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Smartcard authentication - Step 1 (local authentication)

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Problem statement

Smartcard based authentication is another alternative to password based authentication. Other than OTP tokens where all authentication data can be entered at a password prompt Smartcards require special hardware and software to access the credentials stored on the card.

Currently solutions are based on the pam_pkcs11 module which e.g. requires special configuration to map the certificate stored on a Smartcard to a user. Since SSSD already can map certificates to users (see e.g. LookupUsersByCertificate) integration would be much easier. Additionally features like different authentication types per user or per service would only be possible with SSSD.

Use cases

Local authentication

If there is a Smartcard reader connected to a system the user can authenticate to the system by placing his smartcard into the reader, give his name (might not be needed in some cases) and the Smartcard PIN at the login prompt and is authenticated successfully if the certificate on the Smartcard is valid and satisfies other, configurable criteria. This includes authentication at a text or graphical console but local services like su and sudo as well.

Remote authentication with ssh

To avoid password authentication ssh supports public-private key based authentication from the beginning. Since the certificates on the Smartcard are stored together with the PIN protected private key this key material can be used for ssh authentication as well. On the client side a ssh client program is needed which is able to access the Smartcard. On the server side only the public key from the certificate is needed in a suitable format for ssh. With the help of the sss_ssh_authorized_keys utility SSSD can make this information available to the sshd running on the server if the certificate is stored together with the other user data in a central storage, e.g. LDAP.

Overview of the solution

Implementation details

Configuration changes

To enable certificate based authentication in SSSD pam_cert_auth must be set to True in the [pam] section of sssd.conf.

Additional option to tune e.g. the certificate validation will be added later.

How to test

In the following it is assumed that SSSD is running on an IPA client.

Hardware reader and card

Configuring IPA client for local authentication with a Smartcard

The most easy way to test is with a Smartcard reader and a Smartcard. If the Smartcard reader is supported by the coolkey package the needed PKCS#11 modules is already added to the central NSS database at /etc/pki/nssdb during the installation of the package. In case a different PKCS#11 module is needed it can be added with modutil :

modutil modutil -dbdir /etc/pki/nssdb -add "My PKCS#11 modules" -libfile libmypkcs11.so

(if the PKCS#11 modules in not in the default library search path and full path is needed).

Now certutil should ask for a PIN and show your certificate, if the reader is connected and the card is in :

certutil -L -d /etc/pki/nssdb -h all

Most probably the certificate on the card is currently not assigned to an IPA user. To do this the certificate can be extracted with :

certutil -L -d /etc/pki/nssdb -n 'Certificate Nick-Name' -a

which will dump the PEM encoded certificate. Since the ipa utility expected the base64 string from the PEM encoding in a single line :

certutil -L -d /etc/pki/nssdb -n 'Certificate Nick-Name' -a | grep -v -- '----' |tr -d '[\n\r]'

will dump it in a single line. Now ipa user-mod username –certificate=MIIE…… can be used to load the certificate into the user entry. Please note that the –certificate option is only available with FreeIPA 4.2 or later.

If pam_cert_auth = True in the [pam] section of sssd.conf, the card is inserted in the reader and the certificate loaded in the user entry e.g. the console login prompt should now ask for a PIN instead of a password and if the correct PIN is entered the user should be successfully authenticated and logged in.

Running an ssh client with Smartcard support

The ssh client program distributed with Fedora or RHEL contains patches which add Smartcard support to the utility. To activate it the needed PKCS#11 module to talk to the Smartcard reader has to be made available with the -I option :

ssh -I /usr/lib/libmypkcs11.so -l ipauser ipahost.ipa.domain

where the certificate has to be added to the IPA user entry as described above.

Software certificates with libsoftokn3.so

First a certificate together with the private key is needed. Instructions how to create certificates with FreeIPA can e.g. be found at http://www.freeipa.org/page/PKI. Please store the certificate in a NSS database. Since in this this first step the user is looked up with the help of the full certificate any certificate valid for client authentication can be used. This means instead of creating a new one an existing certificate can be used. Please do this only in test environment which will be discarded afterwards. Copying certificates from a production environment is a security breach.

Configuring IPA client for local authentication with a Smartcard

Like in the case with a hardware reader a PKCS#11 module to access the certificate in the NSS DB must be added to the systems NSS DB in /etc/pki/nssdb. The PKCS#11 module for accessing certificates and private keys in a NSS database is libsoftokn3.so. But unfortunately this module needs some configuration option when loaded. Although I guess modutil should work as well I was only able to add the needed parameters with pk11install from the coolkey package. In the following we assume that the certificate and the private key is stored in an NSS DB called my_cert in the home directory of the user. :

pk11install -i -v -p /etc/pki/nssdb 'name=soft parameters="configdir=sql:/home/use/my_cert dbSlotDescription=\"My Slot\" dbTokenDescription=\"My Token\"" library=/usr/lib/libsoftokn3.so'

If pam_cert_auth = True in the [pam] section of sssd.conf, and the certificate loaded in the user entry e.g. the console login prompt should now ask for a PIN instead of a password and if the correct PIN is entered the user should be successfully authenticated and logged in.

Running ssh client with Smartcard support

The PKCS#11 module for accessing certificates and private keys in a NSS database is libsoftokn3.so. But unfortunately this modules needs some configuration option when loaded and there is (AFAIK) currently no way to pass them with the ssh command. Luckily there is p11-kit which can be used to load libsoftokn3.so with options. In the following we assume that the certificate and the private key is stored in an NSS DB call my_cert in the home directory of the user.

To configure p11-kit make sure ~/.config/pkcs11 and ~/.config/pkcs11/modules exists and create the following two files: :

cat ~/.config/pkcs11/pkcs11.conf << EOF_EOF
user-config: only
EOF_EOF

cat ~/.config/pkcs11/modules/my_cert.module << EOF_EOF
module: /usr/lib/libsoftokn3.so
x-init-reserved: configdir='sql:/home/user/my_cert'
critical: yes
EOF_EOF

On 64bit systems you have to use /usr/lib64/libsoftokn3.so.

Now ssh can be called with /usr/lib/p11-kit-proxy.so (or the 64bit version) :

ssh -I /usr/lib/p11-kit-proxy.so -l ipauser ipahost.ipa.domain

Software certificates with libsofthsm2.so

Since the libsoftokn3.so PKCS#11 module requires additional configuration which most consumers like the ssh client (see above) or kinit do not support and the workaround with p11-kit-proxy.so might not always be possible the following section will show how the libsofthsm2.so PKCS#11 module from the OpenDNSSEC project can be used. As above we assume that the certificate and the corresponding private key are available.

Authors

  • Sumit Bose <sbose@redhat.com>