I've noticed that PGP has been described as using a transitive trust
model for certification. I just wanted to clarify this issue a bit.
I don't think the PGP trust model is that transitive. In fact, it
requires that the user decide who he trusts to certify keys, and that
trust is NOT transitive. If Alice trusts Bob to certify a key, and Bob
trusts Charlie to certify a key, PGP does not suggest or encourage
that Alice trust Charlie to certify a key. Each PGP user must decide
whom he *directly* trusts to certify other keys. It doesn't look
transitive to me, assuming I understand what is meant by transitive.
In fact, the user is warned to not apply trust transitively. This follows
normal human common sense.
There is a detailed discussion of this in the PGP documentation, in the
section titled "How does PGP keep track of which keys are valid?"
Philip Zimmermann