A public-key infrastructure (PKI) consists of protocols, services, and standards supporting applications of public-key cryptography. The term PKI, which is relatively recent, is defined variously in current literature. PKI sometimes refers simply to a trust hierarchy based on public-key certificates , and in other contexts embraces encryption and digital signature services provided to end-user applications as well [OG99]. A middle view is that a PKI includes services and protocols for managing public keys, often through the use of Certification Authority (CA) and Registration Authority (RA) components, but not necessarily for performing cryptographic operations with the keys.
Among the services likely to be found in a PKI are the following:
- Key registration: issuing a new certificate for a public key.
- Certificate revocation: canceling a previously issued certificate.
- Key selection: obtaining a party's public key.
- Trust evaluation: determining whether a certificate is valid and what operations it authorizes.
Key recovery has also been suggested as a possible aspect of a PKI.
There is no single pervasive public-key infrastructure today, though efforts to define a PKI generally presume there will eventually be one, or, increasingly, that multiple independent PKIs will evolve with varying degrees of coexistence and interoperability. In this sense, the PKI today can be viewed akin to local and wide-area networks in the 1980's, before there was widespread connectivity via the Internet. As a result of this view toward a global PKI, certificate formats and trust mechanisms are defined in an open and scaleable manner, but with usage profiles corresponding to trust and policy requirements of particular customer and application environments. For instance, it is usually accepted that there will be multiple ``root'' or ``top-level'' certificate authorities in a global PKI, not just one ``root,'' although in a local PKI there may be only one root. Accordingly, protocols are defined with provision for specifying which roots are trusted by a given application or user.
Efforts to define a PKI today are underway in several governments as well as standards organizations. The U.S. Department of the Treasury and NIST both have PKI programs [2,3], as do Canada  and the United Kingdom . NIST has published an interoperability profile for PKI components [BDN97]; it specifies algorithms and certificate formats that certification authorities should support. Some standards bodies which have worked on PKI aspects have included the IETF's PKIX and SPKI working groups [6,7] and The Open Group .
Most PKI definitions are based on X.509 certificates, with the notable exception of the IETF's SPKI.
 PKI - PC Webopedia Definitions and Links:
 Government Information Technology Services, Federal Public key Infrastructure:
 NIST Public key Infrastructure Program:
 The Government of Canada Public key Infrastructure:
 The Open Group Public key Infrastructure, Latest Proposals for an HMG PKI.
 Public key Infrastructure (X.509) (pkix) working group:
 Simple Public key Infrastructure (spki) working group:
 The Open Group Public key Infrastructure: