public key infrastructure

PKI provides "trust services" - in plain terms trusting the actions or outputs of entities, be they people or computers. Trust service objectives respect one or more of the following capabilities: Confidentiality, Integrity and Authenticity (CIA).

Confidentiality: Assurance that no entity can maliciously or unwittingly view a payload in clear text. Data is encrypted to make it secret, such that even if it was read, it appears as gibberish. Perhaps the most common use of PKI for confidentiality purposes is in the context of Transport Layer Security (TLS). TLS is a capability underpinning the security of data in transit, i.e. during transmission. A classic example of TLS for confidentiality is when using a web browser to log on to a service hosted on an internet based web site by entering a password.

Integrity: Assurance that if an entity changed (tampered) with transmitted data in the slightest way, it would be obvious it happened as its integrity would have been compromised. Often it is not of utmost importance to prevent the integrity being compromised (tamper proof), however, it is of utmost importance that if integrity is compromised there is clear evidence of it having done so (tamper evident).

Authenticity: Assurance that an entity has: i) certainty of what it's connecting to; and / or ii) can evidence its own legitimacy when connecting to a protected service. The former is labelled as server certificate authentication, typically employed when logging on at a web server. The latter is designated as client certificate authentication, for instance used when logging on with a smart card hosting a digital certificate and private key.

Public-key cryptography is a cryptographic technique that enables entities to securely communicate on an insecure public network, and reliably verify the identity of an entity via digital signatures.{{cite book|author=Adams, Carlisle |author2=Lloyd, Steve|title=Understanding PKI: concepts, standards, and deployment considerations|publisher=Addison-Wesley Professional|year=2003|isbn=978-0-672-32391-1|pages=11–15|url=https://books.google.com/books?id=ERSfUmmthMYC&pg=PA11}}

A public key infrastructure (PKI) is a system for the creation, storage, and distribution of digital certificates, which are used to verify that a particular public key belongs to a certain entity. The PKI creates digital certificates that map public keys to entities, securely stores these certificates in a central repository and revokes them if needed.{{cite book|author=Trček, Denis|title=Managing information systems security and privacy|publisher=Birkhauser|year=2006|isbn=978-3-540-28103-0|page=69|url=https://books.google.com/books?id=oswvyhAftLsC&pg=PA69}}{{cite book|author=Vacca, Jhn R.|title=Public key infrastructure: building trusted applications and Web services|publisher=CRC Press|year=2004|isbn=978-0-8493-0822-2|page=8|url=https://books.google.com/books?id=3kS8XDALWWYC&pg=PA8}}{{cite book|author=Viega, John|title=Network Security with OpenSSL|publisher=O'Reilly Media|year=2002|isbn=978-0-596-00270-1|pages=61–62|display-authors=etal}}

A PKI consists of:{{cite news|author=McKinley, Barton|title=The ABCs of PKI: Decrypting the complex task of setting up a public key infrastructure|work=Network World|date=January 17, 2001|url=http://www.networkworld.com/research/2000/0117feat.html|url-status=dead|archive-url=https://web.archive.org/web/20120529211639/http://www.networkworld.com/research/2000/0117feat.html|archive-date=May 29, 2012}}{{cite book|author=Al-Janabi, Sufyan T. Faraj|chapter=Combining Mediated and Identity-Based Cryptography for Securing Email|editor=Ariwa, Ezendu |display-editors=etal |title=Digital Enterprise and Information Systems: International Conference, Deis, [...] Proceedings|publisher=Springer|year=2012|isbn=9783642226021|pages=2–3|chapter-url=https://books.google.com/books?id=s-HVzAc_ZqEC&pg=PA2|display-authors=etal}}

• A certificate authority (CA), which stores, issues and signs the digital certificates;
• A registration authority (RA), which verifies the identity of entities requesting their digital certificates to be stored at the CA;
• A central directory, a secure location in which keys are stored and indexed;
• A certificate management system, which manages things like the access to stored certificates or the delivery of the certificates to be issued;
• A certificate policy, which states the PKI's requirements concerning its procedures. Its purpose is to allow outsiders to analyze the PKI's trustworthiness.

{{main|Certificate authority}}

The primary role of the CA is to digitally sign and publish the public key bound to a given user. This is done using the CA's own private key, so that trust in the user key relies on one's trust in the validity of the CA's key. When the CA is a third party separate from the user and the system, then it is called the Registration Authority (RA), which may or may not be separate from the CA."Mike Meyers CompTIA Security+ Certification Passport", by T. J. Samuelle, p. 137. The key-to-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.

{{cite web

|url = http://www.businessdictionary.com/definition/Trusted-Third-Party-Services-TTP-Services.html

|title = Trusted Third Party Service

|date = 4 March 2016

|first = William

|last = Henry

|access-date = 4 March 2016

|archive-date = 4 March 2016

|archive-url = https://web.archive.org/web/20160304211542/http://www.businessdictionary.com/definition/Trusted-Third-Party-Services-TTP-Services.html

|url-status = dead

}}

{{main|Certificate revocation}}

A certificate may be revoked before it expires, which signals that it is no longer valid. Without revocation, an attacker would be able to exploit such a compromised or mis-issued certificate until expiry.{{sfn|Smith|Dickinson|Seamons|2020|p=1}} Hence, revocation is an important part of a public key infrastructure.{{sfn|Sheffer|Saint-Andre|Fossati|2022|loc=7.5. Certificate Revocation}} Revocation is performed by the issuing certificate authority, which produces a cryptographically authenticated statement of revocation.{{sfn|Chung|Lok|Chandrasekaran|Choffnes|2018|p=3}}

For distributing revocation information to clients, timeliness of the discovery of revocation (and hence the window for an attacker to exploit a compromised certificate) trades off against resource usage in querying revocation statuses and privacy concerns.{{sfn|Smith|Dickinson|Seamons|2020|p=10}} If revocation information is unavailable (either due to accident or an attack), clients must decide whether to fail-hard and treat a certificate as if it is revoked (and so degrade availability) or to fail-soft and treat it as unrevoked (and allow attackers to sidestep revocation).{{sfn|Larisch|Choffnes|Levin|Maggs|2017|p=542}}

Due to the cost of revocation checks and the availability impact from potentially-unreliable remote services, Web browsers limit the revocation checks they will perform, and will fail-soft where they do.{{sfn|Smith|Dickinson|Seamons|2020|p=1-2}} Certificate revocation lists are too bandwidth-costly for routine use, and the Online Certificate Status Protocol presents connection latency and privacy issues. Other schemes have been proposed but have not yet been successfully deployed to enable fail-hard checking.{{sfn|Sheffer|Saint-Andre|Fossati|2022|loc=7.5. Certificate Revocation}}

In this model of trust relationships, a CA is a trusted third party – trusted both by the subject (owner) of the certificate and by the party relying upon the certificate.

According to NetCraft report from 2015,{{cite web|url=http://news.netcraft.com/archives/2015/05/13/counting-ssl-certificates.html|title=Counting SSL certificates|date=13 May 2015 }} the industry standard for monitoring active Transport Layer Security (TLS) certificates, states that "Although the global [TLS] ecosystem is competitive, it is dominated by a handful of major CAs — three certificate authorities (Symantec, Sectigo, GoDaddy) account for three-quarters of all issued [TLS] certificates on public-facing web servers. The top spot has been held by Symantec (or VeriSign before it was purchased by Symantec) ever since [our] survey began, with it currently accounting for just under a third of all certificates. To illustrate the effect of differing methodologies, amongst the million busiest sites Symantec issued 44% of the valid, trusted certificates in use — significantly more than its overall market share."

Following major issues in how certificate issuing was managed, all major players gradually distrusted Symantec-issued certificates, starting in 2017 and completed in 2021.{{cite web |title=CA:Symantec Issues |url=https://wiki.mozilla.org/CA:Symantec_Issues |website=Mozilla Wiki |access-date=10 January 2020 |ref=symantec-mozilla}}{{cite web |title=Chrome's Plan to Distrust Symantec Certificates |url=https://security.googleblog.com/2017/09/chromes-plan-to-distrust-symantec.html |website=Google security blog |access-date=10 January 2020 |ref=symantec-chrome}}{{cite web |title=JDK-8215012 : Release Note: Distrust TLS Server Certificates Anchored by Symantec Root CAs |url=https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8215012 |website=Java Bug Database |access-date=10 January 2020 |ref=symantec-java}}{{cite web |date=2023-09-05 |title=Information about distrusting Symantec certificate authorities |url=https://support.apple.com/en-us/HT208860 |access-date=2024-01-16 |website=Apple Support |language=en}}

This approach involves a server that acts as an offline 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.Single Sign-On Technology for SAP Enterprises: What does SAP have to say? {{cite web |url=http://www.secude.com/html/?id=1890 |title=Single Sign-On Technology for SAP Enterprises: What does SAP have to say? | May 2010 | SECUDE AG |access-date=2010-05-25 |url-status=dead |archive-url=https://web.archive.org/web/20110716031415/http://www.secude.com/html/?id=1890 |archive-date=2011-07-16 }}

Starting Sep 2020, TLS Certificate Validity reduced to 13 Months.

{{Main|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 one of 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. If the "web of trust" is completely trusted then, 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 trustworthiness of that enterprise's or domain's implementation of PKI.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 {{Webarchive|url=https://web.archive.org/web/20080905230841/http://mcwg.org/mcg-mirror/cert.htm |date=2008-09-05 }}

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

{{ quote

| 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.

}}

{{main|Simple public-key infrastructure}}

Another alternative, which does not deal with public authentication of public key information, is the simple public key infrastructure (SPKI), which grew out of three independent efforts to overcome the complexities of X.509 and PGP's web of trust. SPKI does not associate users with persons, 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.{{cite web |last=Gonzalez |first=Eloi |title=Simple Public Key Infrastructure |url=https://www.limited-entropy.com/docs/spki.pdf}} This type of PKI is specially useful for making integrations of PKI that do not rely on third parties for certificate authorization, certificate information, etc.; a good example of this is an air-gapped network in an office.

Decentralized identifiers (DIDs) eliminate dependence on centralized registries for identifiers as well as centralized certificate authorities for key management, which is the standard in hierarchical PKI. In cases where the DID registry is a distributed ledger, each entity can serve as its own root authority. This architecture is referred to as decentralized PKI (DPKI).{{cite web |title=Decentralized Identifiers (DIDs) |url=https://www.w3.org/TR/2019/WD-did-core-20191209/ |archive-url=https://web.archive.org/web/20200514014423/https://www.w3.org/TR/2019/WD-did-core-20191209/ |website=World Wide Web Consortium |archive-date=14 May 2020 |date=9 December 2019 |access-date=16 June 2020 |quote=""}}{{cite web |title=Decentralized Public Key Infrastructure |url=https://www.weboftrust.info/papers/#pki |website=weboftrust.info |date=23 December 2015 |access-date=23 June 2020 |quote=""}}

{{Unreferenced section|date=January 2014}}

Developments in PKI occurred in the early 1970s at the British intelligence agency GCHQ, where James Ellis, Clifford Cocks and others made important discoveries related to encryption algorithms and key distribution.{{cite web |last=Ellis |first=James H. |author-link=James H. Ellis |date=January 1970 |url=http://cryptocellar.web.cern.ch/cryptocellar/cesg/possnse.pdf |title=The Possibility of Secure Non-Secret Digital Encryption |archive-url=https://web.archive.org/web/20141030210530/https://cryptocellar.web.cern.ch/cryptocellar/cesg/possnse.pdf |archive-date=2014-10-30}} Because developments at GCHQ are highly classified, the results of this work were kept secret and not publicly acknowledged until the mid-1990s.

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, online access to proprietary databases from web browsers) 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.{{cite web |last=Prodromou |first=Agathoklis |date=2019-03-31 |title=TLS Security 2: A Brief History of SSL/TLS |url=https://www.acunetix.com/blog/articles/history-of-tls-ssl-part-2/ |access-date=2024-05-25 |website=Acunetix |language=en-US}} 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 U.S. states (Utah being the first in 1995) and other jurisdictions throughout the world began to enact laws and adopt regulations. Consumer groups raised questions about privacy, access, and liability considerations, which were more taken into consideration in some jurisdictions than in others.{{cite journal |date=2001 |title=PKI Assessment Guidelines |url=https://theworld.com/~goldberg/pagv30.pdf#page=43 |journal=Information Security Committee |volume=0 |issue=3 |pages=43}}

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

By the first few years of the 21st century, the underlying cryptographic engineering was clearly not easy to deploy correctly. Operating procedures (manual or automatic) were not easy to correctly design (nor even if so designed, to execute perfectly, which the engineering required). The standards that existed were insufficient.

PKI vendors have found a market, but it is not quite the market envisioned in the mid-1990s, and it has grown both more slowly and in somewhat different ways than were anticipated.Stephen Wilson, December 2005, [http://www.china-cic.org.cn/english/digital%20library/200512/3.pdf "The importance of PKI today"] {{webarchive|url=https://web.archive.org/web/20101122134646/http://www.china-cic.org.cn/english/digital%20library/200512/3.pdf |date=2010-11-22 }}, China Communications, Retrieved on 2010-12-13 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.

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 or XML Encryption standards if documents are encoded as XML);
• Authentication of users to applications (e.g., smart card logon, client authentication with SSL/TLS). 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/TLS. In both of these, initial set-up of a secure channel (a "security association") uses asymmetric key—i.e., public key—methods, whereas actual communication uses faster symmetric key—i.e., secret key—methods;
• Mobile signatures are electronic signatures that are created using a mobile device and rely on signature or certification services in a location independent telecommunication environment;Mark Gasson, Martin Meints, Kevin Warwick (2005), [http://www.fidis.net/resources/deliverables/hightechid/#c1785 D3.2: A study on PKI and biometrics], FIDIS deliverable (3)2, July 2005
• Internet of things requires secure communication between mutually trusted devices. A public key infrastructure enables devices to obtain and renew X.509 certificates, which are used to establish trust between devices and encrypt communications using TLS.

• OpenSSL is the simplest form of CA and tool for PKI. It is a toolkit, developed in C, that is included in all major Linux distributions, and can be used both to build your own (simple) CA and to PKI-enable applications. (Apache licensed)
• EJBCA is a full-featured, enterprise-grade, CA implementation developed in Java. It can be used to set up a CA both for internal use and as a service. (LGPL licensed)
• XiPKI,{{cite web|url=https://github.com/xipki/xipki |title=xipki/xipki · GitHub |publisher=Github.com |access-date=2016-10-17}} CA and OCSP responder. With SHA-3 support, implemented in Java. (Apache licensed)
• XCA{{cite web |last=Hohnstädt |first=Christian |title=X - Certificate and Key management |url=https://hohnstaedt.de/xca/ |access-date=2023-12-29 |website=hohnstaedt.de}} is a graphical interface, and database. XCA uses OpenSSL for the underlying PKI operations.
• DogTag is a full featured CA developed and maintained as part of the Fedora Project.
• CFSSL{{cite web|url=https://blog.cloudflare.com/introducing-cfssl/|title=Introducing CFSSL - Cloudflare's PKI toolkit|publisher=CloudFlare|website=CloudFlare's Blog|last=Sullivan|first=Nick|date=10 July 2014|access-date=18 April 2018}}{{cite web|url=https://github.com/cloudflare/cfssl|title=cloudflare/cfssl · GitHub|publisher=Github.com|access-date=18 April 2018}} open source toolkit developed by CloudFlare for signing, verifying, and bundling TLS certificates. (BSD 2-clause licensed)
• Vault{{cite web|url=https://github.com/hashicorp/vault|title=hashicorp/vault · GitHub|publisher=Github.com|access-date=18 April 2018}} tool for securely managing secrets (TLS certificates included) developed by HashiCorp. (Mozilla Public License 2.0 licensed)
• Boulder, an ACME-based CA written in Go. Boulder is the software that runs Let's Encrypt.

{{see also|X.509#Security|Comodo Group#2011 breach incident|Diginotar#Issuance of fraudulent certificates}}

Some argue that purchasing certificates for securing websites by SSL/TLS and securing software by code signing is a costly venture for small businesses.{{cite web |url=https://www.forbes.com/sites/richardstiennon/2013/05/14/should-we-abandon-digital-certificates-or-learn-to-use-them-effectively |title=Should We Abandon Digital Certificates, Or Learn to Use Them Effectively? |work=Forbes}} However, the emergence of free alternatives, such as Let's Encrypt, has changed this. HTTP/2, the latest version of HTTP protocol, allows unsecured connections in theory; in practice, major browser companies have made it clear that they would support this protocol only over a PKI secured TLS connection.{{cite web |url=https://http2.github.io/faq/ |title=HTTP/2 Frequently Asked Questions |via=Github |work=HTTP/2 wiki}} Web browser implementation of HTTP/2 including Chrome, Firefox, Opera, and Edge supports HTTP/2 only over TLS by using the ALPN extension of the TLS protocol. This would mean that, to get the speed benefits of HTTP/2, website owners would be forced to purchase SSL/TLS certificates controlled by corporations.

Currently the majority of web browsers are shipped with pre-installed intermediate certificates issued and signed by a certificate authority, by public keys certified by so-called root certificates. This means browsers need to carry a large number of different certificate providers, increasing the risk of a key compromise.{{cite web |title=Root Certificate vs Intermediate Certificates |url=https://aboutssl.org/root-certificates-vs-intermediate-certificates/ |access-date=2022-05-02 |website=About SSL |language=en-US}}

When a key is known to be compromised, it could be fixed by revoking the certificate, but such a compromise is not easily detectable and can be a huge security breach. Browsers have to issue a security patch to revoke intermediary certificates issued by a compromised root certificate authority.{{cite web |work=Microsoft Security Advisory |title=Fraudulent Digital Certificates could allow spoofing |url=http://support.microsoft.com/kb/2524375 |date=March 23, 2011 |access-date=2011-03-24|publisher=Microsoft}}

• Cryptographic agility (crypto-agility)
• Certificate Management Protocol (CMP)
• Certificate Management over CMS (CMC)
• Simple Certificate Enrollment Protocol (SCEP)
• Enrollment over Secure Transport (EST)
• Automatic Certificate Management Environment (ACME)
• Resource Public Key Infrastructure (RPKI)

{{reflist}}

• {{cite book | doi = 10.1145/3278532.3278543 | chapter-url = https://taejoong.github.io/pubs/publications/chung-2018-ocsp.pdf | chapter = Is the Web Ready for OCSP Must-Staple? | title = Proceedings of the Internet Measurement Conference 2018 | year = 2018 | last1 = Chung | first1 = Taejoong | last2 = Lok | first2 = Jay | last3 = Chandrasekaran | first3 = Balakrishnan | last4 = Choffnes | first4 = David | last5 = Levin | first5 = Dave | last6 = Maggs | first6 = Bruce M. | last7 = Mislove | first7 = Alan | last8 = Rula | first8 = John | last9 = Sullivan | first9 = Nick | last10 = Wilson | first10 = Christo | pages = 105–118 | isbn = 9781450356190 | s2cid = 53223350 }}
• {{cite book | doi = 10.1109/sp.2017.17 | doi-access = free | chapter = CRLite: A Scalable System for Pushing All TLS Revocations to All Browsers | title = 2017 IEEE Symposium on Security and Privacy (SP) | year = 2017 | last1 = Larisch | first1 = James | last2 = Choffnes | first2 = David | last3 = Levin | first3 = Dave | last4 = Maggs | first4 = Bruce M. | last5 = Mislove | first5 = Alan | last6 = Wilson | first6 = Christo | pages = 539–556 | isbn = 978-1-5090-5533-3 | s2cid = 3926509 }}
• {{cite ietf | rfc = 9325 | last1 = Sheffer | first1 = Yaron | last2 = Saint-Andre | first2 = Pierre | last3 = Fossati | first3 = Thomas | date = November 2022 | title = Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) }}
• {{cite book | doi = 10.14722/ndss.2020.24084 | doi-access = free | chapter = Let's Revoke: Scalable Global Certificate Revocation | title = Proceedings 2020 Network and Distributed System Security Symposium | year = 2020 | last1 = Smith | first1 = Trevor | last2 = Dickinson | first2 = Luke | last3 = Seamons | first3 = Kent | isbn = 978-1-891562-61-7 | s2cid = 211268930 }}

• {{cite book |last1=Choudhury |first1=Suranjan |title=Public key infrastructure: implementation and design |last2=Bhatnagar |first2=Kartik |last3=Haque |first3=Wasim |date=2002 |publisher=M&T Books |isbn=978-0-7645-4879-6 |series=Professional mindware |location=New York, NY}}
• {{cite book |last1=Schmeh |first1=Klaus |title=Cryptography and public key infrastructure on the Internet |date=2003 |publisher=Wiley |location=Chichester, West Sussex, England |isbn=0-470-84745-X}}

• [//w3techs.com/technologies/history_overview/ssl_certificate Market share trends for SSL certificate authorities] (W3Techs)

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