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Using Smartcard Insert Event to Bypass Username Prompt in Greeter

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I'm switching over from CentOS 7 to Ubuntu 18.04 and setting up smartcard login.

The smartcard login using pam_pkcs11 with opensc works just as well as it did in CentOS, but the greeter in CentOS (GDM) would identify when a smartcard is inserted and direct the users to enter a PIN without entering a username and leave it to the pam_pkcs11 mapping to find the user account. The username would be derived from the smartcard information.

Is there a way to accomplish this same behavior in Ubuntu's GDM3, LightDM, or SSSD? I've been looking into the source of LightDM, OpenSC, GDM3 to skip the username prompt.

Update: I have found out that an empty string in the username field is an acceptable input to the LightDM greeter. It then checks the smartcard to see if a user can be identified with the provided smartcard information. Now to bypass the username prompt automatically when a smartcard is present.

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