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Cyber-physical systems (CPS) security, as a prevalent concern in all
digital industries, must be implemented on different levels of
abstraction. For example, the development of top-down approaches,
e.g., security models and software architectures is equivalent in
importance to the development of bottom-up solutions like the design
of new protocols and languages. This thesis combines research in the
field of CPS security from both approaches and contributes to the
security models of the two lighthouse examples automotive software
engineering and general password security.
Most existing countermeasures against cyberattacks, e.g., the use of message
cryptography, concentrate on concrete attacks and do not consider the
complexity of the various access options offered by modern cyber-physical systems. This is
mainly due to a solution-oriented approach to security problems. The
model-based technique SAM (Security Abstraction Model) adds to the early
phases of (automotive) software architecture development by explicitly
documenting attacks and managing them with the appropriate security
countermeasures. It additionally establishes the basis for comprehensive
security analysis techniques, e.g., already available attack assessment
methods. SAM thus contributes to an early, problem-oriented and
solution-ignorant understanding combining key stakeholder knowledge. This
thesis provides a detailed overview of SAM and the resulting analyses of our
evaluation show that SAM puts the security-by-design principle into practice
by enabling collaboration between automotive system engineers, system
architects and security experts. The application of SAM aims to reduce costs,
improve overall quality and gain competitive advantages. Based on our
evaluation results, SAM is highly suitable, comprehensible and complete to be
used in the industry.
The bottom-up approach focuses on the area of password hardening encryption
(PHE) services as introduced by Lai et al.~at USENIX 2018. PHE is a password-based
key derivation protocol that involves an oblivious external crypto service
for key derivation. The security of PHE protects against offline brute-force
attacks, even when the attacker has full access to the data server.
The obvious evolution of PHE is the extension of the protocol to use multiple
rate-limiters (guardians) to mitigate the single point of failure introduced by
the original scheme.
In the second part of this thesis, a general overview of the motivation and
use cases of PHE is given, along with a new formalization of the protocol to
help the mentioned scalability and availability issues. Moreover, an implementation
of the resulting threshold-based protocol is briefly explained and evaluated. Our
implementation is furthermore tested and evaluated in a novel use case featuring
password hardened encrypted email.