public key certificate

File:Chain of trust v2.svg.]]

The Transport Layer Security (TLS) protocol – as well as its outdated predecessor, the Secure Sockets Layer (SSL) protocol – ensures that the communication between a client computer and a server is secure. The protocol requires the server to present a digital certificate, proving that it is the intended destination. The connecting client conducts certification path validation, ensuring that:

1. The subject of the certificate matches the hostname (not to be confused with the domain name) to which the client is trying to connect.
2. A trusted certificate authority has signed the certificate.

The Subject field of the certificate must identify the primary hostname of the server as the Common Name.{{Clarify|date=September 2022}} The hostname must be publicly accessible, not using private addresses or reserved domains.{{cite web|url=https://docs.digicert.com/en/certcentral/certificate-tools/discovery-user-guide/tls-ssl-certificate-vulnerabilities/internal-names.html |title=Internal names |publisher=DigiCert Documentation}} A certificate may be valid for multiple hostnames (e.g., a domain and its subdomains). Such certificates are commonly called Subject Alternative Name (SAN) certificates or Unified Communications Certificates (UCC). These certificates contain the Subject Alternative Name field, though many CAs also put them into the Subject Common Name field for backward compatibility. If some of the hostnames contain an asterisk (*), a certificate may also be called a wildcard certificate.

Once the certification path validation is successful, the client can establish an encrypted connection with the server.

Internet-facing servers, such as public web servers, must obtain their certificates from a trusted, public certificate authority (CA).

Client certificates authenticate the client connecting to a TLS service, for instance to provide access control. Because most services provide access to individuals, rather than devices, most client certificates contain an email address or personal name rather than a hostname. In addition, the certificate authority that issues the client certificate is usually the service provider to which client connects because it is the provider that needs to perform authentication. Some service providers even offer free SSL certificates as part of their packages.{{Cite web |title=Free SSL Certificate {{!}} IONOS by 1&1 |url=https://www.ionos.co.uk/security/free-ssl |access-date=2022-07-15 |website=www.ionos.co.uk |language=en-GB |archive-date=2022-07-18 |archive-url=https://web.archive.org/web/20220718121336/https://www.ionos.co.uk/security/free-ssl |url-status=live }}

While most web browsers support client certificates, the most common form of authentication on the Internet is a username and password pair. Client certificates are more common in virtual private networks (VPN) and Remote Desktop Services, where they authenticate devices.

In accordance with the S/MIME protocol, email certificates can both establish the message integrity and encrypt messages. To establish encrypted email communication, the communicating parties must have their digital certificates in advance. Each must send the other one digitally signed email and opt to import the sender's certificate.

Some publicly trusted certificate authorities provide email certificates, but more commonly S/MIME is used when communicating within a given organization, and that organization runs its own CA, which is trusted by participants in that email system.

{{Main|Root certificate|Self-signed certificate}}

A self-signed certificate is a certificate with a subject that matches its issuer, and a signature that can be verified by its own public key.

Self-signed certificates have their own limited uses. They have full trust value when the issuer and the sole user are the same entity. For example, the Encrypting File System on Microsoft Windows issues a self-signed certificate on behalf of the encrypting user and uses it to transparently decrypt data on the fly. The digital certificate chain of trust starts with a self-signed certificate, called a root certificate, trust anchor, or trust root. A certificate authority self-signs a root certificate to be able to sign other certificates.

An intermediate certificate has a similar purpose to the root certificate – its only use is to sign other certificates. However, an intermediate certificate is not self-signed. A root certificate or another intermediate certificate needs to sign it.

An end-entity or leaf certificate is any certificate that cannot sign other certificates. For instance, TLS/SSL server and client certificates, email certificates, code signing certificates, and qualified certificates are all end-entity certificates.

File:Subject Alt Names on Firefox 90 screenshot.png]]

Subject Alternative Name (SAN) certificates are an extension to X.509 that allows various values to be associated with a security certificate using a subjectAltName field.{{cite web

| url=https://www.openssl.org/docs/manmaster/man5/x509v3_config.html#Subject-Alternative-Name

| title=x509v3_config - X509 V3 certificate extension configuration format

| publisher=OpenSSL

| access-date=2020-01-16}} These values are called Subject Alternative Names (SANs). Names include:{{IETF RFC|5280}}: 4.2.1.6. Subject Alternative Name

• Email addresses
• IP addresses
• URIs
• DNS names: this is usually also provided as the Common Name RDN within the Subject field of the main certificate.
• Directory names: alternative Distinguished Names to that given in the Subject.
• Other names, given as a General Name or Universal Principal Name: a registered object identifier followed by a value.

{{IETF RFC|2818}} (May 2000) specifies Subject Alternative Names as the preferred method of adding DNS names to certificates, deprecating the previous method of putting DNS names in the commonName field.{{cite web |url=https://developers.google.com/web/updates/2017/03/chrome-58-deprecations#remove_support_for_commonname_matching_in_certificates |title=Deprecations and Removals in Chrome 58 |last=Medley |first=Joseph |date=March 2017 |publisher=Google Inc. |access-date=2022-01-04 }} Google Chrome version 58 (March 2017) removed support for checking the commonName field at all, instead only looking at the SANs.

As shown in the picture of Wikimedia's section on the right, the SAN field can contain wildcards.{{cite web|url=https://docs.digicert.com/en/certcentral/certificate-tools/certificate-lifecycle-automation-guides/certcentral-managed-automation/common-name--cn--for-a-wildcard-certificate.html |title=Common Name (CN) for a wildcard certificate |publisher=DigiCert Documentation}} Not all vendors support or endorse mixing wildcards into SAN certificates.{{cite web|url=https://www.thawte.com/resources/pdfs/Thawte_Multiuse_SSL_WP.pdf |title=Wildcard and SAN: Understanding Multi-Use SSL Certificates |date=2013 |publisher=Thawte}}

File:Let’s Encrypt example certificate on Firefox 94 screenshot.png: {{code|*}})]]

A public key certificate which uses an asterisk {{code|*}} (the wildcard) in its domain name fragment is called a Wildcard certificate.

Through the use of {{code|*}}, a single certificate may be used for multiple sub-domains. It is commonly used for transport layer security in computer networking.

For example, a single wildcard certificate for {{code|https://*.example.com}} will secure all these subdomains on the {{code|https://*.example.com}} domain:

• {{code|payment.example.com}}
• {{code|contact.example.com}}
• {{code|login-secure.example.com}}
• {{code|www.example.com}}

Instead of getting separate certificates for subdomains, you can use a single certificate for all main domains and subdomains and reduce cost.{{cite web|title=Wildcard Certificate Explained in Simpler Terms|url=http://searchsecurity.techtarget.com/definition/wildcard-certificate|date = 23 May 2016}}

Because the wildcard only covers one level of subdomains (the asterisk doesn't match full stops),

{{cite web

| url= http://tools.ietf.org/html/rfc2818#page-5

| title= RFC 2818 - HTTP Over TLS

| publisher= Internet Engineering Task Force

| date= May 2000

| quote= [...] *.a.com matches foo.a.com but not bar.foo.a.com.

| page= 5

| accessdate= 2014-12-15

}}

these domains would not be valid for the certificates:

{{cite IETF

| url= https://tools.ietf.org/html/rfc2595#page-3

| title= RFC 2595 - Using TLS with IMAP, POP3 and ACAP

| publisher= Internet Engineering Task Force

| date= June 1999

| quote= For example, *.example.com would match a.example.com, foo.example.com, etc. but would not match example.com.

| page= 3

| doi= 10.17487/RFC2595

| rfc=2595

| accessdate= 2014-12-15

| last1= Newman

| first1= C.

| doi-access= free

}}

• {{code|test.login.example.com}}
• {{code|example.com}}

Note possible exceptions by CAs, for example wildcard-plus cert by DigiCert contains an automatic "Plus" property for the naked domain {{code|example.com}}.{{cn|date=September 2024}}

Only a single level of subdomain matching is supported in accordance with {{IETF RFC|2818}}.[https://support.quovadisglobal.com/KB/a60/will-ssl-work-with-multilevel-wildcards.aspx?KBSearchID=10223 Wildcard SSL certificate limitation on QuovadisGlobal.com]

It is not possible to get a wildcard for an Extended Validation Certificate.

{{cite web

| url= https://cabforum.org/wp-content/uploads/EV-V1_5_2Libre.pdf#page=16

| title= Guidelines For The Issuance And Management Of Extended Validation Certificates, Version 1.5.2

| publisher= CA/Browser Forum

| quote= Wildcard certificates are not allowed for EV Certificates.

| page= 10

| date=2014-10-16

| accessdate= 2014-12-15

}}

A workaround could be to add every virtual host name in the Subject Alternative Name (SAN) extension,[https://www.openssl.org/docs/apps/x509v3_config.html#Subject_Alternative_Name_ x509v3_config Subject Alternative Name][https://web.archive.org/web/20120613211438/http://www.symantec.com/theme.jsp?themeid=san-ssl-certificates The SAN option is available for EV SSL Certificates on Symantec.com] the major problem being that the certificate needs to be reissued whenever a new virtual server is added. (See Transport Layer Security § Support for name-based virtual servers for more information.)

Wildcards can be added as domains in multi-domain certificates or Unified Communications Certificates (UCC). In addition, wildcards themselves can have {{code|subjectAltName}} extensions, including other wildcards. For example, the wildcard certificate {{code|*.wikipedia.org}} has {{code|*.m.wikimedia.org}} as a Subject Alternative Name. Thus it secures {{code|www.wikipedia.org}} as well as the completely different website name {{code|meta.m.wikimedia.org}}.[http://www.ssltools.com/certificate_lookup/www.wikipedia.org SSLTools Certificate Lookup of Wikipedia.org's wildcard ssl certificate]

{{IETF RFC|6125}} argues against wildcard certificates on security grounds, in particular "partial wildcards".

{{cite IETF

| url= https://tools.ietf.org/html/rfc6125#section-7.2

| date= March 2011

| publisher= Internet Engineering Task Force

| title= RFC 6125 - Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)

| quote= This document states that the wildcard character '*' SHOULD NOT be included in presented identifiers but MAY be checked by application clients (mainly for the sake of backward compatibility with deployed infrastructure). [...] Several security considerations justify tightening the rules: [...]

| page= 31

| doi= 10.17487/RFC6125

| rfc=6125

| accessdate= 2014-12-10

| last1= Saint-Andre

| first1= P.

| last2= Hodges

| first2= J.

| doi-access= free}}

The wildcard applies only to one level of the domain name. {{code|*.example.com}} matches {{code|sub1.example.com}} but not {{code|example.com}} and not {{code|sub2.sub1.domain.com}}

The wildcard may appear anywhere inside a label as a "partial wildcard" according to early specifications{{Cite IETF |url=https://tools.ietf.org/html/rfc2818.html |title=RFC 2818 - HTTP Over TLS |last1=Rescorla |first1=E. |date=May 2000 |website=tools.ietf.org |doi=10.17487/RFC2818 |rfc=2818 |language=en |accessdate=2019-04-20}}

:{{code|f*.domain.com}} is OK. It will match {{code|frog.domain.com}} but not {{code|frog.super.domain.com}}

:{{code|baz*.example.net}} is OK and matches {{code|baz1.example.net}}

:{{code|*baz.example.net}} is OK and matches {{code|foobaz.example.net}}

:{{code|b*z.example.net}} is OK and matches {{code|buzz.example.net}}

However, use of "partial-wildcard" certs is not recommended. As of 2011, partial wildcard support is optional, and is explicitly disallowed in SubjectAltName headers that are required for multi-name certificates.{{Cite IETF |url=https://tools.ietf.org/html/rfc6125.html |title=RFC 6125 - Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS) |last1 = Saint-Andre |first1 = P. |last2 = Hodges|first2 = J.|date=March 2011 |website=tools.ietf.org |doi=10.17487/RFC6125 |rfc=6125 |language=en |access-date=2019-04-20|doi-access=free }} All major browsers have deliberately removed support for partial-wildcard certificates;{{cite web |url=https://codereview.chromium.org/762013002 |title=Disallow support for a*.example.net, *a.example.net, and a*b.example.net in certificate wildcard handling |publisher=The Chromium Projects, Google Inc.|date=3 December 2014 |accessdate=21 October 2020}}{{cite web |url=https://bugzilla.mozilla.org/show_bug.cgi?id=1107791 |title=Limit wildcard DNS ID support to names of the form *.example.com (not foo*.example.com) |publisher=The Mozilla Foundation |date=10 December 2014 |accessdate=21 October 2020}} they will result in a "SSL_ERROR_BAD_CERT_DOMAIN" error. Similarly, it is typical for standard libraries in programming languages to not support "partial-wildcard" certificates. For example, any "partial-wildcard" certificate will not work with the latest versions of both Python{{cite web |url=https://bugs.python.org/issue23033 |title=Disallow support for a*.example.net, *a.example.net, and a*b.example.net in certificate wildcard handling |publisher=The Python Software Foundation |date=26 November 2017 |accessdate=21 October 2020}} and Go. Thus,

Do not allow a label that consists entirely of just a wildcard unless it is the left-most label

:{{code|sub1.*.domain.com}} is not allowed.

A cert with multiple wildcards in a name is not allowed.

:{{code|*.*.domain.com}}

A cert with {{code|*}} plus a top-level domain is not allowed.

:{{code|*.com}}

Too general and should not be allowed.

:{{code|*}}

International domain names encoded in ASCII (A-label) are labels that are ASCII-encoded and begin with {{code|xn--}}. URLs with international labels cannot contain wildcards.{{cite web|url=https://docs.digicert.com/en/certcentral/manage-certificates/public-certificates---data-entries-that-violate-industry-standards.html#idp433257 |title=Restrictions on data entries for public certificates |publisher=DigiCert Documentation}}

:{{code|xn--caf-dma.com}} is {{code|café.com}}

:{{code|xn--caf-dma*.com}} is not allowed

:{{code|Lw*.xn--caf-dma.com}} is allowed

• EMV certificate: EMV is a payment method based on a technical standard for payment cards, payment terminals and automated teller machines (ATM). EMV payment cards are preloaded with a card issuer certificate, signed by the EMV certificate authority{{cite web|url=https://emvca.com/index.html#EMV-CA|title=EMV CA|date=2 December 2010|publisher=EMV Certificate Authority Worldwide|access-date=January 20, 2020|archive-date=4 July 2020|archive-url=https://web.archive.org/web/20200704133359/https://emvca.com/index.html#EMV-CA|url-status=live}} to validate authenticity of the payment card during the payment transaction.
• Code-signing certificate: Certificates can validate apps (or their binaries) to ensure they were not tampered with during delivery.
• Qualified certificate: A certificate identifying an individual, typically for electronic signature purposes. These are most commonly used in Europe, where the eIDAS regulation standardizes them and requires their recognition.
• Role-based certificate: Defined in the X.509 Certificate Policy for the Federal Bridge Certification Authority (FBCA), role-based certificates "identify a specific role on behalf of which the subscriber is authorized to act rather than the subscriber’s name and are issued in the interest of supporting accepted business practices."{{Cite web |url=https://www.idmanagement.gov/wp-content/uploads/sites/1171/uploads/fpki-x509-cert-policy-fbca.pdf |title=X.509 Certificate Policy For The Federal Bridge Certification Authority (FBCA) |access-date=2021-05-07 |archive-date=2021-03-18 |archive-url=https://web.archive.org/web/20210318002502/https://www.idmanagement.gov/wp-content/uploads/sites/1171/uploads/fpki-x509-cert-policy-fbca.pdf |url-status=live }}
• Group certificate: Defined in the X.509 Certificate Policy for the Federal Bridge Certification Authority (FBCA), for "cases where there are several entities acting in one capacity, and where non-repudiation for transactions is not desired."{{Cite web |url=https://www.idmanagement.gov/wp-content/uploads/sites/1171/uploads/fpki-x509-cert-policy-fbca.pdf |title=X.509 Certificate Policy For The Federal Bridge Certification Authority (FBCA) |access-date=2021-05-07 |archive-date=2021-03-18 |archive-url=https://web.archive.org/web/20210318002502/https://www.idmanagement.gov/wp-content/uploads/sites/1171/uploads/fpki-x509-cert-policy-fbca.pdf |url-status=live }}

{{See also|X.509#Structure of a certificate}}These are some of the most common fields in certificates. Most certificates contain a number of fields not listed here. Note that in terms of a certificate's X.509 representation, a certificate is not "flat" but contains these fields nested in various structures within the certificate.

• Serial Number: Used to uniquely identify the certificate within a CA's systems. In particular this is used to track revocation information.
• Subject: The entity a certificate belongs to: a machine, an individual, or an organization.
• Issuer: The entity that verified the information and signed the certificate.
• Not Before: The earliest time and date on which the certificate is valid. Usually set to a few hours or days prior to the moment the certificate was issued, to avoid clock skew problems.
• Not After: The time and date past which the certificate is no longer valid.
• Key Usage: The valid cryptographic uses of the certificate's public key. Common values include digital signature validation, key encipherment, and certificate signing.
• Extended Key Usage: The applications in which the certificate may be used. Common values include TLS server authentication, email protection, and code signing.
• Public Key: A public key belonging to the certificate subject.
• Signature Algorithm: This contain a hashing algorithm and a digital signature algorithm. For example "sha256RSA" where sha256 is the hashing algorithm and RSA is the signature algorithm.
• Signature: The body of the certificate is hashed (hashing algorithm in "Signature Algorithm" field is used) and then the hash is signed (signature algorithm in the "Signature Algorithm" field is used) with the issuer's private key.

This is an example of a decoded SSL/TLS certificate retrieved from SSL.com's website. The issuer's common name (CN) is shown as SSL.com EV SSL Intermediate CA RSA R3, identifying this as an Extended Validation (EV) certificate. Validated information about the website's owner (SSL Corp) is located in the Subject field. The X509v3 Subject Alternative Name field contains a list of domain names covered by the certificate. The X509v3 Extended Key Usage and X509v3 Key Usage fields show all appropriate uses.

In the European Union, (advanced) electronic signatures on legal documents are commonly performed using digital signatures with accompanying identity certificates. However, only qualified electronic signatures (which require using a qualified trust service provider and signature creation device) are given the same power as a physical signature.

{{Main|Certificate authority}}

File:PublicKeyCertificateDiagram It.svg

In the X.509 trust model, a certificate authority (CA) is responsible for signing certificates. These certificates act as an introduction between two parties, which means that a CA acts as a trusted third party. A CA processes requests from people or organizations requesting certificates (called subscribers), verifies the information, and potentially signs an end-entity certificate based on that information. To perform this role effectively, a CA needs to have one or more broadly trusted root certificates or intermediate certificates and the corresponding private keys. CAs may achieve this broad trust by having their root certificates included in popular software, or by obtaining a cross-signature from another CA delegating trust. Other CAs are trusted within a relatively small community, like a business, and are distributed by other mechanisms like Windows Group Policy.

Certificate authorities are also responsible for maintaining up-to-date revocation information about certificates they have issued, indicating whether certificates are still valid. They provide this information through Online Certificate Status Protocol (OCSP) and/or Certificate Revocation Lists (CRLs). Some of the larger certificate authorities in the market include IdenTrust, DigiCert, and Sectigo.{{Cite web|title=Usage Statistics and Market Share of SSL Certificate Authorities for Websites, May 2020|url=https://w3techs.com/technologies/overview/ssl_certificate|website=w3techs.com|access-date=2020-05-01|archive-date=2022-06-30|archive-url=https://web.archive.org/web/20220630123456/https://w3techs.com/technologies/overview/ssl_certificate|url-status=live}}

Some major software contain a list of certificate authorities that are trusted by default.{{Citation needed|date=September 2022}} This makes it easier for end-users to validate certificates, and easier for people or organizations that request certificates to know which certificate authorities can issue a certificate that will be broadly trusted. This is particularly important in HTTPS, where a web site operator generally wants to get a certificate that is trusted by nearly all potential visitors to their web site.

The policies and processes a provider uses to decide which certificate authorities their software should trust are called root programs. The most influential root programs are:{{Citation needed|date=September 2022}}

• [https://technet.microsoft.com/en-us/library/cc751157.aspx Microsoft Root Program]
• [https://www.apple.com/certificateauthority/ca_program.html Apple Root Program]
• [https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/policy/ Mozilla Root Program]
• [http://www.oracle.com/technetwork/java/javase/javasecarootcertsprogram-1876540.html Oracle Java root program]
• Adobe [https://helpx.adobe.com/acrobat/kb/approved-trust-list2.html AATL] Adobe Approved Trust List and [https://blogs.adobe.com/documentcloud/eu-trusted-list-now-available-in-adobe-acrobat/ EUTL] root programs (used for document signing)

Browsers other than Firefox generally use the operating system's facilities to decide which certificate authorities are trusted. So, for instance, Chrome on Windows trusts the certificate authorities included in the Microsoft Root Program, while on macOS or iOS, Chrome trusts the certificate authorities in the Apple Root Program.{{Cite web|url=https://www.chromium.org/Home/chromium-security/root-ca-policy|title=Root Certificate Policy – The Chromium Projects|website=www.chromium.org|access-date=2017-03-19|archive-date=2017-03-20|archive-url=https://web.archive.org/web/20170320053911/https://www.chromium.org/Home/chromium-security/root-ca-policy|url-status=live}} Edge and Safari use their respective operating system trust stores as well, but each is only available on a single OS. Firefox uses the Mozilla Root Program trust store on all platforms.

The Mozilla Root Program is operated publicly, and its certificate list is part of the open source Firefox web browser, so it is broadly used outside Firefox.{{Citation needed|date=September 2022}} For instance, while there is no common Linux Root Program, many Linux distributions, like Debian,{{Cite web|url=https://launchpad.net/ca-certificates|title=ca-certificates in Launchpad|website=launchpad.net|date=30 April 2010 |language=en|access-date=2017-03-19|archive-date=2017-03-20|archive-url=https://web.archive.org/web/20170320055526/https://launchpad.net/ca-certificates|url-status=live}} include a package that periodically copies the contents of the Firefox trust list, which is then used by applications.

Root programs generally provide a set of valid purposes with the certificates they include. For instance, some CAs may be considered trusted for issuing TLS server certificates, but not for code signing certificates. This is indicated with a set of trust bits in a root certificate storage system.

{{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 misissued 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}}

The most common use of certificates is for HTTPS-based web sites. A web browser validates that an HTTPS web server is authentic, so that the user can feel secure that his/her interaction with the web site has no eavesdroppers and that the web site is who it claims to be. This security is important for electronic commerce. In practice, a web site operator obtains a certificate by applying to a certificate authority with a certificate signing request. The certificate request is an electronic document that contains the web site name, company information and the public key. The certificate provider signs the request, thus producing a public certificate. During web browsing, this public certificate is served to any web browser that connects to the web site and proves to the web browser that the provider believes it has issued a certificate to the owner of the web site.

As an example, when a user connects to https://www.example.com/ with their browser, if the browser does not give any certificate warning message, then the user can be theoretically sure that interacting with https://www.example.com/ is equivalent to interacting with the entity in contact with the email address listed in the public registrar under "example.com", even though that email address may not be displayed anywhere on the web site.{{Citation needed|date=September 2022}} No other surety of any kind is implied. Further, the relationship between the purchaser of the certificate, the operator of the web site, and the generator of the web site content may be tenuous and is not guaranteed.{{Citation needed|date=September 2022}} At best, the certificate guarantees uniqueness of the web site, provided that the web site itself has not been compromised (hacked) or the certificate issuing process subverted.

A certificate provider can opt to issue three types of certificates, each requiring its own degree of vetting rigor. In order of increasing rigor (and naturally, cost) they are: Domain Validation, Organization Validation and Extended Validation. These rigors are loosely agreed upon by voluntary participants in the CA/Browser Forum.{{Citation needed|date=September 2022}}

{{Main|Domain-validated certificate}}

A certificate provider will issue a domain-validated (DV) certificate to a purchaser if the purchaser can demonstrate one vetting criterion: the right to administratively manage the affected DNS domain(s).

A certificate provider will issue an organization validation (OV) class certificate to a purchaser if the purchaser can meet two criteria: the right to administratively manage the domain name in question, and perhaps, the organization's actual existence as a legal entity. A certificate provider publishes its OV vetting criteria through its certificate policy.

{{Main|Extended Validation Certificate}}

To acquire an Extended Validation (EV) certificate, the purchaser must persuade the certificate provider of its legal identity, including manual verification checks by a human. As with OV certificates, a certificate provider publishes its EV vetting criteria through its certificate policy.

Until 2019, major browsers such as Chrome and Firefox generally offered users a visual indication of the legal identity when a site presented an EV certificate. This was done by showing the legal name before the domain, and a bright green color to highlight the change. Most browsers deprecated this feature{{Cite web|url=https://groups.google.com/forum/m/?fromgroups&hl=en#!topic/firefox-dev/6wAg_PpnlY4|title=Firefox-dev Google group - Intent to Ship: Move Extended Validation Information out of the URL bar|website=groups.google.com|access-date=2020-08-03|archive-date=2020-08-12|archive-url=https://web.archive.org/web/20200812031241/https://groups.google.com/forum/m/?hl=en&fromgroups#!topic/firefox-dev/6wAg_PpnlY4|url-status=live}}{{Cite web|url=https://groups.google.com/a/chromium.org/forum/m/#!msg/security-dev/h1bTcoTpfeI/jUTk1z7VAAAJ|title=Chrome Security-dev Google group - Upcoming Change to Chrome's Identity Indicators|website=groups.google.com|access-date=2020-08-03|archive-date=2020-06-07|archive-url=https://web.archive.org/web/20200607075453/https://groups.google.com/a/chromium.org/forum/m/#!msg/security-dev/h1bTcoTpfeI/jUTk1z7VAAAJ|url-status=live}} providing no visual difference to the user on the type of certificate used. This change followed security concerns raised by forensic experts and successful attempts to purchase EV certificates to impersonate famous organizations, proving the inefficiency of these visual indicators and highlighting potential abuses.{{Cite web|url=https://www.troyhunt.com/extended-validation-certificates-are-really-really-dead/|title=Extended Validation Certificates are (Really, Really) Dead|website=troyhunt.com|date=12 August 2019|access-date=2020-08-03|archive-date=2020-07-16|archive-url=https://web.archive.org/web/20200716132825/https://www.troyhunt.com/extended-validation-certificates-are-really-really-dead/|url-status=live}}

A web browser will give no warning to the user if a web site suddenly presents a different certificate, even if that certificate has a lower number of key bits, even if it has a different provider, and even if the previous certificate had an expiry date far into the future.{{Citation needed|date=April 2012}} Where certificate providers are under the jurisdiction of governments, those governments may have the freedom to order the provider to generate any certificate, such as for the purposes of law enforcement. Subsidiary wholesale certificate providers also have the freedom to generate any certificate.

All web browsers come with an extensive built-in list of trusted root certificates, many of which are controlled by organizations that may be unfamiliar to the user.{{cite web|title=List of certificates included by Mozilla|url=https://www.mozilla.org/projects/security/certs/included/|publisher=Mozilla.org|access-date=30 July 2012|archive-date=3 August 2012|archive-url=https://web.archive.org/web/20120803105538/http://www.mozilla.org/projects/security/certs/included/|url-status=live}} Each of these organizations is free to issue any certificate for any web site and have the guarantee that web browsers that include its root certificates will accept it as genuine. In this instance, end users must rely on the developer of the browser software to manage its built-in list of certificates and on the certificate providers to behave correctly and to inform the browser developer of problematic certificates. While uncommon, there have been incidents in which fraudulent certificates have been issued: in some cases, the browsers have detected the fraud; in others, some time passed before browser developers removed these certificates from their software.{{cite web|title=DigiNotar removal by Mozilla|date=2 September 2011 |url=https://blog.mozilla.org/security/2011/09/02/diginotar-removal-follow-up/|publisher=Mozilla.org|access-date=30 July 2012|archive-date=3 June 2012|archive-url=https://web.archive.org/web/20120603095436/http://blog.mozilla.org/security/2011/09/02/diginotar-removal-follow-up/|url-status=live}}{{cite web|title=DigitNotar removal by Google|url=http://googleonlinesecurity.blogspot.com/2011/08/update-on-attempted-man-in-middle.html|access-date=30 July 2012|archive-date=13 September 2011|archive-url=https://web.archive.org/web/20110913024152/http://googleonlinesecurity.blogspot.com/2011/08/update-on-attempted-man-in-middle.html|url-status=live}}

The list of built-in certificates is also not limited to those provided by the browser developer: users (and to a degree applications) are free to extend the list for special purposes such as for company intranets.{{cite web|title=Using certificates article at Mozilla.org|url=https://www.mozilla.org/projects/security/pki/psm/help_21/using_certs_help.html|publisher=Mozilla.org|access-date=30 July 2012|archive-date=12 July 2012|archive-url=https://web.archive.org/web/20120712233633/http://www.mozilla.org/projects/security/pki/psm/help_21/using_certs_help.html|url-status=live}} This means that if someone gains access to a machine and can install a new root certificate in the browser, that browser will recognize websites that use the inserted certificate as legitimate.

For provable security, this reliance on something external to the system has the consequence that any public key certification scheme has to rely on some special setup assumption, such as the existence of a certificate authority.Ran Canetti: Universally Composable Signature, Certification, and Authentication. CSFW 2004, http://eprint.iacr.org/2003/239 {{Webarchive|url=https://web.archive.org/web/20090828160103/http://eprint.iacr.org/2003/239 |date=2009-08-28 }}

In spite of the limitations described above, certificate-authenticated TLS is considered mandatory by all security guidelines whenever a web site hosts confidential information or performs material transactions. This is because, in practice, in spite of the weaknesses described above, web sites secured by public key certificates are still more secure than unsecured http:// web sites.{{cite web |author=Ben Laurie, Ian Goldberg |date=18 January 2014 |title=Replacing passwords on the Internet AKA post-Snowden Opportunistic Encryption |url=https://www.w3.org/2014/strint/papers/46.pdf |access-date=15 November 2014 |archive-date=27 October 2014 |archive-url=https://web.archive.org/web/20141027134430/https://www.w3.org/2014/strint/papers/46.pdf |url-status=live }}

The National Institute of Standards and Technology (NIST) Computer Security Division{{Cite web|url=http://csrc.nist.gov/publications/PubsSPs.html|title=NIST Computer Security Publications – NIST Special Publications (SPs)|website=csrc.nist.gov|access-date=2016-06-19|archive-date=2017-09-17|archive-url=https://web.archive.org/web/20170917101423/http://csrc.nist.gov/publications/PubsSPs.html|url-status=live}} provides guidance documents for public key certificates:

• SP 800-32 Introduction to Public Key Technology and the Federal PKI Infrastructure{{Cite web|url=http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-32.pdf|title=SP 800-32 Introduction to Public Key Technology and the Federal PKI Infrastructure|publisher=National Institute of Standards and Technology|access-date=2016-06-19|archive-date=2018-06-05|archive-url=https://web.archive.org/web/20180605104516/https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-32.pdf|url-status=live}}
• SP 800-25 Federal Agency Use of Public Key Technology for Digital Signatures and Authentication{{Cite web |url=http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-25.pdf |title=SP 800-25 Federal Agency Use of Public Key Technology for Digital Signatures and Authentication |publisher=National Institute of Standards and Technology |access-date=2016-06-19 |archive-date=2018-06-02 |archive-url=https://web.archive.org/web/20180602102217/https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-25.pdf |url-status=live }}

• Authorization certificate
• Pretty Good Privacy

{{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 }}

{{SSL/TLS}}

{{Authority control}}

Category:Public-key cryptography

Category:Key management

Category:Public key infrastructure