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Public Key Infrastructure • Chapter 10 561

can bring her boyfriend, Mike.Although Tim does not know Kate’s boyfriend, he
still has a level of confidence in him because of the chain of trust established first
through his wife, then Kate, and lastly to Kate’s boyfriend.This type of indirect
trust relationship is known as a transitive trust (see Figure 10.6). By a similar chain,
Mike trusts that he will be comfortable at the dinner because Kate has vouched for
Amanda, and Amanda vouches for Tim.
A more technological example of transitive trust is in a Windows Forest, where
a domain will trust any other domain in the forest, by virtue of transitive trust
through their parent domains.

Figure 10.2 A Chain of Trust

Tim trusts Amanda

Amanda trusts Tim
Tim Amanda

Mike trusts Tim by transitive trust Tim trusts Mike by transitive trust Amanda trusts Kate Kate trusts Amanda

Kate trusts Mike

Mike trusts Kate
Mike Kate

Web-of-trust Model

In our example above of transitive trust being developed into a chain of trust, it’s
obvious to see that if Amanda meets Mike at the dinner, and recognizes him as
someone with whom she works,Tim’s trust of Mike will increase – Mike has been
identified as somewhat trustworthy by someone Tim trusts greatly, as well as being
identified as very trustworthy by someone with whom Tim already has a level of
trust with.As more complex relationships occur, we find that we trust individuals
based not only on a direct chain of trust, but on a number of chains of trust, of

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