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mains, with the users now in a flat structure, or a pool
of users. The access control model allows for delega-
tion of access control management through clear role
definitions.
The current life-cycle management of the PRE key
pair hierarchy is based on the relationship between
key pairs in the form of PRE tokens. To allow a user
in a lower tier of the hierarchy to decrypt data, the
encrypted data must first go through a chain of re-
encryptions to pass down through the hierarchy end-
ing at the user. As the collaboration environment
evolves over time, the PRE key pair hierarchy poten-
tially becomes very deep. This is undesirable as the
additional re-encryptions result in an additional com-
putational overhead and expose structural information
about the hierarchy in the form of re-encryption du-
ration. This information could be used for timing
attacks which could potentially reveal cryptographic
details on how data is protected (Kocher, 1996). This
is a subject for future work.
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