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A client authentication certificate is a certificate used to authenticate clients during an SSL handshake. It authenticates users who access a server by exchanging the client authentication certificate.
Client authentication is identical to server authentication, with the exception that the telnet server demands a certificate from the accessing client. This is to verify that the client is who they claim to be. This eliminates the listing of anonymous entries in a database's user activity log when an Internet user accesses the server.
Clients can obtain client authentication certificates from an external certification authority (CA) like VeriSign. Another way is to create a self-signed certificate, which clients can use while waiting for a client certificate from the CA.
A client authentication certificate must be an X.509 certificate signed by a CA trusted by the server. When a certificate is requested by the server, the client can either send the certificate or try to connect without one. The server then permits the connection if it trusts the client certificate. If the client is attempting to establish a connection without a client certificate, the server may permit the connection, but at a lower security range.
Similar to a server certificate, CAs can issue client authentication certificates with distinct classes. The classes reveal the level of investigation done by the CA to verify the identity of the client requesting the client certificate. These are usually out-of-bandwidth procedures like face-to-face interaction with the client. Classes are particularly important when the certificates are issued by an external CA because it is important to ensure that the CA took all the necessary steps to verify the client's identity.
All client authentication certificates include some or all of the following info: