Towards Modelling Information Security with Key-Challenge Petri Nets Teijo Venäläinen

Contents  Introduction  Various modelling methods  Graph based modelling  Key-Challenge Petri Nets

Introduction  Since 7/2006 in Information Technology Research Institute (ITRI), Agora, JYU  Doctoral studies since 2009  Goal is to find a method for measuring information security (IS)  Modelling and Simulation (M&S)

Motivation for testing/modelling  Testing a system in use is not a feasible option => damage  Real system must be replicated (modelled) somehow  Testing is done with the modelled system  How accurately does the model represent the real system?

Resulting information  For the whole system or a single component, the following results are interesting: –Mean time between failure (against attacks) –Success probability of attacks –Damage (performance degradation, money, …) –Attack route i.e. how the attack progresses –And more …

Testing methods  There are different methods, where varies [1] –”target audience” –Human involement during testing –Detail level  Role playing, ”Packet wars”, network design tools  Mathematical modelling, state machines, graph based modelling

Role playing  Scenario-based training exercises  High abstraction level  Test the strategic decision making process of personnel and organizations  Computers not necessary, ”pencil & paper”  Target audience: high level decision makers  Does not provide technical IS information

”Packet wars”  Real network with real users, a dedicated test network in a laboratory  Two teams: attackers and defenders  Highly accurate method but costly  Target audience: IS professionals

Network design tools  Accurate modelling of networks and normal activities  Attack modelling is limited => limited results  No human involvement during testing, only simulation  Target audience: IS professionals, network designers

Mathematical modelling, state machines, graph based models  Also approximations of the real system  Provide results faster through simulation  Cheap  Easily modifyable

Modelling & simulation Model System description Simulation

Graph based modelling  Network attack is usually a series of interdependent actions leading to a goal (= breach in security)  Actions are illustrated using nodes and arcs => an attack graph (AG)  Assign conditions (e.g. probability) on traversing between nodes  Usually attacker’s point of view  Simulate by starting from a node and moving towards the goal node(s)

Attack tree Source [2]

Challenges  The system must be described at adequate level of accuracy. Scalability with large networks?  Valid input parameters (From where? How?)  Usability  Attacker’s and defender’s interaction (game theory?)  Creating graphs is labor intensive => automatic tools

Petri Nets  Place (input/output): holds tokens  Arc: connects places and transitions  Transition: lets token pass through if conditions are met  Token: moves from place to place

Key-Challenge Petri Nets (KCPN)  A modelling method under development  Based on Petri-nets  KCPN graph is created using network and vulnerability information  Conditions for transitions = key-challenge –challenge = security measure –key = means to circumvent/break the security measure

KCPN: overview  Hierarchical i.e. modelling may be performed using various abstration levels  Modular structure  Place = network device or attack action  Arc = physical connection of devices or causal relation of attack actions  Transition = challenge (security measure)

KCPN: simulation  Attacker collects keys that allow him to progress in the graph  Variables may be assigned for transitions –Probability of being detected –Duration of an attack action (time distribution) –Cost, skill level, etc.  It is possible to perform an attack action without required keys but with a greater cost/duration

KCPN: results  Simulation results include: –Probability of success of an entire attack –The most vulnerable attack path –The duration of the entire attack  Results may be used as input data within the model (simulate modules independently)

KCPN: example  Two hierarchy levels: –Topology level (physical world) –Attack action level (abstract world)  Multiple network devices lumped into a single node (Hosts)  Devices with similar connections, OS, software, etc. => lumped together

KCPN: the physical network

KCPN: the graph

Sources  [1] J. Saunders. Simulation Approaches in Information Security Education. Proceedings of 6th National Colloquium for Information System Security Education,  [2] Bruce Schneier. Attack Trees. SANS Network Security rees.pdf

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