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
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
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).