Formally Testing Fail-Safety of Electronic Purse Protocols

Designing and implementing security-critical systems correctly is very difficult. In practice, most vulnerabilities arise from bugs in implementations. We present work towards systematic specification-based testing of securitycritical systems using the CASE tool AutoFocus. Cryptographic systems are formally specified with state transition diagrams, a notation for state machines in the AutoFocus system. We show how to systematically generate test sequences for security properties based on the model that can be used to test the implementation for vulnerabilities. In particular, we focus on the principle of fail-safety. We explain our method at the example of a part of the Common Electronic Purse Specifications (CEPS), a candidate for an international electronic purse standard. Most commonly, attacks address vulnerabilities in the way security mechanisms are used, rather than the mechanisms themselves. Being able to treat security aspects with a general This work was partially supported by the Studienstiftung des deutschen Volkes, and by the German Ministry of Economics within the FairPay project , tel. +44 1865 284104, fax +44 1865 273839 Wolfson Building, Parks Road, Oxford OX1 3QD, Great Britain , tel. +49 89 289 28362, fax +49 89 289 25310 TU München, 80290 München, Germany CASE tool within the context of system development enables detection of such vulnerabilities.