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Smartcards and Multiple Identities

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Problem statement

Although there are other means like e.g. sudo or policy-kit it is still common practice to assign multiple accounts with different privileges to a single person. The typical example is a system administrator who has an ordinary user account for the daily office work and a privileged account for the admin duties. Another example are functional accounts like e.g. a dedicated database administrator account which is used by more than one person.

In the context of Smartcard authentication there are two cases to consider

  • multiple certificates valid for authentication on a single Smartcard
  • a single certfificate is mapped to multiple accounts

Use cases

Multiple certificates on a single Smartcard

To allow to log in to different accounts a user has multiple different certificates which all match the criteria for authentication on a single Smartcard. The user must a able to log in to each account by either giving a user name, selecting a certificate (or both) depending on the login method.

Single certificate for multiple accounts

To allow to log in to different accounts the certificate on the Smartcard of the user is mapped to multiple accounts. The user must a able to log in to each account by giving a user name for the specific account.

Overview of the solution

SSSD will read the the certificates from the Smartcard which matches the criteria for authentication and will use optional additional information, like .e.g. the user name, to determine a unique certificate and a unique user name which can be used for authentication. If there is not sufficient information SSSD might ask to user to select a certificate or provider a user name to proceed with authentication.

Implementation details

In general applications which use PAM for authentication will provide a user name when calling pam_start(). In this case SSSD has both the certificate and a user name which should be sufficient in most cases to determine if the user can authenticate with the certificate. There are the following cases

  • if multiple users are found with the same name this is an error even without Smartcard authentication
  • if the certificate and the user do not map, SSSD will prompt for a password.
  • if the certificate and the user map, SSSD will prompt for a PIN
  • if there are multiple certificates suitable for authentication are on the Smartcard and more than one map to the user SSSD will prompt to select a certificate before asking for a PIN. QUESTION: would it be an information leak to only show the certificates which relate to the user or do we have to display all certificate from the card?

The first and second case already work. For the third case the check if a certificate maps only to a single user must be dropped to support the use case where one certificate is used to log in to different accounts (which of course are identified by different user names). For the forth case a new PAM dialog/conversation is needed.

To my knowledge there are currently two cases where a user name is not available in the first place, InfoPipe lookups by certificate used e.g. by mod_lookup_identity and the GDM Smartcard module which calls pam_start() with an empty user name if a Smartcard was inserted when the GDM login screen is running. Here the following cases are possible:

  • the certificate(s) cannot be mapped to any user, SSSD will just return a suitable error code
  • there are one or more certificates suitable for authentication on the card but only one maps to a user, SSSD will prompt for a PIN
  • there are one or more certificates suitable for authentication on the card but only one maps to multiple users, SSSD will prompt for a user name before asking for a PIN
  • there are more certificates suitable for authentication on the card and more than one map to users, SSSD will prompt to select a certificate, if the selected certificate still maps to multiple users SSSD will go to case three and asks for a user name before asking for a PIN.

Since in the InfoPipe case only one certificate is send to SSSD only the first three cases are valid here and SSSD can e.g. indicate with an error code that either none or multiple users match the certificate. In the latter case the application can ask for a user name.

For the gdm case it might be useful to see how Smartcard authentication is handled on Windows. To illustrate this I prepare two short screencasts (sorry for the raw state, if time permits I will improve them).

The first shows the case where there are two different certificates valid for authentication on the Smartcard. The Windows utility certutil can be used with the -SCInfo option to check the certificates on the card and if privates keys are available as well. When switching to the logon screen Windows shows Icons for each certificate together with some data from the certificate which should help to identify the certificate. In this example the certificates were generated by the AD CS of the domain and hence the displayed data matches the AD user name. In general this does not have to be the case e.g. if certificates are issued by 3rd party CAs. By selecting a specific certificate and entering the PIN the mapped user is logged in.

The second shows the case where there is only one certificate on the card but mapped to two different users. The mapping can be done in AD’s ‘Users and Computers’ utility after enabling the ‘Advanced Features’ in the ‘View’ menu. Now with a right-click ‘Name Mappings’ can be selected from the context menu. After switching to the logon screen Smartcard authentication will fail because Windows does not know which user should be used for login. To solve this the ‘Allow user name hint’ policy setting must be enabled in ‘Computer Configuration\Administrative Templates\Windows Components\Smart Card’ (see ‘Smart Card Group Policy and Registry Settings’ on Technet for details). Now the logon screen displays a ‘Username hint’ prompt in addition to the PIN prompt. If the certificate on the Smartcard is mapped to multiple users the additional username hint makes authentication possible. Please note that the ‘Username hint’ is needed as well if the user is in a trusted domain.

Coming back to gdm, as long as only a ‘Username hint’ is needed pam_sss can send two messages in the PAM conversation, one for the user name and the second for the PIN. It would be nice if gdm can display both messages and prompts at the same time as Windows does to make it more clear to the user what input is expected and why.

If it is needed to select a certificate the typical PAM service will display specific data from the certificates, e.g. the value of the most specific RDN of the subject and the full DN of the issuer, in a numbered list asking the user to enter the number of the certificate which should be used for login. If would be nice if gdm can display this list in a more graphical way and make it possible to select the certificate with a mouse-click. Since SSSD knows that it is called by gdm because of the PAM service name, e.g. gdm-smartcard, it would be possible to send the certificate data with a new messages style, e.g. PAM_SELECTON_LIST_ITEM like the GNU/Linux specific PAM_RADIO_TYPE. Then gdm would be able to display the certificate selection in a more suitable way, e.g. similar to the selection of user names.

Configuration changes

Does your feature involve changes to configuration, like new options or options changing values? Summarize them here. There’s no need to go into too many details, that’s what man pages are for.

How To Test

This section should explain to a person with admin-level of SSSD understanding how this change affects run time behaviour of SSSD and how can an SSSD user test this change. If the feature is internal-only, please list what areas of SSSD are affected so that testers know where to focus.

How To Debug

Explain how to debug this feature if something goes wrong. This section might include examples of additional commands the user might run (such as keytab or certificate sanity checks) or explain what message to look for.

Authors

Give credit to authors of the design in this section.