Using Smart Card Public Key Authentication


Smart card public key authentication is an advanced version of the method described in Using Public Key Authentication. It uses the same basic principle but stores the private key on a smart card, where it can never be extracted. This section will describe how to configure ThinLinc to use it.

General Requirements

  • Smart cards with an appropriate PKCS#11 library. The library included with ThinLinc requires PKCS#15 compliant smart cards and PC/SC libraries on the client system.

Key Generation

The keys on the smart card are generated when the smart card is issued. How this is done is not covered by this guide.

Server Configuration

To use a smart card with ThinLinc, the public key must be extracted off the card and associated with a user on the ThinLinc server. The method for doing this depends on your smart card and your SSH server.

On Linux, with the OpenSSH server and an PKCS#15 compliant smart card, the tool pkcs15-tool (part of the OpenSC suite) is able to extract the public key.

The first step is identifying the certificate on the card:

$ pkcs15-tool --list-certificates
X.509 Certificate [identification]
        Flags    : 0
        Authority: no
        Path     : 3f0050154331
        ID       : 45

The second step is to extract the key, based on the ID number:

$ pkcs15-tool --read-ssh-key 45
1024 65537 918282501237151981353694684191630174855276113858858644490084487922635
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQCCxIx/xtVoDR2qwY4Pym7F6yKmdJsB26MUbbTiGT7o

The second line, starting with ssh-rsa, is the one needed for SSH version 2 authentication. For instructions on how to associate this key with a user, see Using Public Key Authentication.

Client Configuration

The ThinLinc client requires no special configuration to use the smart card.

Automatic Connection

The client is able to automatically connect to the server when a smart card is inserted (see Security tab). It does, however, require that the user is able to log in using the subject name on the card. As that is rarely a valid user name, ThinLinc ships with a special NSS module, called nss-passwdaliases, that enables alternate names for users.

The module is configured by editing the file /etc/passwdaliases. The file is a colon-delimited table of alternate names and their corresponding user ids. Example:


To activate the nss-passwdaliases module, it must be added to the list of NSS modules for the passwd database. This is specified in the file /etc/nsswitch.conf. For example, replace the following line:

passwd: files ldap

with this line:

passwd: files ldap passwdaliases

LDAP Automatic Update (tl-ldap-certalias)

ThinLinc includes the tool tl-ldap-certalias that can automatically update the local databases needed for smart card public key authentication, provided the system uses the OpenSSH server (or any SSH server that uses a compatible format and location for authorized public keys) and standards compliant LDAP servers where users and certificates are stored.

The tl-ldap-certalias command can also perform validation of certificates it finds in LDAP databases. Read more about this in Certificate validation.

  • On invocation, a list of all users and matching certificates is gathered. How is determined by the certificate_user_match configuration variable. If allow_invalid_certificates is no, only matching valid certificates will be gathered.

  • The user’s home directory, as well as the ~/.ssh directory, are created if they are required and do not already exist. tl-ldap-certalias reuses the /vsmagent/make_homedir_mode configuration variable from vsmagent for determining the default permissions of newly created home directories.

  • Any old public keys added by tl-ldap-certalias are removed from the ~/.ssh/authorized_keys file and the keys from the current set of certificates are added.

  • The file /etc/passwdaliases is updated with a list of subject names and user id:s, to allow for login without usernames. See Automatic Connection for more information.


It should be noted that any custom entries in ~/.ssh/authorized_keys will be retained, but custom changes to /etc/passwdaliases will be overwritten each time tl-ldap-certalias is run.

After deployment, tl-ldap-certalias is meant to be run from cron at regular intervals, for example every 15 minutes. This makes sure that the ThinLinc system automatically keeps all user certificates updated. However, please note that if you’re using certificate validation, downloading and parsing certificate revocation lists may take a long time (up to 5 minutes each). This is mitigated by caching the data from the CRL:s, but the first run and whenever the CRL needs to be updated may take a long time. Thus, if you have certificates from a lot of different certificate authorities, don’t run tl-ldap-certalias too often.

Since the default use of this tool is to be run from cron, the default behaviour is to produces no output other than error messages. If you want more output from tl-ldap-certalias, see options in Command line options.


The root user must able to write to the users’ home directories for tl-ldap-certalias to be able to update the ~/.ssh/authorized_keys files.

Command line options

tl-ldap-certalias accepts a number of different command line options that affects how the program interacts with its environment.

-v, --verbose

Turn on program status output to standard output. This is off by default.

-d, --debug

Turn on extra debugging putput to standard output. This is off by default.

-s, --simulate

Dry run mode. Specifying this option tells tl-ldap-certalias to avoid writing any changes to disk. This is off by default.

-h, --help

Show usage information and exit.


tl-ldap-certalias uses the /utils/tl-ldap-certalias hiveconf folder for configuration purposes. On a standard ThinLinc installation, it’s located in /opt/thinlinc/etc/conf.d/tl-ldap-certalias.hconf. See Parameters in /utils/tl-ldap-certalias/ for details about the available parameters.

Certificate validation

tl-ldap-certalias can perform validation of certificates found in LDAP databases by the following methods if allow_invalid_certificates is set to yes:

Certificate validity and expiry dates

tl-ldap-certalias now checks the certificate validity and expiry dates and rejects certificates that are not valid yet or have expired.

Matching certificate to certificate issuers

Place the CA certificates you wish to trust certificates from in /opt/thinlinc/etc/ca/. The CA certificates must be in DER form. If tl-ldap-certalias finds a certificate with an issuer that does not match any of the certificates in /opt/thinlinc/etc/ca/, the certificate will be considered invalid and ignored.

Certificate revocation lists

tl-ldap-certalias searches the certificates it encounter for certificate revocation lists (CRL), to make sure that the certificate has not been revoked by its issuer. Once downloaded, the CRL will be cached until the time for the next scheduled update found in the CRL list has passed.


tl-ldap-certalias can only handle CRL lists distributed with HTTP.

Validation of certificate signatures.

tl-ldap-certalias can verify that the certificate signature is valid and thus assures that the certificate has not been tampered with.