CHAPTER 27 Security and Privacy for Mobile Health-Care (m-Health) Systems 697 the group-signature-based counterpart in terms of both security and efficiency. The detailed com- parisons can be found in Ref. [48]. We have mentioned that if revocation is used, the outsourcing of PEKS-encrypted data enabling distributed storage would be intractable, in that the outside storage servers cannot be trusted to execute the revocation mechanism. If the storage server is totally public which is outside the EHR system, it is impossible to apply any technique for such server (due to the difficulty in authen- tication) to exercise revocation. Therefore, the option of outsourcing to a totally public server is feasible only when revocation is not needed. In systems where distributed data storage/retrieval is attractive, we can leverage the storage server at each delegatee's organization, or a public server designated for patient data storage within the EHR system (i.e., the public storage server shown in Figure 27-1), to host the PEKS storage and present in Ref. [14] where the physician can directly query the server. Moreover, the distribu- tions of the secret keys in privilege assignment and the nounce in emergency health information retrieval are through secure encryption schemes to provide confidentiality for sensitive messages. The confidentiality of the patient data shared between the delegator and delegatee to facilitate cooperation is assured by the PEKS primitive, which essentially protects patients' health data privacy. Furthermore, secret information con- tained in message exchanges remains confidential by using encryption schemes (i.e., HIBE, IBE). Fail-Open: We developed family-based and P- device-based approaches as the backup mecha- nisms for emergency situations. Both approaches are effective in successfully retrieving the needed PHI in the absence of the patient and preserve the privacy properties as described above. Access Control: The fact that in our HIPS