Editing Pretty Good Privacy (section)

===Web of trust===
{{Main|Web of trust}}
Both when encrypting messages and when verifying signatures, it is critical that the public key used to send messages to someone or some entity actually does 'belong' to the intended recipient. Simply downloading a public key from somewhere is not a reliable assurance of that association; deliberate (or accidental) impersonation is possible. From its first version, PGP has always included provisions for distributing user's public keys in an '[[Public key certificate|identity certification]]', which is also constructed cryptographically so that any tampering (or accidental garble) is readily detectable. However, merely making a certificate that is impossible to modify without being detected is insufficient; this can prevent corruption only after the certificate has been created, not before. Users must also ensure by some means that the public key in a certificate actually does belong to the person or entity claiming it. A given public key (or more specifically, information binding a user name to a key) may be digitally signed by a third-party user to attest to the association between someone (actually a user name) and the key. There are several levels of confidence that can be included in such signatures. Although many programs read and write this information, few (if any) include this level of certification when calculating whether to trust a key.

The web of trust protocol was first described by Phil Zimmermann in 1992, in the manual for PGP version 2.0:

{{quotation|As time goes on, you will accumulate keys from other people that you may want to designate as trusted introducers.  Everyone else will each choose their own trusted introducers.  And everyone will gradually accumulate and distribute with their key a collection of certifying signatures from other people, with the expectation that anyone receiving it will trust at least one or two of the signatures.  This will cause the emergence of a decentralized fault-tolerant web of confidence for all public keys.}}

The web of trust mechanism has advantages over a centrally managed [[public key infrastructure]] scheme such as that used by [[S/MIME]] but has not been universally used. Users have to be willing to accept certificates and check their validity manually or have to simply accept them. No satisfactory solution has been found for the underlying problem.