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Diagram of a public key infrastructure
The 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.

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 for each CA. 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.


Broadly speaking, there are 3 approaches to getting this trust: Certificate Authorities (CAs), Web of Trust (WoT), and Simple public key infrastructure (SPKI).

Certificate Authorities

CAs are established, often commercial, web sitesThe 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.

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

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.

Additionally, PKI supports message encryption and digital signatures that further enhance transactional security. While essential services such as certificate validation and revocation, key backup and recovery, and simultaneous update of key pairs minimize the administrative workload for a PKI, features such as audit of key history and time-stamping enhance security control and management. And last but not least, the PKI supports cross-certification, which is key to creating a truly federated identity by enabling seamless integration among circles of trust.

In comparison to Kerberos, PKI provides enhanced security, greater scalability and easier administration, control and management of the infrastructure. As a result, PKI enables a much larger community of users, consumers and partners to communicate and transact more dynamically, securely, reliably and cost-effectively.


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 URL); 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 (Utahmarker 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.

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 standard as existed were in some respects inadequate to the purposes to which they were being put.

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. 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 several solutions on the market:

  • KEYNECTIS: Offers a product called Sequoia as well as hosted solutions

  • Comodo: Is the second-largest Certificate Authority for high-assurance digital certificates. It offers PKI and certificate management products, as well as Two-factor Authentication using Public Key infrastructure.

  • Entrust: Offer a product called Entrust Authority. Entrust offers PKI software and a managed service options mainly in the .gov and financial services space

  • GlobalSign: Offers TrustedRoot, a PKI CA Rootstore chaining program (Root Sign) which allows you to get immediate trust for your SSL, S/MIME and code signing certificates by chaining your Microsoft CA or Inhouse CA Root Certificate to the pre-trusted GlobalSign root certificate.

  • IBM: Offers PKI Services for z/OS.

  • Novell: Offers the Novell Certificate Server, which is integrated into the eDirectory. Alternatively, the eDirectory add-on product cv act PKIntegrated (provided by a third party vendor at additional costs) can be used.

  • Red Hat Certificate System: Formerly the Netscape Certificate Server. Its now fully open source. See

  • RSA Security: Offers a product called RSA Certificate Manager (Previously known as Keon).

  • Safelayer: Offers a family of PKI software products called KeyOne and a PKI-broker called TrustedX.

  • "SECUDE": Offers secure single sign-on solutions for SAP, based on PKI technology through SECUDE Secure Login

  • TrustAlert: Offers her RESEPT solution. This solution automatically provides client side certificates without the need for CRLs.

  • VeriSign: Offers a managed service called VeriSign® Managed PKI Service. VeriSign® Managed PKI Service is a flexible, hosted platform enabling complete management of digital certificates for authentication, encryption and digital signing.

  • Verizon Business: Verizon Business Offers outsourced and in-house PKI offerings. UniCERT PKI software can be deployed on premise or customers may operate their PKI in a completely outsourced hosted model. Verizon Business customers include Governments and Enterprises around the world.

  • SeguriData: Offers a product called SeguriServer. SeguriData offers a whole suite of PKI based solutions.

  • OpenTrust: Offers a product called OpenTrust PKI.

  • Signicat: Offers an online identity broker and digital signature service called id.signicat.

  • Djigzo email encryption: Open source email encryption gateway with support for S/MIME and PDF encryption with one-time-password via SMS.

  • Echoworx: All data is encrypted using industry trusted standard PKI (Public Key Infrastructure) and S/MIME technologies for strong encryption and digital signatures.

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


External links

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