Trust is first of all a mental state, an attitude towards another agent (usually a social attitude). Delegation necessarily is an action, a result of a decision, and it also creates and is a (social) relation among X, Y, and the action A.
Considerations on UMA Trust Model
UMA trust model can be showed through the tree phases of the entire process:
The first two phases can be consolidated in a unique and fundamental element that we call Bootstrapping Trust.
Through UMA, which is an user centric Identity management system, a Subject is able to control what information will be revealed for what purpose with which parties. Based on these elements, the following table describes the main factors that we consider relevant to measure the level of trustworthiness for each party involved in the protocol:
Bootstrapping trust in UMA is referred to the initial process to create a trust relationship among the three main parties: Authorizing User, Host and Authorization Manager.
The following picture shows the possible alternative approaches applicable to UMA protocol:
Bootstrapping Trust is the fundamental phase where the subject builds his (trust) mental state towards other agent (Host, AM), evaluating the trustworthiness factors.
Evaluating data sharing trust
UMA Data sharing process is based on three actions of delegation:
Delegation can be represented as a vector in a Cartesian plan (X-axes: Level of trustworthiness between the delegator and delegated agent, Y-axes: level of delegation, based on the iterative process of the protocol), which has initial point (delegator), a terminal point (delegated agent), a length and a direction.
The level of trustworthiness of UMA actors is calculated based on trustworthiness factors. For instance, in the person-self sharing constellation, we assume that the distance between Authorization Manager (AM) and Authorizing User (AU) is equal to 0, this is the highest level of trust that AU relays on AM, considering all the trustworthiness factors (qualitative and quantitative), which are evaluated during the Bootstrapping trust phase.
In the picture are represented the following delegation’s vectors:
The degree of trust of the entire process of data sharing can be calculated as length of resulting vector from summing delegation’s vectors (V1+V2+V3):
Without direct Authorizing User Consent
V1+V2 = (2,1)+(-1,1) = (1,2)
Degree = Length (V1+V2) = 2.23
With direct Authorizing User Consent
V1+V2+V3= (2,1)(-1,1)(0,-2) = (1, 0)
Degree = Length (V1+V2+V3) = 1
 T3 Trust theory http://www.istc.cnr.it/T3/trust/pages/delegation.html
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