This paper elaborates HCI (Human-Computer Interaction) requirementsfor making cloud data protection tools comprehensible and trustworthy.The requirements and corresponding user interface design principles are derivedfrom our research and review work conducted to address in particular the followingHCI challenges: How can the users be guided to better comprehend theflow and traces of data on the Internet and in the cloud? How can individual endusers be supported to do better informed decisions on how their data can be usedby cloud providers or others? How can the legal privacy principle of transparencyand accountability be enforced by the user interfaces of cloud inspectiontools? How can the user interfaces help users to reassess their trust/distrust inservices? The research methods that we have used comprise stakeholderworkshops, focus groups, controlled experiments, usability tests as well as literatureand law reviews. The derived requirements and principles are groupedinto the following functional categories: (1) ex-ante transparency, (2) exercisingdata subject rights, (3) obtaining consent, (4) privacy preference management,(5) privacy policy management, (6) ex-post transparency, (7) audit configuration,(8) access control management, and (9) privacy risk assessment. This broadcategorization makes our results accessible and applicable for any developerwithin the field of usable privacy and transparency-enhancing technologies forcloud service chains.