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Diagram of a public key infrastructure

Public Key Infrastructure (PKI) is a set of hardware, software, people, policies, and procedures needed to create, manage, distribute, use, store, and revoke digital certificates [1]. In cryptography, a PKI is an arrangement that binds public keys with respective user identities by means of a certificate authority (CA). The user identity must be unique within each CA domain. The binding is established through the registration and issuance process, which, depending on the level of assurance the binding has, may be carried out by software at a CA, or under human supervision. The PKI role that assures this binding is called the Registration Authority (RA) . For each user, the user identity, the public key, their binding, validity conditions and other attributes are made unforgeable in public key certificates issued by the CA.

The term trusted third party (TTP) may also be used for certificate authority (CA). The term PKI is sometimes erroneously used to denote public key algorithms, which do not require the use of a CA.

Alternatives

Broadly speaking, there are three approaches to getting this trust: Certificate Authorities (CAs), Web of Trust (WoT), and Simple public key infrastructure (SPKI).

Certificate Authorities

The primary role of the CA is to publish the key bound to a given user. This is done using the CA's own key, so that trust in the user key relies on one's trust in the validity of the CA's key. The mechanism that binds keys to users is called the Registration Authority (RA), which might or might not be separate from the CA. The key-user binding is established, depending on the level of assurance the binding has, by software or under human supervision.

The term trusted third party (TTP) may also be used for certificate authority (CA). Moreover, PKI is itself often used as a synonym for a CA implementation.

Web of Trust

An alternative approach to the problem of public authentication of public key information is the web of trust scheme, which uses self-signed certificates and third party attestations of those certificates. The singular term Web of Trust does not imply the existence of a single web of trust, or common point of trust, but rather any number of potentially disjoint "webs of trust". Examples of implementations of this approach are PGP (Pretty Good Privacy) and GnuPG (an implementation of OpenPGP, the standardized specification of PGP). Because PGP and implementations allow the use of e-mail digital signatures for self-publication of public key information, it is relatively easy to implement one's own Web of Trust. One of the benefits of the Web of Trust, such as in PGP, is that it can interoperate with a PKI CA fully-trusted by all parties in a domain (such as an internal CA in a company) that is willing to guarantee certificates, as a trusted introducer. Only if the "web of trust" is completely trusted, and because of the nature of a web of trust, trusting one certificate is granting trust to all the certificates in that web. A PKI is only as valuable as the standards and practices that control the issuance of certificates and including PGP or a personally instituted web of trust could significantly degrade the trustability of that enterprise's or domain's implementation of PKI.[2]

The web of trust concept was first put forth by PGP creator Phil Zimmermann in 1992 in the manual for PGP version 2.0:

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.

Simple public key infrastructure

Another alternative, which however does not deal with public authentication of public key information, is the simple public key infrastructure (SPKI) that grew out of 3 independent efforts to overcome the complexities of X.509 and PGP's web of trust. SPKI does not bind people to keys, since the key is what is trusted, rather than the person. SPKI does not use any notion of trust, as the verifier is also the issuer. This is called an "authorization loop" in SPKI terminology, where authorization is integral to its design.

History

The public disclosure of both secure key exchange and asymmetric key algorithms in 1976 by Diffie, Hellman, Rivest, Shamir, and Adleman changed secure communications entirely. With the further development of high speed digital electronic communications (the Internet and its predecessors), a need became evident for ways in which users could securely communicate with each other, and as a further consequence of that, for ways in which users could be sure with whom they were actually interacting.

Assorted cryptographic protocols were invented and analyzed within which the new cryptographic primitives could be effectively used. With the invention of the World Wide Web and its rapid spread, the need for authentication and secure communication became still more acute. Commercial reasons alone (e.g., e-commerce, on-line access to proprietary databases from Web browsers, etc.) were sufficient. Taher Elgamal and others at Netscape developed the SSL protocol ('https' in Web URLs); it included key establishment, server authentication (prior to v3, one-way only), and so on. A PKI structure was thus created for Web users/sites wishing secure communications.

Vendors and entrepreneurs saw the possibility of a large market, started companies (or new projects at existing companies), and began to agitate for legal recognition and protection from liability. An American Bar Association technology project published an extensive analysis of some of the foreseeable legal aspects of PKI operations (see ABA digital signature guidelines), and shortly thereafter, several US states (Utah being the first in 1995) and other jurisdictions throughout the world, began to enact laws and adopt regulations. Consumer groups and others raised questions of privacy, access, and liability considerations which were more taken into consideration in some jurisdictions than in others.

The enacted laws and regulations differed, there were technical and operational problems in converting PKI schemes into successful commercial operation, and progress has been far slower than pioneers had imagined it would be.[citation needed]

By the first few years of the 21st century, it had become clear that the underlying cryptographic engineering was not easy to deploy correctly, that operating procedures (manual or automatic) were not easy to correctly design (nor even if so designed, to execute perfectly, which the engineering required), and that such standards as existed were in some respects inadequate to the purposes to which they were being put.[citation needed]

PKI vendors have found a market, but it is not quite the market envisioned in the mid-90s, and it has grown both more slowly and in somewhat different ways than were anticipated [3]. PKIs have not solved some of the problems they were expected to, and several major vendors have gone out of business or been acquired by others. PKI has had the most success in government implementations; the largest PKI implementation to date is the Defense Information Systems Agency (DISA) PKI infrastructure for the Common Access Cards program.

PKI software

When deploying a PKI, the most important part is an appropriate CA piece of software. There are many solutions on the market:

Usage examples

PKIs of one type or another, and from any of several vendors, have many uses, including providing public keys and bindings to user identities which are used for:

See also

References

  1. ^ "LPKI - A Lightweight Public Key Infrastructure for the Mobile Environments", Proceedings of the 11th IEEE International Conference on Communication Systems (IEEE ICCS'08), pp.162-166, Guangzhou, China, Nov. 2008.
  2. ^ Ed Gerck, Overview of Certification Systems: x.509, CA, PGP and SKIP, in The Black Hat Briefings '99, http://www.securitytechnet.com/resource/rsc-center/presentation/black/vegas99/certover.pdf and http://mcwg.org/mcg-mirror/cert.htm
  3. ^ Stephen Wilson, Dec 2005, "The importance of PKI today", China Communications, Retrieved on 2010-12-13
  4. ^ Mark Gasson, Martin Meints, Kevin Warwick (2005), D3.2: A study on PKI and biometrics, FIDIS deliverable (3)2, July 2005


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