In our project Inverse Transparency, we research ways to securely track and make available information about data accesses to data owners. One approach to guarantee tamper-proof logging without necessitating trust in a central authority is using blockchain for storing the logs. Yet, blockchain is by design fundamentally at odds with the GDPR privacy legislation. To overcome this conflict, we have designed P³, a pseudonym provisioning system for secure usage logs including a protocol for recording new usages. With it, we enable GDPR-compliant logging in any blockchain without requiring a trusted third party.