User Authentication with Certificates

Certificate authentication is technically a part of the public-key authentication method. The signature created with the private key and the verification of the signature using the public key (contained in the X.509 certificate when doing certificate authentication) are done identically with conventional public keys and certificates. The major difference is in determining whether a specific user is allowed to log in with a specific public key or certificate. With conventional public keys, every server must have every user's public key, whereas with certificates the users' public keys do not have to be distributed to the servers - distributing the public key of the CA (self-signed certificate) is enough.

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 (and external resources as required) to check that the user's certificate is valid.
The server verifies that the user has a valid private key by checking the signature in the initial packet.
The server matches the user certificate against the rules in the server configuration file to decide whether login is allowed or not.

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, although this does not take effect until the next CRL update and requires that every other authentication method has been disabled. The status of a certificate can be checked either by using the Online Certificate Status Protocol (OCSP) or a certificate revocation list (CRL), which can be published either in an LDAP or HTTP repository.

OCSP is used if the certificate contains a valid Authority Info Access extension or if an ocsp-responder has been defined in the ssh-server-config.xml file. If no OCSP responder is defined or the OCSP connection fails, CRLs are used. The certificate should contain a valid CRL Distribution Point extension or an LDAP server for CRL fetching should be defined in the ssh-server-config.xml file.