YubiHSM PAM module backend
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Copyright 2011 Google Inc.

YubiHSM for unix authentication.

You can use YubiHSM to encrypt all your passwords using a key that's
only stored inside the YubiHSM. There is then no way to snarf
/etc/shadow and do offline password cracking.

Installation instructions

1. Set up your YubiHSM (if you haven't already)
(this is a quick rehash of chapter 8 of the YubiHSM reference manual)

Plug it in and run "minicom -D /dev/ttyACM0".

If the prompt is "HSM>" then it's already set up.

If the prompt is "WSAPI>" then it's in WSAPI mode. Run "hsm" and set
it up according to the instructions in chapter 8.4 of the YubiHSM
reference manual.

If you don't get a prompt at all, then unplug it and hold down the
configuration button while inserting it, and try again.

2. Set up a crypto key for YHSMPAM
Go to the YubiHSM configuration mode (unplug, hold down config button
while re-inserting).

Run "minicom -D /dev/ttyACM0". You should get a "HSM>" prompt.

List your existing keys using "keylist". Either pick an existing key
number or generate a new using "keygen 8000 1 20" (to generate key
with handle 8000).

Type "exit" to make the YubiHSM go into HSM mode.

3. Configure YHSMPAM
$ sudo mkdir -m 700 /etc/yhsmpam
$ sudo mkdir -m 700 /etc/yhsmpam/users
$ sudo tee /etc/yhsmpam/yhsmpam.conf > /dev/null
key_handle 8000
device /dev/ttyACM0

If your YubiHSM is /dev/ttyACM0 then you can skip that in the
config. It's the default.

Mode 700 on the directories may prevent pam_externalpass from seeing
the user config files for some PAM-enabled programs such as su. A fix
is being worked on.

4. Set password for a test user
First copy yhsmpam.py somewhere nice in your admin PATH, such as
/usr/local/sbin/yhsmpam (without .py). It will be used by the PAM
module, so don't put it in your home directory.

$ sudo yhsmpam set testuser
User password: secret

Verify that it was written:
$ sudo cat /etc/yhsmpam/users/testuser
071234002600 3821309218309821038213218392103213123123ee12312893182903

5. Test the password
% sudo yhsmpam check testuser
User password: secret
Password correct

6. Install pam_externalpass
Compile and install pam_externalpass:

  git clone https://github.com/ThomasHabets/pam_externalpass.git
  cd pam_externalpass
  make install

7. Configure PAM
Add this to /etc/pam.d/common-auth before the pam_unix line:
  auth sufficient /usr/local/lib/libpam_externalpass.so \
             exec=/usr/local/sbin/yhsmpam \
             prompt=HSM_Password:_ \

8. Try logging in as the test user
Use ssh, su or whatever. If you put the line in /etc/pam.d/common-auth
it should work for everything (except su, see above).