P2Photo is a mobile application that allows users to store photos on mobile devices without using centralized storage. Users can store photos, create albums, share albums with other users, provide photo storage for other users and use other users’ devices for photo storage. The basic architecture of P2Photo relies on a central server and a client mobile application.
The P2Photo client is a mobile Android application that users install and run on their devices and allow users to perform the following functions:
- F1. Sign up.
- F2. Log in/out.
- F3. Create albums.
- F4. Find users.
- F5. Add photos to albums.
- F6. Add users to albums.
- F7. List user’s albums.
- F8. View album.
The sign up operation (F1) allows users to create a new user account in the system; the user must enter a username and password. The client then contacts the P2Photo server, which must ensure that the new username is unique and adds the user to the user database. If the operation is successful, the user can then log in and start a new session on the client device. To perform useful functions on P2Photo, the user must log into the system (F2) with his account credentials (username and password), which must be validated by the server. If they are valid, the server must generate a new session id, keep an internal record associating that session id to the username and return the new session id to the client. However, if the credentials are invalid, the server must return an error. Only users with valid session ids will be allowed to perform additional functions (F3-F8). The log out function (F2) ends the current session. Once a session is active, users are able to manage photos and albums. Users can create an album (F4). This operation creates an album catalog file in the P2Photo server containing the URL of an album-slice catalog of the creating user. The album-slice is the part of an album that contains the photos contributed by a user to an album. An album-slice is stored in its owner’s cloud storage. It includes all the photos that user had contributed to an album and a text file (the catalog) grouping all the URLs pointing to those photos. For example, if Alice creates an album “Fun”, automatically there is a album-slice in Alice’s cloud storage (e.g. Dropbox account) for that album “Fun” and a URL for that album-slice catalog. The album-slice URL points to an album-slice catalog which is a file in Alice’s cloud storage with a list of all the URLs of all the photos in Alice’s “Fun” album-slice. P2Photo allows logged in users to find other users to create albums (F4). With the usernames returned by the server, a user can add those other users to the album membership (F6). Once a user is invited to an album, the application adds an album-slice to the album. If Alice invites Bob to join the “Fun” album, the catalog for that album (in the P2Photo server) will now include URLs for Alice’s album-slice catalog (in Alice’s cloud storage) and Bob’s album-slice catalog (in Bob’s cloud storage).
In this version of the application the photos are maintained in storage space allocated on the cloud. When the user signs in to P2Photo for the first time, he associates the P2Photo account with an account on Dropbox, Google Drive, or similar cloud storage provider, which P2Photo will use for private storage. When a user publishes a photo on a given album, that photo will be stored on the user's private storage. In order to retrieve that photo, the other members of the group with access permissions to that album will be given a direct URL to the photo. This mechanism will allow members to publish and read photos without involving the P2Photo server. The P2Photo server only needs to maintain the metadata about group membership of each album as well as a list of all P2Photo users. That metadata includes a list of user ids, a list of each user’s albums and for each user’s album, a URL that points to a special file located in the user's private storage space. This special file, named catalog, contains the list of the URLs that point to the photos published by that user in that specific album. Thus, in order for a user to list all the photos available on a given album, the P2Photo mobile application needs simply to retrieve the album membership from the P2Photo server, and then, for each member, download its respective catalog, parse it, and download all the photos published by that member using the URL contained in the catalog. We assume that the application provider is not malicious and does not willingly retrieve the catalogs and respective photos from the users' private stores. It is assumed that photos cannot be removed, albums deleted or users removed from an album’s membership.
In this version of the application, the photos are maintained in storage space allocated on album members devices and the photos are exchanged opportunistically in a P2P fashion between devices over WiFi Direct. The basic protocol is as follows. Each device manages local storage containing the photos published by the device owner and a catalog file which lists the set of photos associated with a given album. Whenever a set of co-located devices forms an ad-hoc wireless network, the devices broadcast their catalog files so as to inform all other members about the photos posted by their respective owners in each catalog. After assembling this information, each device has the necessary means to contact each device and download a copy of each photo and display it to the local user.
The server is a web server which is in charge of maintaining the user list, managing album membership and maintaining the album catalogs. Students should start by implementing all the project using only the web server and cloud storage. Later, the solution should transition to the more advanced solution where metadata is stored on the server but files are exchanged based on WiFi Direct.
-
A) Security: Design and implementation of a security mechanism to encrypt the catalog files and prevent a malicious application provider from retrieving users' catalogs and photos from their private stores.
-
B) Availability: Design and implementation of a replication protocol for increasing photo availability in the presence of disconnections. A simple solution would be for each device to maintain a reserved space for caching, and use it for keeping replicas of photos published by other users. However, solutions that involve the partial or total replication of the albums can be considered.