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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 unforgettable 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 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[]

Main article: 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: Template:Quotation

Temporary Certificates & Single Sign-On[]

This approach involves a server that acts as an online certificate authority within a single sign-on system. A single sign-on server will issue digital certificates into the client system, but never stores them. Users can execute programs, etc. with the temporary certificate. It is common to find this solution variety with x.509-based certificates.[3]

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.


The concepts and use of Public Key Infrastructure were discovered by British scientists in GCHQ in 1969 with Ellis. After the re-discovery and commercial use of PKI by Rivest, Shamir, Diffie and others, the British government considered releasing the records of GCHQ's successes in this field. However, the untimely publication of Spycatcher meant that the government once again issued a gag order and full details of GCHQ achievement were never revealed.

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.Template: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.Template: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 [4]. 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:

  • Ascertia: The Company remains independent from Certification Authority products and vendors and therefore its Professional Service is free to provide objective advice for the deployment of PKI.
  • 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.
  • Cryptomathic: Offers a product called CCA.
  • DigiCert: is a certificate authority that offers a managed PKI service called DigiCert® Enterprise Managed PKI for SSL/TLS certificates.
  • 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.
  • EJBCA: Open source, platform independent solution implemented in Java EE.
  • Entrust: Offer a product suite called Entrust Authority. Entrust's offering includes off-the-shelf PKI software as well as a hosted, managed solution mainly in the government and financial industry spaces.
  • 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.
  • KEYNECTIS: Offers a product called Sequoia as well as hosted solutions
  • Linux: Linux supports OpenSSL and OpenCA, which are two open source CA solutions.
  • Microsoft: Windows 2000 Server, Windows Server 2003, Windows Server 2008 and Windows Server 2008 R2 all contain CA software, Active Directory Certificate Services (ADCS) which is integrated into the Active Directory and doesn't require additional license fees.
  • Nexus: Offers the open standards based Nexus Certificate Manager, which enables deployment of multiple virtual CAs with different policies, operator groups, certificate and smart card procedures, certificate distribution, etc on a single platform. It is part of the Nexus portfolio of integrated PKI solutions that are available in both software and ready-to-use virutal appliance forms.
  • 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.
  • OpenTrust: Offers a product called OpenTrust PKI.
  • OpenXPKI: Offers a flexible and modular enterprise-grade Open Source PKI server.
  • PKTP Public Key Transaction Processor: Provides for digitally signed metal exchanges.
  • Red Hat Certificate System: Formerly the Netscape Certificate Server. Its now fully open source and known as Dogtag Certificate System. 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
  • SeguriData: Offers a product called SeguriServer. SeguriData offers a whole suite of PKI based solutions.
  • Signicat: Offers an online identity broker and digital signature service called id.signicat.
  • Thursby Software: Offers a Mac-based Public Key Infrastructure Client for Microsoft Active Directory/CAC/PIV called ADmitMac PKI.
  • TrustAlert: Offers their 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.
  • Voltage Security: Offers the next generation PKI called Identity-Based Encryption as well as an online service (Voltage Security Network).
  • Zix Corporation: Offers a hosted and shared public email encryption key directory with millions of keys, and software (ZixMail) or appliance (ZixVPM/Gateway) implementation.
  • S3: SCAN Shared Security Services (S#) a single enterprise-wide architecture, providing identity management, authentication and secured transactions. S3 is a PKI solution to address all security needs for businesses in an integrated manner and provides automation for easier and cost-effective way to manage daily security administration and operations for organizations
  • e-sign: e-sign is a service provider under Access technical Solutions Pvt Ltd who provides services like Electronic signatures or digital signatures and SSL certificates to national and international clients. We are a licensed certifying body under the Information Technology Act of India authorized to issue digital certificates in and outside the country.
  • eMudhra: eMudhra Digital Signature Certificates an Initiative by 3i Infotech Consumer Services Limited is a licensed CA in India offering Class1,2 & 3 Level DSCs as per the business or individual requirements.

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:

  • Encryption and/or sender authentication of e-mail messages (e.g., using OpenPGP or S/MIME).
  • Encryption and/or authentication of documents (e.g., the XML Signature [2] or XML Encryption [3] standards if documents are encoded as XML).
  • Authentication of users to applications (e.g., smart card logon, client authentication with SSL). There's experimental usage for digitally-signed HTTP authentication in the Enigform and mod_openpgp projects.
  • Bootstrapping secure communication protocols, such as Internet key exchange (IKE) and SSL. In both of these, initial set-up of a secure channel (a "security association") uses asymmetric key (a.k.a. public key) methods, whereas actual communication uses faster symmetric key (a.k.a. secret key) methods.
  • Mobile signatures[5] are electronic signatures that are created using a mobile device and rely on signature or certification services in a location independent telecommunication environment.


  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, and
  3. Single Sign-On Technology for SAP Enterprises: What does SAP have to say? [1]
  4. Stephen Wilson, Dec 2005, "The importance of PKI today", China Communications, Retrieved on 2010-12-13
  5. Mark Gasson, Martin Meints, Kevin Warwick (2005), D3.2: A study on PKI and biometrics, FIDIS deliverable (3)2, July 2005

External links[]

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