A certificate contains an identity, a public key, and signatures attesting that

Question

A   certificate   contains   an   identity,   a   public   key,   and   signatures   attesting   that   the public   key   belongs   to   the   identity.   Other   fields   that   may   be   present   include   the organization   (for   example,   university,   company,   or   government)   to   which   that identity   belongs   and   perhaps   suborganizations   (college,   department,   program,   branch, office).   What   security   purpose   do   these   other   fields   serve,   if   any?   Explain   your answer.

Explanation / Answer

In cryptography, a public key certificate (also known as a digital certificate or identity certificate) is an electronic document used to prove ownership of a public key. The certificate includes information about the key, information about its owner's identity, and the digital signature of an entity that has verified the certificate's contents are correct. If the signature is valid, and the person examining the certificate trusts the signer, then they know they can use that key to communicate with its owner.

In a typical public-key infrastructure (PKI) scheme, the signer is a certificate authority (CA), usually a company which charges customers to issue certificates for them. In a web of trust scheme, the signer is either the key's owner (a self-signed certificate) or other users ("endorsements") whom the person examining the certificate might know and trust.

Certificates are an important component of Transport Layer Security (TLS, sometimes called by its older name SSL, Secure Sockets Layer), where they prevent an attacker from impersonating a secure website or other server. They are also used in other important applications, such as email encryption and code signing

Certificate authorities

Main article: Certificate authority

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 [1], the industry standard for monitoring Active TLS certificates, states that "Although the global [TLS] ecosystem is competitive, it is dominated by a handful of major CAs — three certificate authorities (Symantec, Comodo, 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."

Certificates and website security

The most common use of certificates is for HTTPS-based web sites. A web browser validates that a TLS (Transport Layer Security) 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 provider (a CA that presents as a commercial retailer of certificates) with a certificate signing request. The certificate request is an electronic document that contains the web site name, contact email address, company information and the public key (for security reasons the private key is not part of the request and is not sent to the certificate authority). 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.

Before issuing a certificate, the certificate provider will request the contact email address for the web site from a public domain name registrar, and check that published address against the email address supplied in the certificate request. Therefore, an https web site is only secure to the extent that the end user can be sure that the web site is operated by someone in contact with the person who registered the domain name.

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[edit]

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.[citation needed] 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.

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