In Figure 18.4, each entry in the public-key ring contains an Owner Trust field that indicates the degree of trust associated with this public-key owner.Why is that not enough? That is, if this owner is trusted and this is supposed to be the owner’s public key, why is that trust not enough to permit PGP to use this public key?

Timestamp Timestamp Key ID* T PU, mod 264 T Key ID* PU; mod 264 PU Private-Key Ring Public Key trust_flag PU Public-Key Ring Public Key Owner Trust User ID* . Encrypted Private Key E(H(P), PR) User i Key Legitimacy trust_flag = field used to index table Figure 18.4 General Structure of Private- and Public-Key Rings User ID* . User i Signature(s) Signature Trust(s)