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Confidence Valuation in a Public-key Infrastructure Based on Uncertain Evidence

Reto Kohlas and Ueli Maurer

Public-key authentication based on public-key certificates is a special case of the general problem of verifying a hypothesis (that a public key is authentic), given certain pieces of evidence. Beginning with PGP, several authors have pointed out that trust is often an uncertain piece of evidence and have proposed ad hoc methods, sometimes referred to as trust management, for dealing with this kind of uncertainty. These approaches can lead to counter-intuitive conclusions as is demonstrated with examples in the PGP trust management. For instance, an introducer marginally trusted by a user can make him accept an arbitrary key for any other user. In this paper we take a general approach to public-key authentication based on uncertain evidence, where not only trust, but also other pieces of evidence (e.g. entity authentication) can be uncertain. First, we formalize the assignment and the valuation of confidence values in the general context of reasoning based on uncertain evidence. Second, we propose a set of principles for sound confidence valuation. Third, we analyze PGP and some other previous methods for dealing with uncertainty in the light of our principles.

\vspace{1ex} \noi \textbf{Key words.} Public-key certification, public-key infrastructure (PKI), web of trust, Pretty Good Privacy (PGP), evidence theory, reasoning with uncertainty.