sshd(8) can be configured to use sss_ssh_authorizedkeys for public key user authentication if it is compiled with support for "AuthorizedKeysCommand" option. Please refer to the sshd_config(5) man page for more details about this option.
AuthorizedKeysCommand /usr/bin/sss_ssh_authorizedkeys AuthorizedKeysCommandUser nobody
In addition to the public SSH keys for user USER sss_ssh_authorizedkeys can return public SSH keys derived from the public key of a X.509 certificate as well.
To enable this the "ssh_use_certificate_keys" option must be set to true (default) in the [ssh] section of sssd.conf. If the user entry contains certificates (see "ldap_user_certificate" in sssd-ldap(5) for details) or there is a certificate in an override entry for the user (see sss_override(8) or sssd-ipa(5) for details) and the certificate is valid SSSD will extract the public key from the certificate and convert it into the format expected by sshd.
Besides "ssh_use_certificate_keys" the options
can be used to control how the certificates are validated (see sssd.conf(5) for details).
The validation is the benefit of using X.509 certificates instead of SSH keys directly because e.g. it gives a better control of the lifetime of the keys. When the ssh client is configured to use the private keys from a Smartcard with the help of a PKCS#11 shared library (see ssh(1) for details) it might be irritating that authentication is still working even if the related X.509 certificate on the Smartcard is already expired because neither ssh nor sshd will look at the certificate at all.
sssd(8), sssd.conf(5), sssd-ldap(5), sssd-krb5(5), sssd-simple(5), sssd-ipa(5), sssd-ad(5), sssd-files(5), sssd-sudo(5), sssd-session-recording(5), sss_cache(8), sss_debuglevel(8), sss_obfuscate(8), sss_seed(8), sssd_krb5_locator_plugin(8), sss_ssh_authorizedkeys(8), sss_ssh_knownhostsproxy(8), sssd-ifp(5), pam_sss(8). sss_rpcidmapd(5) sssd-systemtap(5)
The SSSD upstream - https://github.com/SSSD/sssd/