  It is practically impossible for users to memorize a large portfolio of
strong and individual passwords for their online accounts. A solution is to
generate passwords randomly and store them. Yet, storing passwords instead of
memorizing them bears the risk of loss, e.g., in situations where the device on
which the passwords are stored is damaged, lost, or stolen. This makes the
creation of backups of the passwords indispensable. However, placing such
backups at secure locations to protect them as well from loss and unauthorized
access and keeping them up-to-date at the same time is an unsolved problem in
practice.
  We present PASCO, a backup solution for passwords that solves this challenge.
PASCO backups need not to be updated, even when the user's password portfolio
is changed. PASCO backups can be revoked without having physical access to
them. This prevents password leakage, even when a user loses control over a
backup. Additionally, we show how to extend PASCO to enable a fully
controllable emergency access. It allows a user to give someone else access to
his passwords in urgent situations. We also present a security evaluation and
an implementation of PASCO.
