In cryptography, a certificate authority or certification authority (CA) is an entity that issues digital certificates. A digital certificate certifies the ownership of a public key by the named subject of the certificate. This allows others (relying parties) to rely upon signatures or on assertions made by the private key that corresponds to the certified public key. 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. Many[quantify] public-key infrastructure (PKI) schemes feature CAs.
Trusted certificates are typically used to make secure connections to a server over the Internet. A certificate is required in order to avoid the case that a malicious party which happens to be on the path to the target server pretends to be the target. Such a scenario is commonly referred to as a man-in-the-middle attack. The client uses the CA certificate to verify the CA signature on the server certificate, as part of the checks before establishing a secure connection. Usually, client software—for example, browsers—include a set of trusted CA certificates. That makes sense in as much as users need to trust their client software: A malicious or compromised client can skip any security check and still fool its users into believing otherwise.
The customers of a CA are server administrators who need a certificate that their servers will present to clients. Commercial CAs charge to issue certificates, and their customers expect the CA's certificate to be included by most web browsers, so that secure connections to the certified server work smoothly out of the box. The number of web browsers and other devices and applications that trust a particular certificate authority is referred to as ubiquity. Mozilla, which is a non-profit organization, distributes several commercial CA certificates with its products. While Mozilla developed their own policy, the CA/Browser Forum developed similar guidelines for CA trust. A single CA certificate may be shared among multiple CAs or their resellers. A root CA certificate may be the base to issue multiple intermediate CA certificates with varying validation requirements.
Aside from commercial CAs, some providers issue digital certificates to the public at no cost; a noteworthy example is CAcert. Large institutions or government entities may have their own PKIs, each including their own CAs. Formally, any site using self-signed certificates acts as its own CA too. At any rate, decent clients allow users to add or remove CA certificates at will. While server certificates usually last for a rather short period, CA certificates last much longer, so, for frequently visited servers, it is less error-prone to import and trust the CA that issues their certificates rather than confirm a security exception every time the server's certificate is renewed.
A less frequent usage of trusted certificates is for encrypting or signing messages. CAs issue end-user certificates too, which can be used with S/MIME. However, encryption requires the recipient's public key and, since authors and recipients of encrypted messages presumably know one another, the usefulness of a trusted third party remains confined to the signature verification of messages sent to public mailing lists.
The commercial CAs that issue the bulk of certificates that clients trust for email servers and public HTTPS servers typically use a technique called "domain validation" to authenticate the recipient of the certificate. Domain validation involves sending an email containing an authentication token or link, to an email address that is known to be administratively responsible for the domain. This could be the technical contact email address listed in the domain's WHOIS entry, or an administrative email like postmaster@ or root@ the domain. The theory behind domain validation is that only the legitimate owner of a domain would be able to read emails sent to these administrative addresses.
Domain validation suffers from certain structural security limitations. In particular, it is always vulnerable to attacks that allow an adversary to observe the domain validation emails that CAs send. These can include attacks against the DNS, TCP, or BGP protocols (which lack the cryptographic protections of TLS/SSL), or the compromise of routers. Such attacks are possible either on the network near a CA, or near the victim domain itself.
Some Certificate Authorities offer Extended Validation (EV) certificates as a more rigorous alternative to domain validated certificates. One limitation of EV as a solution to the weaknesses of domain validation is that attackers could still obtain a domain validated certificate for the victim domain, and deploy it during an attack; if that occurred, the only difference observable to the victim user would be a blue HTTPS address bar rather than a green one. Few users would be likely to recognise this difference as indicative of an attack being in progress.
Domain validation implementations have also sometimes been a source of security vulnerabilities. In one instance, security researchers showed that attackers could obtain certificates for webmail sites because a CA was willing to use an email address like SSLCertificates@domain.com for domain.com, but not all webmail systems had reserved the "SSLCertificates" username to prevent attackers from registering it.
Issuing a certificate
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A CA issues digital certificates that contain a public key and the identity of the owner. The matching private key is not made available publicly, but kept secret by the end user who generated the key pair. The certificate is also a confirmation or validation by the CA that the public key contained in the certificate belongs to the person, organization, server or other entity noted in the certificate. A CA's obligation in such schemes is to verify an applicant's credentials, so that users and relying parties can trust the information in the CA's certificates. CAs use a variety of standards and tests to do so. In essence, the certificate authority is responsible for saying "yes, this person is who they say they are, and we, the CA, certify that".
If the user trusts the CA and can verify the CA's signature, then (s)he can also assume that a certain public key does indeed belong to whoever is identified in the certificate.
Public-key cryptography can be used to encrypt data communicated between two parties. This can typically happen when a user logs on to any site that implements the HTTP Secure protocol. In this example let us suppose that the user logs on to his bank's homepage www.bank.example to do online banking. When the user opens www.bank.example homepage, he receives a public key along with all the data that his web-browser displays. The public key could be used to encrypt data from the client to the server but the safe procedure is to use it in a protocol that determines a shared symmetric encryption key; messages in such protocol are ciphered with the public key and only the bank server has the private key to read them. The rest of the communication proceeds using the new (disposable) symmetric key, so when the user enters some information to the bank's page and submits the page (sends the information back to the bank) then the data the user has entered to the page will be encrypted by his web browser. Therefore, even if someone can access the (encrypted) data that was communicated from the user to www.bank.example, such eavesdropper cannot read or decipher it.
This mechanism is only safe if the user can be sure that it is the bank that he sees in his web browser. If the user types in www.bank.example, but his communication is hi-jacked and a fake web-site (that pretends to be the bank web-site) sends the page information back to the user's browser, the fake web-page can send a fake public key to the user (for which the fake site owns a matching private key). The user will fill the form with his personal data and will submit the page. The fake web-page will get access to the user's data.
A certificate authority (CA) is an organization that stores public keys and their owners, and every party in a communication trusts this organization (and knows its public key). When the user's web browser receives the public key from www.bank.example it also receives a digital signature of the key (with some more information, in a so-called X.509 certificate). The browser already possesses the public key of the CA and consequently can verify the signature, trust the certificate and the public key in it: since www.bank.example uses a public key that the certification authority certifies, a fake www.bank.example can only use the same public key. Since the fake www.bank.example does not know the corresponding private key, it cannot create the signature needed to verify its authenticity.
The problem of assuring correctness of match between data and entity when the data are presented to the CA (perhaps over an electronic network), and when the credentials of the person/company/program asking for a certificate are likewise presented, is difficult. This is why commercial CAs often use a combination of authentication techniques including leveraging government bureaus, the payment infrastructure, third parties' databases and services, and custom heuristics. In some enterprise systems, local forms of authentication such as Kerberos can be used to obtain a certificate which can in turn be used by external relying parties. Notaries are required in some cases to personally know the party whose signature is being notarized; this is a higher standard than is reached by many CAs. According to the American Bar Association outline on Online Transaction Management the primary points of US Federal and State statutes enacted regarding digital signatures has been to "prevent conflicting and overly burdensome local regulation and to establish that electronic writings satisfy the traditional requirements associated with paper documents." Further the US E-Sign statute and the suggested UETA code  help ensure that:
- a signature, contract or other record relating to such transaction may not be denied legal effect, validity, or enforceability solely because it is in electronic form; and
- a contract relating to such transaction may not be denied legal effect, validity or enforceability solely because an electronic signature or electronic record was used in its formation.
Despite the security measures undertaken to correctly verify the identities of people and companies, there is a risk of a single CA issuing a bogus certificate to an imposter. It is also possible to register individuals and companies with the same or very similar names, which may lead to confusion. To minimize this hazard, the certificate transparency initiative proposes auditing all certificates in a public unforgeable log, which could help in the prevention of phishing.
In large-scale deployments, Alice may not be familiar with Bob's certificate authority (perhaps they each have a different CA server), so Bob's certificate may also include his CA's public key signed by a different CA2, which is presumably recognizable by Alice. This process typically leads to a hierarchy or mesh of CAs and CA certificates.
Authority revocation lists
An authority revocation list (ARL) is a form of CRL containing certificates issued to certificate authorities, contrary to CRLs which contain revoked end-entity certificates.
Worldwide, the certificate authority business is fragmented, with national or regional providers dominating their home market. This is because many uses of digital certificates, such as for legally binding digital signatures, are linked to local law, regulations, and accreditation schemes for certificate authorities.
However, the market for SSL certificates, a kind of certificate used for website security, is largely held by a small number of multinational companies. This market has significant barriers to entry due to the technical requirements, while not legally required new providers may choose to undergo annual security audits (such as WebTrust for Certification Authorities in North American and ETSI in Europe) to be included in the list of web browser trusted authorities. More than 50 root certificates are trusted in the most popular web browser versions. A W3Techs survey from December 2014 shows:
- Symantec (which bought VeriSign's SSL interests and owns Thawte and Geotrust) with 35.7% market share
- Comodo SSL with 26.9%
- GlobalSign with 14.9%
- Go Daddy with 13.0%
- DigiCert with 3.4%
- Entrust with 0.5%
On November 18, 2014, a group of companies and nonprofit organizations, including the Electronic Frontier Foundation, Mozilla, Cisco, and Akamai, announced "Let's Encrypt," a new nonprofit certificate authority that plans to provide free SSL certificates, as well as software to enable installation and maintenance of certificates. Let's Encrypt will be operated by the newly formed Internet Security Research Group, a California nonprofit whose application for federal tax exemption under Section 501(c)(3) was pending at the time of the Let's Encrypt announcement.
- Certificate Authority Security Council (CASC) - In February 2013, the CASC was founded as an industry advocacy organization dedicated to addressing industry issues and educating the public on internet security. The founding members are the seven largest Certificate Authorities.
- Common Computing Security Standards Forum (CCSF) - In 2009 the CCSF was founded to promote industry standards that protect end users. Comodo Group CEO Melih Abdulhayoğlu is considered the founder of the CCSF. 
- CA/Browser Forum - In 2005, a new consortium of Certificate Authorities and web browser vendors was formed to promote industry standards and baseline requirements for internet security. Comodo Group CEO Melih Abdulhayoğlu organized the first meeting and is considered the founder of the CA/Browser Forum.
If the CA can be subverted, then the security of the entire system is lost; even as widely as all the entities that trust the compromised CA.
For example, suppose an attacker, Eve, manages to get a CA to issue to her a certificate that claims to represent Alice. That is, the certificate would publicly state that it represents Alice, and might include other information about Alice. Some of the information about Alice, such as her employer name, might be true, increasing the certificate's credibility. Eve, however, would have the all-important private key associated with the certificate. Eve could then use the certificate to send digitally signed email to Bob, tricking Bob into believing that the email was from Alice. Bob might even respond with encrypted email, believing that it could only be read by Alice, when Eve is actually able to decrypt it using the private key.
A notable case of CA subversion like this occurred in 2001, when the certificate authority VeriSign issued two certificates to a person claiming to represent Microsoft. The certificates have the name "Microsoft Corporation", so could be used to spoof someone into believing that updates to Microsoft software came from Microsoft when they actually did not. The fraud was detected in early 2001. Microsoft and VeriSign took steps to limit the impact of the problem.
In 2011 fraudulent certificates were obtained from Comodo and DigiNotar, allegedly by Iranian hackers. There is evidence that the fraudulent DigiNotar certificates were used in a man-in-the-middle attack in Iran.
In 2012, it became known that Trustwave issued a subordinate root certificate that was used for transparent traffic management (man-in-the-middle) which effectively permitted an enterprise to sniff SSL internal network traffic using the subordinate certificate.
Open source implementations
There exist several open source implementations of certificate authority software. Common to all is that they provide the necessary services to issue, revoke and manage digital certificates.
Some open source implementations are:
- OpenSSL, an SSL/TLS library that comes with tools allowing its use as a simple certificate authority
- EasyRSA, OpenVPN's command line CA utilities using OpenSSL.
- TinyCA, which is a perl gui on top of some CPAN modules.
- Automated Certificate Management Environment (ACME), Let's Encrypt's protocol for communications between its certificate authority and servers. Let's Encrypt also provides node-acme, a Node.js implementation of ACME, and lets-encrypt-preview, a Python-based test implementation of server certificate management software using the ACME protocol.
- Certificate revocation list
- Certificate server
- Extended Validation Certificate
- Intermediate certificate authorities
- Robot certificate authority
- Root Key Ceremony
- SAFE-BioPharma Association
- Self-signed certificate
- Server gated cryptography
- Web of trust
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