Trust - Peer-to-Peer Computing: Applications, Architecture, Protocols, and Challenges - page 137

credential negotiation, or the selection of the negotiation policy, Bertino et
al. [Bertino et al., 2004] proposed a comprehensive solution called Trust-χ, an
XML-based system addressing all the phases of a negotiation and providing
novel features with respect to existing approaches.
The first component of Trust-χ is an XML-based language, named χ-TNL,
for composing certificates and policies. Trust-χ certificates are either creden-
tials or declarations. Here, a credential states personal characteristics of its
owner, certified by a Credential Authority (CA), whereas declarations collect
personal information about its owner that do not need to be certified (such
as, for instance, specific preferences) but may help in better customizing the
offered service. A novel aspect of χ-TNL is the support for trust tickets, which
are issued upon the successful completion of a negotiation and can be used to
speed up subsequent negotiations for the same resource. Additionally, χ-TNL
allows the specification of a wide range of policies and provides a mechanism
for policy protection, based on the notion of policy preconditions.
A Trust-χ negotiation consists of a set of phrases to be sequentially exe-
cuted. A salient feature of Trust-χ is that it provides a variety of strategies for
trust negotiations, which allow a peer to better trade off between e ciency
and protection requirements. The motivation behind this design is that, since
trust negotiations can be executed for several types of resources and by a
variety of entities having various security requirements and needs, a single
approach to perform negotiation processes may not be adequate in all the cir-
cumstances. As a result, Trust-χ is very flexible and can support negotiations
in a variety of scenarios, involving entities like business, military and scientific
partners, or companies and their cooperating partners or customers.
As shown in Figure 6.5, according to the design rationales underlying
Trust-χ, each entity in the P2P system can be the controller of one or more
resources, a third-party credential issuer, or a requester. Typically, a negotia-
tion involves two entities: the entity providing negotiated resources, referred
to as the controller, and the entity wishing to access the resources, referred to
as requester. Note that the controller does not necessarily coincide with the
owner of the resource, it may be the manager of the resource entitled by the
real owner.
Each entity, characterized by a Trust-χ profile of certificates, can act as
a requester in one negotiation and as a controller in another. During a ne-
gotiation, mutual trust might be established between the controller and the
requester. Specifically, the requester has to show its certificates to obtain the
resource, and the controller, whose honesty is not always assured, submits cer-
tificates to its counterpart in order to establish trust before receiving sensitive
information. Release of information is regulated by disclosure policies, which
are exchanged to inform the other party of the trust requirements that need
to be satisfied to advance the state of the negotiation. Trust-χ participants
are both considered equally important. Thus, each party has an associated
system managing negotiation and always has a complete view of the state of
the negotiation process.