Editing Pretty Good Privacy (section)

===Certificates===
{{Main|Public key certificate}}
In the (more recent) OpenPGP specification, ''trust signatures'' can be used to support creation of [[certificate authority|certificate authorities]]. A trust signature indicates both that the key belongs to its claimed owner and that the owner of the key is trustworthy to sign other keys at one level below their own. A level 0 signature is comparable to a web of trust signature since only the validity of the key is certified. A level 1 signature is similar to the trust one has in a certificate authority because a key signed to level 1 is able to issue an unlimited number of level 0 signatures. A level 2 signature is highly analogous to the trust assumption users must rely on whenever they use the default certificate authority list (like those included in web browsers); it allows the owner of the key to make other keys certificate authorities.

PGP versions have always included a way to cancel ('[[certificate revocation|revoke]]') public key certificates. A lost or compromised private key will require this if communication security is to be retained by that user. This is, more or less, equivalent to the [[certificate revocation list]]s of centralised PKI schemes. Recent PGP versions have also supported certificate expiration dates.

The problem of correctly identifying a public key as belonging to a particular user is not unique to PGP. All public key/private key cryptosystems have the same problem, even if in slightly different guises, and no fully satisfactory solution is known. PGP's original scheme at least leaves the decision as to whether or not to use its endorsement/vetting system to the user, while most other PKI schemes do not, requiring instead that every certificate attested to by a central [[certificate authority]] be accepted as correct.