Public Key Infrastructure (Digital Cert )

In cryptography, a public key certificate (also known as a digital certificate oridentity certificate) is an electronic document which uses a digital signature to bind a public key with an identity — information such as the name of a person or an organization, their address, and so forth. The certificate can be used to verify that a public key belongs to an individual.

In a typical public key infrastructure (PKI) scheme, the signature will be of a certificate authority (CA). In a web of trust scheme, the signature is of either the user (a self-signed certificate) or other users ("endorsements"). In either case, the signatures on a certificate are attestations by the certificate signer that the identity information and the public key belong together.

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.

Certificates can be created for Unix-based servers with tools such as OpenSSL's command. or SuSE's gensslcert. These may be used to issue unmanaged certificates, Certification Authority (CA) certificates for managing other certificates, and user and/or computer certificate requests to be signed by the CA, as well as a number of other certificate related functions.

Similarly, Microsoft Windows 2000 Server and Windows Server 2003 contain a Certification Authority (CA) as part of Certificate Services for the creation of digital certificates. In Windows Server 2008 the CA may be installed as part of Active Directory Certificate Services. The CA is used to manage and centrally issue certificates to users and/or computers. Microsoft also provides a number of different certificate utilities, such as SelfSSL.exe for creating unmanaged certificates, and Certreq.exe for creating and submitting certificate requests to be signed by the CA, and certutil.exe for a number of other certificate related functions.

Certificates and web site security

The most common use of certificates is for HTTPS-based web sites. A web browser validates that an SSL (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, and company information. 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 his browser, if the browser gives no 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.

http://en.wikipedia.org/wiki/Public_key_certificate