  This paper considers password generators, i.e. systems designed to generate
site-specific passwords on demand. Such systems are an alternative to password
managers. Over the last 15 years a range of password generator systems have
been described. This paper proposes the first general model for such systems,
and critically examines options for instantiating this model; options
considered include all those previously proposed as part of existing schemes as
well as certain novel possibilities. The model enables a more objective and
high-level assessment of the design of such systems; it has also been used to
sketch a possible new scheme, AutoPass, intended to incorporate the best
features of the prior art whilst also addressing many of the most serious
shortcomings of existing systems through the inclusion of novel features.
