In cryptography, a public key certificate, also known as a digital certificate or identity certificate, is an electronic document used to prove the ownership 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 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 RFC 5280.
Types of certificate
TLS/SSL server certificate
In TLS (formerly known as SSL), a server is required to present a certificate as part of the initial connection setup. A client connecting to that server will verify at least two things:
- The subject of the certificate matches the hostname to which the client is trying to connect.
- The certificate is signed by a trusted certificate authority.
Hostnames are listed in the Subject Alternative Name field of the certificate, though many CAs will also put a hostname in the Subject Common Name field for convenience or backwards compatibility. Certificates can be valid for multiple hostnames. Such certificates are commonly called SAN certificates or UCC certificates. If some of the hostnames contain an asterisk (*), a certificate may also be called a wildcard certificate.
A TLS server may be configured with a self-signed certificate. When that is the case, clients will generally be unable to verify the certificate, and will terminate the connection unless certificate checking is disabled.
TLS/SSL client certificate
Client certificates are less common than server certificates, and are used to 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. Also, because authentication is usually managed by the service provider, client certificates are not usually issued by a public CA that provides server certificates. Instead, the operator of a service that requires client certificates will generally operate their own internal CA to issue them. Client certificates are supported by many web browsers, but most services use passwords and cookies to authenticate users, instead of client certificates.
Client certificates are more common in RPC systems, where they are used to authenticate devices to ensure that only authorized devices can make certain RPC calls.
Email certificate
In the S/MIME protocol for secure email, senders need to discover which public key to use for any given recipient. They get this information from an email 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.
Code signing certificate
Certificates can also be used to validate signatures on programs to ensure they were not tampered with during delivery. Authenticode is one example of a code signing scheme.
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.
Root certificate
A self-signed certificate used to sign other certificates. Also sometimes called a trust anchor.
Intermediate certificate
A certificate used to sign other certificates, which is not self-signed. An intermediate certificate must be signed by another intermediate certificate, or a root certificate.
End-entity or leaf certificate
Any certificate that cannot be used to sign other certificates. For instance, TLS/SSL server and client certificates, email certificates, code signing certificates, and qualified certificates are all end-entity certificates.
Self-signed certificate
A certificate with a subject that matches its issuer, and a signature that can be verified by its own public key. Most types of certificate can be self-signed. Self-signed certificates are also often called snake oil certificates to emphasize their untrustworthiness.
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.
- 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: The algorithm used to sign the public key certificate.
- Signature: A signature of the certificate body by the issuer's private key.
Usage in the European Union
In the European Union, electronic signatures on legal documents are commonly performed using digital signatures with accompanying identity certificates. This is largely because such signatures are granted the same enforceability as handwritten signatures under eIDAS, an EU regulation.
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).
Root programs
Some major software contains 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:
- Microsoft Root Program
- Apple Root Program
- Mozilla Root Program
- Oracle Java root program
- Adobe AATL and 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. 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.
Certificates and 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 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. 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 Validation (DV) class certificate to a purchaser if the purchaser can demonstrate one vetting criterion: the right to administratively manage a domain name.
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.
Browsers will generally offer users a visual indication of the legal identity when a site presents an EV certificate. Most browsers show the legal name before the domain, and use a bright green color to highlight the change. In this way, the user can see the legal identity of the owner has been verified.
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. However a change from an EV certificate to a non-EV certificate will be apparent as the green bar will no longer be displayed. 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:
- SP 800-32 Introduction to Public Key Technology and the Federal PKI Infrastructure
- SP 800-25 Federal Agency Use of Public Key Technology for Digital Signatures and Authentication
See also
- Authorization certificate
- Pretty Good Privacy