  Passwords are widely used for client to server authentication as well as for
encrypting data stored in untrusted environments, such as cloud storage. Both,
authentication and encrypted cloud storage, are usually discussed in isolation.
In this work, we propose AuthStore, a flexible authentication framework that
allows users to securely reuse passwords for authentication as well as for
encrypted cloud storage at a single or multiple service providers. Users can
configure how secure passwords are protected using password stretching
techniques. We present a compact password-authenticated key exchange protocol
(CompactPAKE) that integrates the retrieval of password stretching parameters.
A parameter attack is described and we show how existing solutions suffer from
this attack. Furthermore, we introduce a password manager that supports
CompactPAKE.
