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Also certificate authentication is performed via the public-key authentication method. Instead of the client sending just a public key, it sends a certificate containing a public key.
In brief, certificate authentication works in the following way:
The client sends the user certificate (which includes the user's public key) to the server. The packet also contains random data unique to the session and signed by the user's private key.
The server uses the CA certificate to check that the user's certificate is valid.
The server uses the user certificate to check from its mapping file(s) whether login is allowed or not.
Finally, if connection is allowed, the server verifies that the user has a valid private key by checking the signature in the initial packet.
Compared to traditional public-key authentication, this method is more secure because the system checks that the user certificate was issued by a trusted CA. In addition, certificate authentication is more convenient because no local database of user public keys is required on the server.
It is also easy to deny a user's access to the system by revoking his or her certificate. The status of a certificate can be checked either by using the Online Certificate Status Protocol (OCSP) or certificate revocation lists (CRLs), which can be published either in an LDAP or HTTP repository.
OCSP is used if the certificate contains a valid Authority Info Access extension. Correspondingly, CRLs are used if the certificate contains a valid CRL Distribution Point extension. If LDAP is used as the CRL publishing method, the LDAP repository location can be also defined in the sshd2_config file (see below).