Digital certificates (or simply 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, such as personal certificates, used by individuals, and server certificates, used to establish secure sessions between the server and clients through secure sockets layer (SSL) technology. For more information on SSL, see About Secure Sockets Layer.
Certificates are based on public key cryptography, which uses pairs of digital keys (very long numbers) to encrypt, or encode, information so it 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; it is always kept secret. Because the keys are mathematically related, data encrypted with one key can be decrypted only with the other key in the pair.
A certificate is like a passport: it identifies the holder and provides other important information. Certificates are issued by a trusted third party called a Certification Authority (CA). The CA is analogous to passport office: it validates the certificate holder's identity and signs the certificate so that it cannot be forged or tampered with. Once 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. Like a passport binds a photograph to personal information about its holder, a certificate binds a public key to information about its owner.
In addition to the public key, a certificate typically includes information such as:
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.
Digital 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.
Web browsers are preconfigured with a set of root CA certificates that the browser automatically trusts. Any certificates from elsewhere must come with a certificate chain to verify their validity. A certificate chain is series of certificates issued by successive CA certificates, eventually ending in a root CA certificate.
When a certificate is first generated, it is a self-signed certificate. A self-signed certificate is one for which the issuer (signer) is the same as the subject (the entity whose public key is being authenticated by the certificate). When the owner sends a certificate signing request (CSR) to a CA, then imports the response, the self-signed certificate is replaced by a chain of certificates. At the bottom of the chain is the certificate (reply) issued by the CA authenticating the subject's public key. The next certificate in the chain is one that authenticates the CA's public key. Usually, this is a self-signed certificate (that is, a certificate from the CA authenticating its own public key) and the last certificate in the chain.
In other cases, the CA can return a chain of certificates. In this case, the bottom certificate in the chain is the same (a certificate signed by the CA, authenticating the public key of the key entry), but the second certificate in the chain is a certificate signed by a different CA, authenticating the public key of the CA to which you sent the CSR. Then, the next certificate in the chain is a certificate authenticating the second CA's key, and so on, until a self-signed root certificate is reached. Each certificate in the chain (after the first) thus authenticates the public key of the signer of the previous certificate in the chain.