Public key certificate

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In cryptography, a public key certificate (also known as a digital certificate or identity certificate) is an electronic document which uses a digital signature to bind together 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.^[1]

Certificates can be created for Unix-based servers with tools such as OpenSSL's ssl-ca.^[2] or SuSE's gensslcert. Similarly, Microsoft Windows 2003 contains Certificate Authority for the creation of digital certificates. In Windows Server 2008 the capability is in Active Directory Certification Authority.

Contents of a typical digital certificate

Serial Number: Used to uniquely identify the certificate.

Subject: The person, or entity identified.

Signature Algorithm: The algorithm used to create the signature.

Issuer: The entity that verified the information and issued the certificate.

Valid-From: The date the certificate is first valid from.

Valid-To: The expiration date.

Key-Usage: Purpose of the public key (encryption, verifying signatures, or both).

Public Key: The public key to encrypt a message to the named subject or to verify a signature from the named subject.

Thumbprint Algorithm: The algorithm used to hash the certificate.

Thumbprint: The hash itself to ensure that the certificate has not been tampered with.

Classes

VeriSign introduced the concept of classes of digital certificates^{[citation needed]}:

• Class 1 for individuals, intended for email
• Class 2 for organizations, for which proof of identity is required
• Class 3 for servers and software signing, for which independent verification and checking of identity and authority is done by the issuing certificate authority
• Class 4 for online business transactions between companies
• Class 5 for private organizations or governmental security

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 their 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 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. 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 that registered the domain name.

As an example, when a user connects to https://www.example.com/ with their 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.

See also

• Authorization certificate
• Certificate server
• OpenPGP

References

1. ^ Ran Canetti: Universally Composable Signature, Certification, and Authentication. CSFW 2004, http://eprint.iacr.org/2003/239
2. ^ OpenSSL: Contribution, Misc

External links

• RFC 2459 Internet X.509 Public Key Infrastructure Certificate and CRL Profile