This document proposes a shared authority based privacy-preserving authentication protocol (SAPA) for cloud storage. SAPA allows for anonymous access requests and attribute-based access control, while encrypting data to preserve privacy. It aims to address privacy issues when a user requests data sharing from other users and the cloud server, where the request itself could reveal private information. The protocol uses anonymous matching of access requests, encryption of data, and temporary authorized data sharing between users through proxy re-encryption. It is designed to simultaneously provide access control, sharing of access authority, and privacy preservation for collaborative cloud applications.