These proposals look towards a future when the Internet (and encryption) are as ubiquitous as the telephone. Finding a user's public key should be as easy (and as reliable) as looking up a friend's phone number. Shopping via the Web should be as easy as buying merchandise over the phone (and less subject to fraud, if possible). Interstate -- and even global -- commercial transactions should be easy; this implies interoperability of software components and some kind of policy coordination.
All of this implies a set of standards for applications that use encryption: what algorithms do they use, and how do they use them? In what format should public keys be stored or served? What format should a lookup request take? On the policy level, what minimum requirements should a public key server or certificate authority meet to pass a test of reliability?
To date, several groups have proposed standards and policies. This document concentrates on groups working within the U.S. legal framework.
The IETF X509 PKI Working Group has produced a proposed Architecture for Public-Key Infrastructure based on X.509 certificates.
The National Institute of Standards and Technology (NIST) has produced a set of documents describing Requirements for the Federal Public Infrastructure and related issues. The IETF SPKI (Simple Public Key Infrastructure) Working Group has produced specifications for a non-X.509 certificate format and accompanying infrastructure.
Ron Rivest created a completely different protocol which is speficied in his paper on a Simple Distributed Security Infrastructure.
The The Open Group PKI Task Group has produced a proposal for an Architecture for Public-Key Infrastructure.