Best trust is reinforced by complete transparency
Often your users and your partners require efforts beyond regular "just make it secure". They demand transparency and secure audit logging that provides sufficient confidence in your system. PCI DSS audit trails are one side of the coin; settlement logs and system transparency for marketplaces and intermediaries is another.
Modern cryptography offers a whole set of protocols and approaches to back your claims and provide ways to prove your system's behaviour.
Transparent, auditable systems are hard
Audit logging is hard to blend
Dealing with 3rd party sensitive data requires audit logging while preserving the security of sensitive data.
Trusted multi-party logs are crucial
Making audit logs transparent to multiple participants yet preserving sufficient distributed trust and privacy is hard.
Provable logs are hard
Collecting logs through all components and 3rd parties is only the first step. All these logs should be protected against tampering to prove the system's behaviour.