===== xfce4-screensaver with SmartCard login =====

**Issue:** When using sssd with smartcard login, xfce4-screensaver (or lightdm, mate, etc...) don't ask for the PIN Code, while some services (sudo, su, gdm..) do ask.

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**Rootcause:** Before sssd-2.0 or sssd-1.16.4, the list of pam services allowed to use Smartcard (aka P11 or pkcs11) was hardcoded and cannot be overriden with configuration (https://github.com/SSSD/sssd/blob/sssd-1_16_3/src/responder/pam/pamsrv_p11.c#L230-L232) 
From these 2 releases, the configuration "pam_p11_allowed_services" in section [pam] of /etc/sssd/sssd.conf allows to add more services, like XFCE elements:

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**Resolution:** the user will need to update sssd.conf accordingly: 

  # /etc/sssd/sssd.conf
  [pam]
  pam_p11_allowed_services = +xfce4-screensaver

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**Scenario:**

  - xfce4-screensaver (or any other pam enabled application) wants to auth the user
  - Calls pam_start (service xfce4-screensaver)
  - in /etc/pam.d/xfce4-screensaver (or included files) there must be a line with "auth pam_sss.so"
  - pam_sss.so will talk to the sss daemon through /var/lib/sss/pipes/pam socket. In this conversation, pam_sss will provide the name of the pam service being serviced
  - sss_pam will check if the service is allowed to use smartcard auth. If so, It'll spawn /usr/libexec/sssd/p11_child to work with the pkcs11 stack (/usr/share/p11-kit/modules)
  - "p11_child --pre" will communicate with pcscd through /var/run/pcscd/pcscd.comm to read a smartcard through the USB device and list available certificates.
  - sss_pam will filter these certificates and if one matches, ask the user for the PIN Code (through pam_message)
  - "echo -n 12345 | p11_child --auth --pin  ..." will return 0 if referenced certificate is valid for sssd, 1 if any error occurs
  - pam sequence continues according to configuration.

**Source:**
    * [[https://bugzilla.xfce.org/show_bug.cgi?id=16123|]]