Jan 26, 2023

More FIDO2 hardware auth/key uses on a linux machine and their quirks

As I kinda went on to replace a lot of silly long and insecure passwords with FIDO2 USB devices - aka "yubikeys" - in various ways (e.g. earlier post about password/secret management), support for my use-cases was mostly good:

  • Webauthn - works ok, and been working well for me with U2F/FIDO2 on various important sites/services for quite a few years by now.

    Wish it worked with NFC reader in Firefox on Linux Desktop too, but oh well, maybe someday, if Mozilla doesn't implode before that.

  • pam-u2f to login with the token using much simplier and hw-rate-limited PIN (with pw fallback).

    Module itself worked effortlessly, but had to be added to various pam services properly, so that password fallback is available as well, e.g. system-local-login:

    # system-login
    auth required pam_shells.so
    auth requisite pam_nologin.so
    # system-auth + pam_u2f
    auth required pam_faillock.so preauth
    # auth_err=ignore will try same string as password for pam_unix
    -auth [success=2 authinfo_unavail=ignore auth_err=ignore] pam_u2f.so \
      origin=pam://my.host.net authfile=/etc/secure/pam-fido2.auth \
      userpresence=1 pinverification=1 cue
    auth [success=1 default=bad] pam_unix.so try_first_pass nullok
    auth [default=die] pam_faillock.so authfail
    auth optional pam_permit.so
    auth required pam_env.so
    auth required pam_faillock.so authsucc
    # auth    include   system-login
    account   include   system-login
    password  include   system-login
    session   include   system-login

    "auth" section is an exact copy of system-login and system-auth lines from the current Arch Linux, with pam_u2f.so line added in the middle, jumping over pam_unix.so on success, or ignoring failure result to allow for entered string to be tried as password there.

    Using Enlightenment Desktop Environment here, also needed to make a trivial "include system-local-login" file for its lock screen, which uses "enlightenment" PAM service by default, falling back to basic system-auth or something like that, instead of system-local-login.

  • sk-ssh-ed25519 keys work out of the box with OpenSSH.

    Part that gets loaded in ssh-agent is much less sensitive than the usual private-key - here it's just a cred-id blob that is useless without FIDO2 token, and even that can be stored on-device with Discoverable/Resident Creds, for some extra security or portability.

    SSH connections can easily be cached using ControlMaster / ControlPath / ControlPersist opts in the client config, so there's no need to repeat touch presence-check too often.

    One somewhat-annoying thing was with signing git commits - this can't be cached like ssh connections, and doing physical ack on every git commit/amend is too burdensome, but fix is easy too - add separate ssh key just for signing. Such key would naturally be less secure, but not as important as an access key anyway.

    Github supports adding "signing" ssh keys that don't allow access, but Codeberg (and its underlying Gitea) currently does not - access keys can be marked as "Verified", but can't be used for signing-only on the account, which will probably be fixed, eventually, not a huge deal.

  • Early-boot LUKS / dm-crypt disk encryption unlock with offline key and a simplier + properly rate-limited "pin", instead of a long and hard-to-type passphrase.

    systemd-cryptenroll can work for that, if you have typical "Full Disk Encryption" (FDE) setup, with one LUKS-encrypted SSD, but that's not the case for me.

    I have more flexible LUKS-on-LVM setup instead, where some LVs are encrypted and needed on boot, some aren't, some might have fscrypt, gocryptfs, some other distro or separate post-boot unlock, etc etc.

    systemd-cryptenroll does not support such use-case well, as it generates and stores different credentials for each LUKS volume, and then prompts for separate FIDO2 user verification/presence check for each of them, while I need something like 5 unlocks on boot - no way I'm doing same thing 5 times, but it is unavoidable with such implementation.

    So had to make my own key-derivation fido2-hmac-boot tool for this, described in more detail separately below.

  • Management of legacy passwords, passphrases, pins, other secrets and similar sensitive strings of information - described in a lot more detail in an earlier "FIDO2 hardware password/secret management" post.

    This works great, required an (simple) extra binary, and integrating it into emacs for my purposes, but also easy to setup in various other ways, and a lot better than all alternatives (memory + reuse, plaintext somewhere, crappy third-party services, paper, etc).

  • One notable problem with FIDO2 devices is that they don't really show what it is you are confirming, so as a user, I can think that it wants to authorize one thing, while whatever compromised code secretly requests something else from the token.

    But that's reasonably easy to mitigate by splitting usage by different security level and rarity, then using multiple separate U2F/FIDO2 tokens for those, given how tiny and affordable they are these days - I ended up having three of them (so far!).

    So using token with "ssh-git" label, you have a good idea what it'd authorize.

Aside from reasonably-minor quirks mentioned above, it all was pretty common sense and straightforward for me, so can easily recommend migrating to workflows built around cheap FIDO2 smartcards on modern linux as a basic InfoSec hygiene - it doesn't add much inconvenience, and should be vastly superior to outdated (but still common) practices/rituals involving passwords or keys-in-files.

Given how all modern PC hardware has TPM2 chips in motherboards, and these can be used as a regular smartcard via PKCS#11 wrapper, they might also be a somewhat nice malware/tamper-proof cryptographic backend for various use-cases above.

From my perspective, they seem to be strictly inferior to using portable FIDO2 devices however:

  • Soldered on the motherboard, so can't be easily used in multiple places.

  • Will live/die, and have to be replaced with the motherboard.

  • Non-removable and always-accessible, holding persistent keys in there.

    Booting random OS with access to this thing seem to be a really bad idea, as ideally such keys shouldn't even be physically connected most of the time, especially to some random likely-untrustworthy software.

  • There is no physical access confirmation mechanism, so no way to actually limit it - anything getting ahold of the PIN is really bad, as secret keys can then be used freely, without any further visibility, rate-limiting or confirmation.

  • Motherboard vendor firmware security has a bad track record, and I'd rather avoid trusting crappy code there with anything extra. In fact, part of the point with having separate FIDO2 device is to trust local machine a bit less, if possible, not more.

So given that grabbing FIDO2 device(s) is an easy option, don't think TPM2 is even worth considering as an alternative to those, for all the reasons above, and probably a bunch more that I'm forgetting at the moment.

Might be best to think of TPM2 to be in the domain and managed by the OS vendor, e.g. leave it to Windows 11 and Microsoft SSO system to do trusted/measured boot and store whatever OS-managed secrets, being entirely uninteresting and invisible to the end-user.

As also mentioned above, least well-supported FIDO2-backed thing for me was early-boot dm-crypt / LUKS volume init - systemd-cryptenroll requires unlocking each encrypted LUKS blkdev separately, re-entering PIN and re-doing the touch thing multiple times in a row, with a somewhat-uncommon LUKS-on-LVM setup like mine.

But of course that's easily fixable, having following steps with a typical systemd init process:

  • Starting early on boot or in initramfs, Before=cryptsetup-pre.target, run service to ask for FIDO2 token PIN via systemd-ask-password, then use that with FIDO2 token and its hmac-secret extension to produce secure high-entropy volume unlock key.

    If PIN or FIDO2 interaction won't work, print error and repeat the query, or exit if prompt is cancelled to fallback to default systemd passphrase unlocking.

  • Drop that key into /run/cryptsetup-keys.d/ dir for each volume that it needs to open, with whatever extra per-volume alterations/hashing.

  • Let systemd pass cryptsetup.target, where systemd-cryptsetup will automatically lookup volume keys in that dir and use them to unlock devices.

    If any keys won't work or missing, systemd will do the usual passphrase-prompting and caching, so there's always a well-supported first-class fallback unlock-path.

  • Run early-boot service to cleanup after cryptsetup.target, Before=sysinit.target, to remove /run/cryptsetup-keys.d/ directory, as everything should be unlocked by now and these keys are no longer needed.

I'm using common dracut initramfs generator with systemd here, where it's easy to add a custom module that'd do all necessary early steps outlined above.

fido2_hmac_boot.nim implements all actual asking and FIDO2 operations, and can be easily run from an initramfs systemd unit file like this (fhb.service):


# Should be ordered same as stock systemd-pcrphase-initrd.service
Conflicts=shutdown.target initrd-switch-root.target
Before=sysinit.target cryptsetup-pre.target cryptsetup.target
Before=shutdown.target initrd-switch-root.target systemd-sysext.service

ExecStart=/sbin/fhb /run/initramfs/fhb.key
ExecStart=/bin/sh -c '\
  key=/run/initramfs/fhb.key; [ -e "$key" ] || exit 0; \
  mkdir -p /run/cryptsetup-keys.d; while read dev line; \
  do cat "$key" >/run/cryptsetup-keys.d/"$dev".key; \
  done < /etc/fhb.devices; rm -f "$key"'

With that fhb.service file and compiled binary itself installed via module-setup.sh in the module dir:


check() {
  require_binaries /root/fhb || return 1
  return 255 # only include if asked for

depends() {
  echo 'systemd crypt fido2'
  return 0

install() {
  # fhb.service starts binary before cryptsetup-pre.target to create key-file
  inst_binary /root/fhb /sbin/fhb
  inst_multiple mkdir cat rm
  inst_simple "$moddir"/fhb.service "$systemdsystemunitdir"/fhb.service
  $SYSTEMCTL -q --root "$initdir" add-wants initrd.target fhb.service

  # Some custom rules might be relevant for making consistent /dev symlinks
  while read p
  do grep -qiP '\b(u2f|fido2)\b' "$p" && inst_rules "$p"
  done < <(find /etc/udev/rules.d -maxdepth 1 -type f)

  # List of devices that fhb.service will create key for in cryptsetup-keys.d
  # Should be safe to have all "auto" crypttab devices there, just in case
  while read luks dev key opts; do
    [[ "${opts//,/ }" =~ (^| )noauto( |$) ]] && continue
    echo "$luks"
  done <"$dracutsysrootdir"/etc/crypttab >"$initdir"/etc/fhb.devices
  mark_hostonly /etc/fhb.devices

Module would need to be enabled via e.g. add_dracutmodules+=" fhb " in dracut.conf.d, and will include the "fhb" binary, service file to run it, list of devices to generate unlock-keys for in /etc/fhb.devices there, and any udev rules mentioning u2f/fido2 from /etc/udev/rules.d, in case these might be relevant for consistent device path or whatever other basic device-related setup.

fido2_hmac_boot.nim "fhb" binary can be built (using C-like Nim compiler) with all parameters needed for its operation hardcoded via e.g. -d:FHB_CID=... compile-time options, to avoid needing to bother with any of those in systemd unit file or when running it anytime on its own later.

It runs same operation as fido2-assert tool, producing HMAC secret for specified Credential ID and Salt values. Credential ID should be created/secured prior to that using related fido2-token and fido2-cred binaries. All these tools come bundled with libfido2.

Since systemd doesn't nuke /run/cryptsetup-keys.d by default (keyfile-erase option in crypttab can help, but has to be used consistently for each volume), custom unit file to do that can be added/enabled to main systemd as well:


ExecStart=rm -rf /run/cryptsetup-keys.d


And that should do it for implementing above early-boot unlocking sequence.

To enroll the key produced by "fhb" binary into LUKS headers, simply run it, same as early-boot systemd would, and luksAddKey its output.

Couple additional notes on all this stuff:

  • HMAC key produced by "fhb" tool is a high-entropy uniformly-random 256-bit (32B) value, so unlike passwords, does not actually need any kind of KDF applied to it - it is the key, bruteforcing it should be about as infeasible as bruteforcing 128/256-bit master symmetric cipher key (and likely even harder).

    Afaik cryptsetup doesn't support disabling KDF for key-slot entirely, but --pbkdf pbkdf2 --pbkdf-force-iterations 1000 options can be used to set fastest parameters and get something close to disabling it.

  • cryptsetup config --key-slot N --priority prefer can be used to make systemd-cryptsetup try unlocking volume with this no-KDF keyslot quickly first, before trying other slots with memory/cpu-heavy argon2id and such proper PBKDF, which should almost always be a good idea to do in this order, as it should take almost no time to try 1K-rounds PBKDF2 slot.

  • Ideally each volume should have its own sub-key derived from one that fhb outputs, e.g. via simple HMAC-SHA256(volume-uuid, key=fhb.key) operation, which is omitted here for simplicitly.

    fhb binary includes --hmac option for that, to use instead of "cat" above:

    fhb --hmac "$key" "$dev" /run/cryptsetup-keys.d/"$dev".key

    Can be added to avoid any of LUKS keys/keyslots being leaked or broken (for some weird reason) to have any effect on other keys - reversing such HMAC back to fhb.key to use it for other volumes would still be cryptographically infeasible.

Custom fido2_hmac_boot.nim binary/code used here is somewhat similar to an earlier fido2-hmac-desalinate.c that I use for password management (see above), but a bit more complex, so is written in an easier and much nicer/safer language (Nim), while still being compiled through C to pretty much same result.