Enhancing Transparency with Distributed Privacy-Preserving Logging

Transparency of data processing is often a requirement for compliance to legislation and/or business requirements. Furthermore, it has recognised as a key privacy principle, for example in the European Data Protection Directive. At the same time, transparency of the data processing should be limited to the users involved in order to minimise the leakage of sensitive business information and privacy of the employees (if any) performing the data processing.

We propose a cryptographic logging solution, making the resulting log data publicly accessible, that can be used by data subjects to gain insight in the data processing that takes place on their personal data, without disclosing any information about data processing on other users’ data. Our proposed solution can handle arbitrary distributed processes, dynamically continuing the logging from one data processor to the next. Committing to the logged data is irrevocable, and will result in log data that can be verified by the data subject, the data processor and a third party with respect to integrity. Moreover, our solution allows data processors to offload storage and interaction with users to dedicated log servers. Finally, we show that our scheme is applicable in practice, providing performance results for a prototype implementation.