Public key certificate
In cryptography, a public key certificate, also known as a digital certificate or identity certificate, is an electronic document used to prove the validity of a public key. The certificate includes information about the key, information about the identity of its owner (called the subject), and the digital signature of an entity that has verified the certificate's contents (called the issuer). If the signature is valid, and the software examining the certificate trusts the issuer, then it can use that key to communicate securely with the certificate's subject. In email encryption, code signing, and e -signature systems, a certificate's subject is typically a person or organization. However, in Transport Layer Security (TLS) a certificate's subject is typically a computer or other device, though TLS certificates may identify organizations or individuals in addition to their core role in identifying devices. TLS, sometimes called by its older name Secure Sockets Layer (SSL), is notable for being a part of HTTPS, a protocol for securely browsing the web. In a typical public-key infrastructure (PKI) scheme, the certificate issuer is a certificate authority (CA), usually a company that charges customers to issue certificates for them. By  contrast, in a web of trust scheme, individuals sign each other's keys directly, in a format that performs a similar function to a public key certificate. The most common format for public key certificates is defined by X.509. Because X.509 is very general, the format is further constrained by profiles defined for certain use cases, such as Public Key Infrastructure (X.509) as defined in . Types of certificate. TLS/SSL server certificate. The Transport Layer Security (TLS) protocol – as well as its outdated predecessor, the Secure Sockets Layer (SSL) protocol – ensure that the communication between a client computer and a  server is secure.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: The Subject field of the certificate must identify the primary host name of the server as the Common Name. A certificate may be valid for multiple host names (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 host names 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). TLS/SSL client certificate. 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 host name . 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. 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. Email certificate. 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. Self-signed and root certificates. 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. For most purposes, such a self-signed certificate is worthless. However , 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 certificate. However, an intermediate certificate is not self-signed. A root certificate or another intermediate certificate need 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. Common fields. 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. Example. 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 codice_1, identifying this as an Extended Validation (EV) certificate. Validated information about the website's owner (SSL  Corp) is located in the codice_2 field. The codice_3 field contains a list of domain names covered by the certificate. The codice_4 and codice_5 fields show all appropriate uses. Usage in the European Union. 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. Certificate authorities. 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. Root programs. Some major software contain a list of certificate authorities that are trusted by default. 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: 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. 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. For instance, while there is no common Linux Root Program, many Linux distributions, like Debian, 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. Website security. 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 codice_6 with their browser, if the browser does not give any certificate warning message, then the user can be theoretically sure that interacting with codice_6 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. 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. 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. Validation levels. Domain validation. 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). Organization validation. 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. Extended validation. 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 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. Weaknesses. 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. 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. 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. 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. 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. Usefulness versus unsecured web sites. 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. Standards. The National Institute of Standards and Technology (NIST) Computer Security Division provides guidance documents for public key certificates: