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pam_pcsc

pam_pcsc is a NFC-based PAM authentification module.

Using pam_pcsc, you can use NFC (Near Field Communication) cards / tags to autohrise users into PAM (Pluggable Authentication Modules).

How does it work?

Basics

It uses pscs-lite framework (http://ludovic.rousseau.free.fr/softwares/) to communicate with the reader so should work with any reader supported by the framework.

It was tested with Mifare Ultralight cards, but should work with any memory card that your reader can read and write.

First of all you register your card for authentication with pam_registercard.pl <user> as root. The program will asks you the password which will be checked by usual systemwide password check.

Then it will detect your card, encrypt the password, write it to the card and check that it is readable.

It will print you a line to put to /etc/rfshadow file. This line contains the encryption key for your password, stored on the card.

In order to authenticate with cards you need to put this module to your PAM configuration files in /etc/pam.d:

auth optional pam_pcsc.so

somewere before this one:

auth required pam_unix.so try_first_pass nullok

Be sure to put the pam_rfid line before the pam_unix in order to avoid standard password prompt.

When the module is sollicitated during the PAM authentication process it will read the card search a key in rfshadow file and try to decrypt the password. In success the password will be sent down to the PAM stack like if you typed it. If not, PAM will just continue and give you a standard password prompt.

The module checks that the authentication request is from local host in order to avoid compromising remote security.

Password encryption

For encrypting the password a RSA key is generated and stored in the rfshadow file. This key is random and unique for each card.

Build and install

Dependencies:

Perl module for PCSC is included into pcsc-lite.

Build command: autoreconf -vfi

Debian packaging files are also present. You can build a debian package with

debuild -i -us -uc -b

Usage

Place your card to the reader before to use anything (enroll or authentication). The code don't wait for a card.

To register a card

Put your card to the reader and launch:

pam_registercard.pl <user>

the program will ask for a password, write it encrypted to the card then output a line you should copy and paste to /etc/rfshadow file. pam_registercard.pl tool is not sufficiently tested for allowing it to write this quite important file. This file must be owned by root and must have at most 640 permission. In this line:

  • user
  • service - PAM service this card authenticates. By default pam_registercard.pl put "*" in it, which means any service. You can restrict this card to one service, for example su.
  • hashed card ID
  • method - currently the only supported method by pam_pcsc is mc. id method is for pam_rfid module which will just match user with Card ID and return PAM_SUCCESS. In current version it is not tested at all. No password will be stored with this method.
  • authentication data - for mc it is encryption key, and empty for id

File shoud contain only one line for user/service/ID combination. If not, only first line will be used and a message will be logged.

It will also write card detection information to /var/pam_pcsc/card.red. It contains total number of writable blocks, written blocks and size of encrypted password. PAM module need this infomation in order correctly read the card.

Authentication

Put the card to the reader before login. PAM Module will not wait for a card.

Configuration

Card detection uses /usr/share/pam_pcsc/mc.def file to identify the card. It contains regexes for known card's ATR and some useful information about card structure. Tools and modules use /etc/pcsc.conf file for Card reader configuration. Configuration includes the reader's number. If you have more than one reader on the system you should put the number of the reader you will use. pcsc_scan tool from pscs-lite package is useful for understanding your configuration and cards and correctly adjust this file.

The configuration file also contains APDUs for reading and writing cards. By default there are standard APDUs but it could be different for your reader. A good tool for checking APDUs is gscriptor from pscs-lite.

To do

  1. pam_registercard.pl is a real mess. Need to clean up the code and implement all features: write to rfshadow, keep lines unique, remove lines, update lines and password.
  2. Protect used blocks on the card by internal key.