In cryptography, a public key certificate (or identity certificate) is an electronic document which incorporates 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.
Principles
Certificates are useful for large-scale public-key cryptography. Securely exchanging secret keys amongst users becomes impractical to the point of effective impossibility for anything other than quite small networks. Public key cryptography provides a way to avoid this problem. In principle, if Alice wants others to be able to send secret messages, she needs only to publish her public key. Anyone possessing it can then send secure information to her. Unfortunately, David could publish a different public key (for which he knows the related private key) claiming that it is Alice's public key. In so doing, David could intercept and read at least some of the messages meant for Alice. But if Alice builds her public key into a certificate and has it digitally signed by a trusted third party (Trent), anyone who trusts Trent can merely check the certificate to see whether Trent thinks the embedded public key is Alice's. In typical public-key infrastructures (PKIs), Trent will be a CA, who is trusted by all participants. In a web of trust, Trent can be any user, and whether to trust that user's attestation that a particular public key belongs to Alice will be up to the person wishing to send a message to Alice. In large-scale deployments, Alice may not be familiar with Bob's certificate authority (perhaps they each have a different CA — if both use employer CAs, different employers would produce this result), so Bob's certificate may also include his CA's public key signed by a "higher level" CA2, which might be recognized by Alice. This process leads in general to a hierarchy of certificates, and to even more complex trust relationships. Public key infrastructure refers, mostly, to the software that manages certificates in a large-scale setting. In X.509 PKI systems, the hierarchy of certificates is always a top-down tree, with a root certificate at the top, representing a CA that is so central to the scheme that it does not need to be authenticated by some trusted third party. A certificate may be revoked if it is discovered that its related private key has been compromised, or if the relationship (between an entity and a public key) embedded in the certificate is discovered to be incorrect or has changed; this might occur, for example, if a person changes jobs or names. A revocation will likely be a rare occurrence, but the possibility means that when a certificate is trusted, the user should always check its validity. This can be done by comparing it against a certificate revocation list (CRL) — a list of revoked or cancelled certificates. Ensuring that such a list is up-to-date and accurate is a core function in a centralized PKI, one which requires both staff and budget and one which is therefore sometimes not properly done. To be effective, it must be readily available to any who needs it whenever it is needed and must be updated frequently. Another way to check a certificate validity is to query the certificate authority using a protocol such as the Online Certificate Status Protocol (OCSP), in which a third party server parses the revocation list and returns an answer to the client. A certificate typically includes:
- The public key being signed.
- A name, which can refer to a person, a computer or an organization.
- A validity period.
- The location (URL) of a revocation center.
- The digital signature of the certificate, produced by the CA's private key.
The most common certificate standard is the ITU-T X.509. X.509 is being adapted to the Internet by the IETF PKIX working group.
Classes
VeriSign introduced the concept of classes of digital certificates:
- 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 (CA).
- Class 4 for online business transactions between companies
- Class 5 for private organizations or governmental security
