an expert system for automatic software protection

An Expert System forAutomatic Software Protection

Doctoral Program in Computer and Control Engineering

XXXI cycle

DAUIN – Department of Control and Computer Engineering

Leonardo Regano

Tutor:

Prof. Antonio Lioy

Cataldo Basile, Ph.D.

Coordinator:

Prof. Matteo Sonza Reorda

Why is software security needed?

• software is used in (almost) every aspect of everyday life• e-banking, entertainment, e-government and many more

• attacks on weakly or non-protected software have a great impact on software companies• loss of intellectual property• loss of revenues: 46 billions $ in 20181

• using unlicensed software is dangerous• malware usually contained in pirated software• disclosure of sensitive data and/or identity theft

12018 BSA Global Security Survey: https://gss.bsa.org/

1/19

Software protection techniques

• objective: safeguard security requirements of software assets• assets: algorithms IP, license schemes, users´ data…

• security requirements: confidentiality, integrity

• Man At The End (MATE) scenario• attacker has white-box access to application

• no perfect software protection exists• but protections can defer attacks

2/19

How to protect the software?

• protections decided and applied manually/empirically: several issues• long and complex vulnerability analysis

• high expertise needed to choose the best protections

• different platform+OS require different analysis

• an automatic approach? desirable• for the expert: can provide a good starting point

• for the beginner: one click and do everything

3/19

Expert system for Software Protection (ESP)

• objective: provide an optimal protection solution for a given application• decide protections best able to safeguard the application assets

• preserving the user experience

• can drive automatic protection tools• for a fully automated protection workflow

• implemented as a set of Eclipse plug-ins

4/19

Automated software protection workflow

Source code analysis

Risk assessment

Asset protection

Application source code

Asset hiding

Solution deployment

Protected binary Knowledge Base

Expert knowledge

Attacks against assets

Protection solutions

Application structure

Functions Variables

Call graph Assets

5/19

Automated software protection workflow

Source code analysis

Risk assessment

Asset protection

Application source code

Asset hiding

Solution deployment

Protected binary Knowledge Base

Expert knowledge

Attacks against assets

Protection solutions

Application structure

Protection techniques

Protection tools

Attack toolsAttacker profiles

Security requirements

Attack step types

5/19

Software security meta-model4

• formalizes all data handled by expert system• software security experts’

general knowledge• application-specific data• results of expert system

• OWL2 ontology

• classes and associations to describe:• application structure• assets and security

requirements• attacks against assets• protections

Datum Type

Code

Call

Call Parameter

File

Application Part

Datum

has type

accesses

refers to4C. Basile, D. Canavese, L. Regano, P. Falcarin, B. De Sutter, A meta-model for software protections andreverse engineering attacks, Journal of Systems and Software, Volume 150, 2019

6/19

Application Part

Security Requirement

Attack Path

Asset

threatens affects

Attacker Expertise

requires

Attack Step Type

Attack Tool

Attack Step

has

has type

implements

Software security meta-model4

• formalizes all data handled by expert system• software security experts’

general knowledge• application-specific data• results of expert system

• OWL2 ontology

• classes and associations to describe:• application structure• assets and security

requirements• attacks against assets• protections

4C. Basile, D. Canavese, L. Regano, P. Falcarin, B. De Sutter, A meta-model for software protections andreverse engineering attacks, Journal of Systems and Software, Volume 150, 2019

6/19

Software security meta-model4

• formalizes all data handled by expert system• software security experts’

general knowledge• application-specific data• results of expert system

• OWL2 ontology

• classes and associations to describe:• application structure• assets and security

requirements• attacks against assets• protections

enforces

Application Part

Security Requirement

Asset

Attack Step Type

ProtectionTool

ProtectionInstance

Protection

has

enforces mitigates

applied on

AppliedProt. Instance

Solution

has type

has type

4C. Basile, D. Canavese, L. Regano, P. Falcarin, B. De Sutter, A meta-model for software protections andreverse engineering attacks, Journal of Systems and Software, Volume 150, 2019

6/19

Risk assessment phase5

• infers possible attacks• on the unprotected application

• able to breach assets’ security requirements

• attack steps = simple attacker actions• expressed as Prolog inference rules

• attack paths = ordered sequences of attack steps• against actual assets

5L. Regano, D. Canavese, C. Basile, A. Viticchié, A. Lioy, Towards Automatic Risk Analysis and Mitigation of Software Applications, 2016 Workshop in Information Security Theory and Practice (WISTP), 2016

7/19

Risk assessment phase:attack paths

integrity of license_check() is breached

license_check() is changed skip license_check()

statically change license_check()

statically locate license_check()

dynamically change license_check()

dynamically locate license_check()

statically changemain()

statically locatemain()

dynamically change main()

dynamically locatemain()

8/19

Asset protection phase

• infers the optimal protection solution best able to defer attack paths

• takes into account:• structure of application• assets+security requirements• attack paths from risk assessment phase• interactions among protection techniques• protected application slow-down

• decision based on• experts knowledge• quantitative asset metrics (e.g. cyclomatic complexity)

9/19

Asset protection phase:protections vs. attacks

execution correctness of license_check() is breached

license_check() is changed skip license_check()

statically change license_check()

statically locate license_check()

dynamically change license_check()

dynamically locate license_check()

statically changemain()

statically locatemain()

dynamically change main()

dynamically locate main()

Anti-debugginglicense_check()

Anti-debuggingmain()

10/19

Asset protection phase:protections vs. attacks

execution correctness of license_check() is breached

license_check() is changed skip license_check()

statically change license_check()

statically locate license_check()

dynamically change license_check()

dynamically locate license_check()

statically changemain()

statically locatemain()

dynamically change main()

dynamically locate main()

Code mobilitylicense_check()

Code mobilitymain()

10/19

Asset protection phase:protections vs. attacks

execution correctness of license_check() is breached

license_check() is changed skip license_check()

statically change license_check()

statically locate license_check()

dynamically change license_check()

dynamically locate license_check()

statically changemain()

statically locatemain()

dynamically change main()

dynamically locate main()

SW attestationlicense_check()

SW attestationmain()

10/19

Asset protection phase:valid protection solutions

• must be able to defer all attack paths

• business logic of the application must remain unaltered

• ordering among protections applied on the same asset is important

• protected application slow-down must be below user-defined limits

11/19

Asset protection phase:game-theoretic approach

root1

2

Protection Index

Asset

12/19

Asset protection phase:game-theoretic approach

root

solution

1

2

Protection Index

Asset

12/19

Asset protection phase:game-theoretic approach

root

solution

attack

1

2

Protection Index

Asset

12/19

Asset protection phase:game-theoretic approach

root

solution

attack

attack

1

2

Protection Index

Asset

12/19

Asset protection phase:game-theoretic approach

root

solution

attack attack

attack

1

2

Protection Index

Asset

12/19

Asset protection phase:game-theoretic approach

root

solution

attack attack

attackattack

1

2

Protection Index

Asset

12/19

Asset protection phase:game-theoretic approach

root

solution

attack attack

attackattack

solution

1

2

Protection Index

Asset

12/19

Asset protection phase:game-theoretic approach

root

solution

attack attack

attackattack

solution

1

2

Protection Index

Asset

attack

12/19

Asset protection phase:game-theoretic approach

root

solution

attack attack

attackattack

solution

1

2

Protection Index

Asset

attack

attack

12/19

Asset protection phase:game-theoretic approach

root

solution solution solution

attackattackattack

attack attack attackattack attack

attackattack 1

attack

attack

12/19

Asset protection phase:game-theoretic approach

root

solutionoptimalsolution

solution

attackattack

attack

attack 1

attack

attack

12/19

Asset hiding phase6

• problem: software protections might expose a ''fingerprint''• fingerprints: code patterns, peculiar behaviors, etc.

• attackers locate assets looking for protection fingerprints

• solution: Asset Hiding (AH) phase• apply protections to hide fingerprints

• trade-off between fingerprint hiding and overhead

• state of the art: manually obfuscate as much code as possible

6L. Regano, D. Canavese, C. Basile, A. Lioy, Towards Optimally Hiding Protected Assets in Software Applications, 2017 IEEE International Conference on Software Quality, Reliability and Security (QRS), 2017

13/19

Fingerprint example:Control Flow Flattening

…

BB 2 BB 3

BB 4

BB 5

…

BB 1

14/19

Fingerprint example:Control Flow Flattening

…

loop head

BB 1 BB 2 BB 3

…

BB 4 BB 5

14/19

fingerprint

Fingerprint example:Control Flow Flattening

14/19

Asset hiding phase:strategies

• Asset Hiding strategies:• fingerprint replication• protected area enlargement• fingerprint shadowing

• deciding AH protections is difficult:• not all strategies are useful to

hide all protections• some strategies may lower AP

protections security• overhead must be taken into

account

asset

binary code

15/19

asset

Asset hiding phase:approach

• objective: maximize the confusion index• confusion index: how much the attacker is expected do be delayed

by the AH in finding the assets

• applying an AH protection increases the confusion index

• custom Mixed Integer-Linear problem• based on the well-known Knapsack Problem

• capacity constraints: overhead limits (e.g. CPU time, memory)

16/19

Validation by experts

• ESP tested on three real-life use-cases• OTP generator, application licensing scheme, DRM video player

• ESP results validated by software security experts• attack paths cover real attacks by tiger teams

• protection solutions effectively block attacks

• protection solutions leave applications business logic unaltered

• protection solutions introduce limited overhead

17/19

Experimental results

0

20

40

60

80

100

120

140

160

4 8 12 16 20

seco

nd

s

Protection Instances

Application A

Risk assessment

Asset protection

Asset hiding

Total

0

50

100

150

200

250

300

350

400

4 8 12 16 20

seco

nd

s

Protection Instances

Application B

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

4 8 12 16 20

seco

nd

s

Protection Instances

Application C

Application SLOC Functions Assets

A 443 18 4

B 1029 47 15

C 3749 178 39

18/19

Conclusions and future work

• completely automated workflow for software protection• user must only identify assets and security requirements• infers attacks against assets• decides best protection to defer attacks• deploys protections by driving automatic protection tools

• results validated by software security experts

• future work: empirical assessment of software protections• master students asked to attack protected applications…• …to assess how much attackers are deferred by protections• useful data to drive ESP reasoning processes

19/19

Thank youfor your attention!

Questions?