Certificates

A certificate is a kind of digital identity card that associates the public key with a unique digital thumbprint identifying an individual, a server, or any other entity.

Now that Georges is able to send a secured message to his bank, there is still a problem. How can Georges be sure that the server he is connected to is really the bank's server and not a malicious server?

Georges must be sure that the public key he is using to encrypt his message corresponds to the bank's private key. Similarly, the bank needs to verify that the message signature it receives corresponds to Georges' signature.

To identify a remote peer, use a certificate - a kind of digital identity card that associates the public key with a unique digital thumbprint identifying an individual, a server, or any other entity (known as the subject). It also includes the identification and signature of the Certificate Authority that issued the certificate, and the period of time during which the certificate is valid. It may have additional information (or extensions) as well as administrative information for the Certificate Authority's use, such as a serial number.

A standard X.509 certificate contains the following standard fields:

• Certificate version
• Serial number of the certificate
• The distinguished name of the certificate issuer
• The distinguished name of the certificate owner
• The validity period of the certificate
• The public key
• The digital signature of the issuer
• Signature algorithm used
• Zero or more certificate extensions