Private group photo sharing with efficient revocation using proxy re-encryption

With the ubiquity of smartphone cameras and photo storage sites such as Google Photos and Apple Photos, the culture and expectation of easy photo sharing has become pervasive. The problem is that many of these services have serious privacy concerns. At a minimum there is the concern that the photo hosting service has access to all the photos they host. There are systems that use forms of cryptography to encrypt photos end-to-end including while at rest on third party servers. These services prevent hosting organizations from viewing the photos they host, however the systems don't meet all the requirements for dynamic photo sharing. They often require a user to download, decrypt, encrypt, and then upload a photo again to change the group of people who have access. I implemented a photo sharing system that uses proxy re-encryption to prevent unauthorized users from viewing photos. My usage of proxy re-encryption enables users of the system to grant or revoke another user's access to different collections of photos in constant computation and communication complexity.