Certificates, also called digital certificates, are electronic files that uniquely identify people and resources on the Internet. Certificates also enable secure, confidential communication between two entities. There are different kinds of certificates:
Personal certificates are used by individuals.
Server certificates are used to establish secure sessions between the server and clients through secure sockets layer (SSL) technology.
Certificates are based on public key cryptography, which uses pairs of digital keys (very long numbers) to encrypt, or encode, information so the information can be read only by its intended recipient. The recipient then decrypts (decodes) the information to read it. A key pair contains a public key and a private key. The owner distributes the public key and makes it available to anyone. But the owner never distributes the private key, which is always kept secret. Because the keys are mathematically related, data encrypted with one key can only be decrypted with the other key in the pair.
Certificates are issued by a trusted third party called a Certification Authority (CA). The CA is analogous to a passport office: it validates the certificate holder's identity and signs the certificate so that it cannot be forged or tampered with. After a CA has signed a certificate, the holder can present it as proof of identity and to establish encrypted, confidential communications. Most importantly, a certificate binds the owner's public key to the owner's identity.
In addition to the public key, a certificate typically includes information such as the following:
The name of the holder and other identification, such as the URL of the web server using the certificate, or an individual's email address
The name of the CA that issued the certificate
An expiration date
Certificates are governed by the technical specifications of the X.509 format. To verify the identity of a user in the certificate realm, the authentication service verifies an X.509 certificate, using the common name field of the X.509 certificate as the principal name.