`` taking into account the **quality** of an indicator; and a time-dependant ``score``, which decreases the more time passes.

A simplified version would be something like this:
```
score = base_score * P
```
Where ``P`` is composed of ``parameters``:
- ``lifetime``: The lifetime of the IOC or the time at which the score of the *Attribute*'s score will be 0
- ``decay_speed``: The speed at which the decay happens or the speed at which an *Attribute* will loose score

**⚠** **It should be noted that everytime a [*Sightings*](https://www.circl.lu/doc/misp/sightings/) is added to an *Attribute*, the ``score`` is refresh to the ``base_score`` and a new decay is initiated from that point.**

# Polynomial Decaying Model built-in in MISP
We still have to see how the ``base_score`` is actually computed. In the built-in version of the *Decaying Model* in MISP, the ``base_score`` is computed from the *Taxonomies* and some weigths. Weights are a mean to prioritize extracted ``numerical_values`` from *Taxonomies* over others.

To give the intuition of how the ``base_score`` computation works, let's look at two examples. In these examples, the two *Taxonomies* used are 
[*phishing*](https://github.com/MISP/misp-taxonomies/blob/master/phishing/machinetag.json) and [*admiralty-scale*](https://github.com/MISP/misp-taxonomies/blob/master/admiralty-scale/machinetag.json). Both of them contain *Tags* that have a ``numerical_value`` associated to them:
- <img src="../assets/images/misp/blog/decaying//tag-as-A.png" alt="admiraly-scale:source-reliability = Completely reliable" width="300"/>, ``numerical_value = 100``
- <img src="../assets/images/misp/blog/decaying//tag-as-D.png" alt="admiraly-scale:source-reliability = Not usually reliable" width="300"/>, ``numerical_value = 25``
- <img src="../assets/images/misp/blog/decaying//tag-p-H.png" alt="phishing:psychological-acceptability = high" width="250"/>, ``numerical_value = 75``

So, if an *Attribute* only have one *Tag* attached, let's say ``admiralty-scale:source-reliability="Completely reliable"``, the ``base_score`` would be:
```
base_score = 100
```

Weights come into action when multiple *Tags* are attached to an *Attribute*. To make things a bit easier, let's suppose that both *Taxonomies* should have the same importance in regards to the *Attribute*'s score. Thus, the total weigth (100) will be shared, assigning both *Taxonomy* a weight of 50.
```
admiralty-scale = 50
phishing        = 50
---------------------
sum              100
```
If an *Attribute* has the *Tags* <img src="../assets/images/misp/blog/decaying/tag-as-A.png" alt="admiraly-scale:source-reliability = Completely reliable" width="300"/> and <img src="../assets/images/misp/blog/decaying//tag-p-H.png" alt="phishing:psychological-acceptability = high" width="250"/> attached, the computation steps would look like this:

<img src="../assets/images/misp/blog/decaying//bs-computation-steps.png" alt="base_score comnputation steps" width="600"/>

Thus, the ``base_score`` of this *Attribute* will be ``87.50``.


# Short tutorial
Now that we've seen the basic concepts, let's have a look at how MISP implents these components. For these examples, we are using the default [phishing model](https://github.com/MISP/misp-decaying-models/blob/master/models/phishing-model.json) model on a **test** *Event*.

## Practical integration in MISP

### Endpoint: ``events/view``
At the *Event* level, a new filtering button has been added to attach the real-time computed ``score`` of any *Attributes* that has been mapped to a *Model*.

<img src="../assets/images/misp/blog/decaying//dm-event.png" alt="Decaying Model index" width="700"/>

### Endpoint: ``attribute/restSearch``
The ``attribute/restSearch`` endpoint has been updated and now supports four new parameters to filter out expired *Attributes* or play with the different available models.
- ``includeDecayScore`` **[bool]**: Attach the real-time computed ``score`` of the *Attribute* along with *Model(s)* informations
- ``excludeDecayed`` **[bool]**: Filter out all expired IOC
- ``decayingModel`` **[list]**: List of *Model(s)*, which will be attached to the *Attribute*
- ``modelOverrides`` **[dict]**: JSON that can be used to on-the-fly modify *Model(s)* parameters

Example
```
// attribute/restSearch query that gets every `ip-src` attributes being tagged with tlp or phishing,
// not being expired,
// with a overriden model threshold of 30 for the two models with id 84 and 12.
{
    "type": "ip-src",
    "tags": ["tlp:%","phishing:%"],
    "includeDecayScore": 1,
    "excludeDecayed": 1,
    "modelOverrides": {
        "threshold": 30
    }
    "decayingModel": [84, 12],
}
```

## Default and Custom Models
In MISP, Some *Decaying Models* called **Default Models** will be supplied by default. Similarly to *Taxonomies*, *Galaxies* or *misp-objects*, *Decaying Models* will have their [own repository](https://github.com/MISP/misp-decaying-models) and will have the possibility to be updated directly from the UI via a single click. **Default Models** are available to everyone, meaning that they can been viewed and customized by any users having a presence on the MISP instance.

**Custom Models** are user-defined models that are shared to other users. However, if desired, they can be hidden by turning off the sharing flag, similarly to the *Tag Collection* feature.

## Decaying Fine Tuning Tool: Setting parameters and mapping model to *Attribute* types
When creating a new *Decaying Model*, setting a parameters and viewing its impact should be as easy and straighforward as possible. To do so, few widgets are shipped with the latest version of MISP.

### Customizing lifetime and decay speed parameters
<video src="../assets/images/misp/blog/decaying//dm-tool.mp4" title="Decaying Model Fine Tuning Tool - Parameters" width="800" height="450" controls autoplay loop>
  Your browser does not support the video tag.
</video> 

### Setting the ``base_score``: Customizing Taxonomies' weigth
<video src="../assets/images/misp/blog/decaying//dm-bs.mp4" type="video/mp4" title="Decaying Model Fine Tuning Tool - Base score" width="800" height="450" controls autoplay loop>
  Your browser does not support the video tag.
</video> 

### Viewing scores and Simulating the model
<video src="../assets/images/misp/blog/decaying//dm-simulation.mp4" type="video/mp4" title="Decaying Model Simulation Tool" width="800" height="450" controls autoplay loop>
  Your browser does not support the video tag.
</video> 

# Developer perspective: Creating a model using a different algorithm
The Built-in Polynomial *Decaying Model* implemented in MISP allows any user to customize various components to achieve fine-grained decay behaviors. Still, it is possible that our model doesn't encompass your specific use-case. Thanks to the implemented architecture, any other formulas or algorithms can be added and used in a straightforward way.

Steps to create a new decay algorithm:
- Create a new file ``$filename`` in ``app/Model/DecayingModelsFormulas/``
- Extend the **Base** class ``DecayingModelBase``
- Implement the two functions ``computeScore`` and ``isDecayed`` with you own formula/algorithm
- Create a *Model* and set the ``formula`` field to ``$filename``

```
<?php
include_once 'Base.php';

class Polynomial extends DecayingModelBase
{
    public const DESCRIPTION = 'The description of your new decaying algorithm';

    public function computeScore($model, $attribute, $base_score, $elapsed_time)
    {
       // algorithm returning a numerical score
    }

    public function isDecayed($model, $attribute, $score)
    {
        // algorithm returning a boolean stating if the attribute is expired or not
    }
}
?>
```

# Outcomes
Evaluating **quality** and **freshness** of IOCs is a problem commonly found in Threat Intelligence Platforms. We tried to solve it using a simple yet customizable system.

Upon release, MISP will be shipped with few models that could fit most use-cases. Still, we are eagerly waiting for contributions, fine-tunings or feedbacks from users. This would opens up plenty of opportunities includings improved *Models*' precision, parameters tweaking or even integration of machine learning as a new *Model* algorithm.

Furthermore, we are not done yet! There are already improvements cooking in the MISP-Project oven,
- Integration of ``False Positive`` and ``Expiration`` *Sightings*
- Formula tweakings to provide better control on how to reset the ``base_score`` once a *Sighting* is created
- Per-user Taxonomies' ``numerical_value`` overrides
- Weights on *Tag*'s predicate level