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pam-gnupg

Unlock GnuPG keys on login

What is it?

A PAM module that hands over your login password to gpg-agent, which can be useful if you are using a GnuPG-based password manager like pass.

Requires GnuPG 2.1, and probably only works on Linux.

Disclaimer

The code was written mainly by looking at and occasionally copying from Gnome Keyring's PAM module and pam_mount and is based on a somewhat mediocre understanding of the details of both PAM and C. You should be aware that there may be potentially dangerous bugs lurking.

Installation

Packages

Manually

The usual

./autogen.sh
./configure
make
make install

should work. configure takes an option --with-moduledir to set the installation path of the PAM module. It defaults to /lib/security, but your distribution might use a different path.

Usage

Setup guide

  • For services that open a new session (gdm, sddm, login, ...), add the lines

    auth     optional  pam_gnupg.so store-only
session  optional  pam_gnupg.so

    at the end (¹) of the corresponding file in /etc/pam.d, or in one of the files included from there, e.g. system-local-login on Arch.

    When opening the session, gpg-agent will be autostarted if necessary. If you want to start it by other means, e.g. as a systemd user service, make sure it is up before pam_gnupg.so is run, e.g. by putting the lines somewhere below pam_systemd.so, and (optionally) add no-autostart to the session line.

    (¹): The end is usually a good place, but details depend on your PAM setup. In particular, modules declared sufficient can terminate the PAM stack early. At least, pam_gnupg.so should come after pam_unix.so, pam_systemd_home.so, pam_systemd.so and pam_env.so in case you use those modules.

  • For services that only authenticate (i3lock, physlock, ...), use

    auth     optional  pam_gnupg.so

    For screen lockers, this only really makes sense if you arrange for the password cache to be cleared prior to locking the screen by calling

    gpg-connect-agent --no-autostart reloadagent /bye

    During authentication, the agent will never be autostarted.

  • Add

    allow-preset-passphrase

    to ~/.gnupg/gpg-agent.conf. Optionally, customize the cache timeout via max-cache-ttl, e.g. set

    max-cache-ttl 86400

    to have it expire after a day.

  • Run

    gpg -K --with-keygrip

    The output should look something like this:

    sec   rsa2048 2018-11-16 [SC]
      9AB5DD43C5E5FD40475FA6DA0D776275F7F5B2E7
      Keygrip = 6F4ABB77A88E922406BCE6627AFEEE2363914B76
uid           [ultimate] Chris Ruegge <mail@cxcs.de>
ssb   rsa2048 2018-11-16 [E]
      Keygrip = FBDEAD7B0C484CDC85F1CF70352833EB0C921D58

    Write the keygrip for the encryption subkey marked [E] – shown in boldface in the output above – into ~/.pam-gnupg. If you want to unlock multiple keys or subkeys, add all keygrips on separate lines.

    Keygrips are exactly 40 characters in length. Leading whitespace, lines starting with # and everything after the keygrip is ignored.

    If ~/.pam-gnupg does not exists, $XDG_CONFIG_HOME/pam-gnupg will be tried, with XDG_CONFIG_HOME defaulting to ~/.config as usual. If you want to customize this variable, read the section on environment variables below.

  • Set the same password for your gpg key and your user account. All pam-gnupg does is to send the password as entered to gpg-agent. It is therefore not compatible with auto-login of any kind; you actually have to type your password for things to work.

GNUPGHOME

If you change your gnupg directory from the default ~/.gnupg by setting GNUPGHOME, this variable needs to be made available to pam-gnupg when presetting. Since PAM usually runs before your init scripts, it needs to obtain the variable in a different way.

To set it, add the path to the config file on a separate line before any keygrips, either as absolute path or starting with ~/ for paths relative to the home directory. The connection to the agent will be opened when the first keygrip is read, so setting GNUPGHOME after that will have no effect.

Note that the variable is only used for connecting to and optionally autostarting the agent. It is not passed down to your login shell or desktop session, so you also need to set it in your init scripts. Additionally, if you start the agent via systemd, you need to adjust the various service and socket units separately.

Alternatives

If you use systemd-homed, you can modify env vars via homectl --setenv, and they will be made available to PAM by pam_systemd_home.so.

Another way is to run pam_env.so with user_readenv=1 before pam_gnupg.so, so you can set env vars from ~/.pam_environment, e.g.

GNUPGHOME DEFAULT=@{HOME}/path/to/your/gnupg

You can also modify XDG_CONFIG_HOME this way. Unfortunately, user_readenv is deprecated and will go away in some future version of pam_env.

SSH Keys

SSH key support is indirect via gpg-agent's built-in SSH support (there's no SSH specific code in pam-gnupg). The full details are in gpg-agent(1), but here's a basic step-by-step guide:

  • Add

    enable-ssh-support

    to ~/.gnupg/gpg-agent.conf. (This is not actually strictly necessary in all setups, but doesn't hurt either.)

  • Set the SSH_AUTH_SOCK variable to gpg-agent's SSH socket by putting

    export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)

    into your relevant init script.

  • Add your SSH keys to the agent using ssh-add as usual. This only needs to be done once. The agent will re-encrypt the keys and store them in ~/.gnupg, independent of the ones in ~/.ssh.

  • Get the SSH keygrips using

    gpg-connect-agent 'keyinfo --ssh-list' /bye

    The output should look like

    S KEYINFO DBB0B60CFE5F23716ABEE8787C6184C27E2486E1 D - - - P - - S
OK

    with one keygrip per line. Alternatively, get them from ~/.gnupg/sshcontrol.

  • Add the keygrips to ~/.pam-gnupg the same way as for the gpg keys.

Debug output

Both the auth and the session module take a debug option to enable some basic debug logging to syslog / journal.

Known issues

  • Using pass during startup of systemd user services has a racing condition even if the service declares After=gpg-agent.socket, because systemd does not know about pam-gnupg and will start the service right after the socket is up, but maybe before the key has been unlocked. Until I figure out a cleaner solution, you can circumvent this by adding a small startup delay to the service, e.g.

    ExecStartPre=/usr/bin/sleep 5

  • Screen lockers need to call pam_setcred after authentication to actually send the passphrase. Those who don't will not work with pam-gnupg.

  • Specifically for suckless' slock with the pam-auth patch, you have to set user and group to your user name and your primary group (as displayed by id -gn) in slock's config.h, which will therefore not work for multiple users. Alternatively, you can try the (untested) steps outlined in this issue comment.

Contact

  • Email: mail@cxcs.de, gpg key. The gpg -K output above is real, so the second line is the actual fingerprint.