The thing that strikes me is that the PGP web of trust idea is appropriate for very close-knit communities, where reputations matter and people mostly know one another. A key signed by Carl Ellison or Jon Callas actually means something to me, because I know those people. But transitive trust is just always a slippery and unsatisfactory sort of thing--the fact that Jon Callas trusts Fred Smith trusts John Jones to sign a key doesn' t really tell me whether or not I should trust him--by the time we're about three hops away, you'd have to be God to know enough to have your signature mean anything.
PGP .... or other similar account-based mechanisms provide trust between parties that have established relationship .... on a purely pair-wise, bilaterial basis. It does allow some direct trust operations to diffuse out to other parties. It isn't so much a close-knit community .... it is how far every specific entities's trust operation diffuse out across other individuals.
If the entity is called a certification authority .... and it provides an online service ... then the diffusing of specific trust operation might propogate out to a wide community. The issue of course is what trust attributes are propagating/diffusing and the diligence that the entity used in establishing the information to be trusted.
If the entity is called a certification authority, and it manufactures certificates (basically stale, static copies of some CA internal account record) then those certificates will presumably contains some information that is bound to the public key ... where there is some degree of confidence (aka trust) with regard to the binding between the information and the public key.
One issue is what meaning is there between having absolute certainty between something like an email address and a public key. Let's say it is an email address. Typically, email addresses at random are meaningless to me unless they are part of some specific context .... like somebody I have an established relationship with. However, if I have an established relationship with the entity, then it is back to the PGP scenario. In a broad context, businesses run on established relationships; aka financial institutions. The whole existing payment infrastructure effectively has the PGP scenario without needing certificates, and not exactly being considered a very close-knit community.
The primary difference between a financial institution actiing as an entity in a PGP web-of-trust paradigm (say payment cards, credit, debit, etc) and individual .... is the typical scope of the reputation of the financial institution is larger than an individual, and therefor the propagation/diffusing of trust is likely to have a much further reach. To a larger degree ... the trust radius of an entity is somewhat independent of whether it is operating in the PGP manner w/o certificates or in certificate paradigm.
The primary difference in the certificate paradigm is not the scope of the entity's trust .... it is the design point of delivering the trust. The certificate paradigm of trust delivery was targeted at an offline environment for relying parties that had no previous relationship (and had no online and/or direct recourse to the trust entity.
The payment card industry established a certificateless nearly world-wide scope of trust, in part by providing an extensive online network.
The certificate-based design point was to be able to provide an infrastructure for propogating trust between relying parties that had no previous relationship, were unlikely to need future relationship, and had no online or direct recourse to the trust enttity.
Anne & Lynn Wheeler http://www.garlic.com/~lynn/
Internet trivia 20th anv http://www.garlic.com/~lynn/rfcietff.htm
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