## Security and Privacy Challenges in e-Health Cloud
The approach also proposes a flexible data access policy that allows changes especially in emergency cases within which the regular access control policies could be broken to allow a type of \"break-glass access to PHR.\" However, apart from the computation overhead on the patients for key distribution and data or user management, this approach does not address the risks of privacy exposure by the Cloud owner. Some methods can be devised to resolve the challenge of heavy computation attached to the previous structure within which the owner is required to carry all the operations of data and user management in addition to the tasks of re-encryption in case of user revocation and at the same time to protect data privacy against cloud owners. One way to do that is by allowing the data owner to delegate most of the computation tasks involved in fine-grained data access control to the cloud owner without disclosing the original data content [65]. This can be accomplished through utilizing combined advanced cryptographic techniques: Key-Policy Attribute-Based Encryption (KP-ABE), Proxy Re-Encryption (PRE) and lazy re-encryption. Each record is associated with a set of attributes and each user is assigned an expressive access structure that is defined over these attributes. To enforce the attribute-based access control, KP-ABE is used to guide data encryption keys of data record. PRE is combined with KP-ABE to enable the data owner to delegate most of the computation operations to the cloud owner but without disclosing the underlying record contents. By this, data privacy is maintained as the cloud owner is not able to decipher any \"plaintext medical data.\" In this approach, scalability is also achieved using a lazy re-encryption technique to allow cloud owners to \"aggregate\" the computation tasks of multiple system operations. This causes the computation complexity to be either \"proportional to the number of system attributes\", or \"linear to the size of the user access tree\",which is independent of the number of users in the system. However, this approach does not address the need for data access policies management and the mechanism to serve emergency scenarios. Moreover, in [66] the authors argue that forensic methods are the best to use to ensure privacy, confidentiality and tracking of activities at the Cloud owner end of the e-Health Cloud infrastructure. The approach proposes computer forensic tools (CFT) as an acceptable security model for health records over the Cloud. It involves installing data loggers and sniffers on the cloud machines to capture any volatile information. At the same time, nonvolatile information has to be protected through different authentication, authorization and access control mechanisms. The approach also recommends supporting the e-Health Cloud architecture with legal mechanisms and periodic certification by a Third Party Audit (TPA). The forensic and anti-forensic tools and studies are performed by TPA to dynamically check the activities of the Cloud owners. Although e-Health Cloud with CFT has the potential to earn the clients' confidence; the approach watches and reports the volatile actions, but it does not physically prevent them from hanging in the first place. From a different angle, [21] provides a technical solution to particularly address the security issues of end-user platforms and external storage. End users do not use their platform to only access health records in the Cloud but also for other applications. Therefore, and with the limitations of security features within operating systems, end users platforms become very vulnerable to malware attacks which can obtain the user's passwords and secret data. This is in addition to the issue of transferring medical data to mobile storage units which take them away from any security control in the Cloud. To address all these issues, this approach proposes constructing trusted privacy domains (TVD) for the patients' medical data.