  In this paper, we introduce PASSAT, a practical system to boost the security
assurance delivered by the current cloud architecture without requiring any
changes or cooperation from the cloud service providers. PASSAT is an
application transparent to the cloud servers that allows users to securely and
efficiently store and access their files stored on public cloud storage based
on a single master password. Using a fast and light-weight XOR secret sharing
scheme, PASSAT secret-shares users' files and distributes them among n publicly
available cloud platforms. To access the files, PASSAT communicates with any k
out of n cloud platforms to receive the shares and runs a secret-sharing
reconstruction algorithm to recover the files. An attacker (insider or
outsider) who compromises or colludes with less than k platforms cannot learn
the user's files or modify the files stealthily. To authenticate the user to
multiple cloud platforms, PASSAT crucially stores the authentication
credentials, specific to each platform on a password manager, protected under
the user's master password. Upon requesting access to files, the user enters
the password to unlock the vault and fetches the authentication tokens using
which PASSAT can interact with cloud storage. Our instantiation of PASSAT based
on (2, 3)-XOR secret sharing of Kurihara et al., implemented with three popular
storage providers, namely, Google Drive, Box, and Dropbox, confirms that our
approach can efficiently enhance the confidentiality, integrity, and
availability of the stored files with no changes on the servers.
