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dicult to implement. For that reason we developed the biometric observer or
four-eyes principle, which shall enable a flexible and ecient protection. With
this principle, an arbitrary user which is already enrolled, the so-called observer,
vouches for the authenticity of the enrollment process and can guarantee for the
originality of the biometric profile. The validation of the user identity can be
tied to different guidelines. A schematic flow is shown in Figure 1.
Fig. 1. Schematic Flow (Observation)
1. A user wants to create a biometric profile. Therefore he starts the enrollment
process, where the name and, if necessary, various identity-related attributes
are handed over.
2. To ensure the authenticity of the profile, an already enrolled user, the ob-
server, acts as trusted instance and checks the identity of the user.
3. The observer logs in with his biometric profile, verifies the identity of the
enrolling user and, if required, specifies by which means this verification was
conducted.
4. The user starts providing his biometric data (enrollment).
5. When the enrollment process is completed, the observer approves the accu-
racy of the process.
By means of this method, trust can be established across several steps. If Alice
observed Bob for example, she can trust Carol's and Dave's profiles transitively,
whose enrollment processes were observed by Bob. The level of trust however
decreases in this coherence. These trust relations can be described within a
directed graph. Every profile is represented through a node in the graph and the
relations are directed edges. In this scenario, the distance of two nodes is crucial
for the level of reliability.
In a model where Alice observes Bob during the enrollment process (Fig-
ure 2), Bob in contrast just is observed and does not prove the identity of his
observer (Alice), there is only a one-way relationship. Hereby every single user
builds his own tree of trust with himself being the anchor. As a consequence,
Alice will never be part of Bob's tree of trust, since there are only trust relations
to one's followers. From a global perspective this leads to a hierarchy, a tree
with the system administrator on top of it as global trust anchor that enrolled
at the beginning without observation. To establish a Web of Trust, in which
all nodes can potentially trust each other, a subsequent approval of a profile's
 
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