Script taken from https://blog.nimamoh.net/yubi-key-gpg-wsl2/ and improved

gpg-agent-relay

#!/usr/bin/env bash
# Inspired by https://blog.nimamoh.net/yubi-key-gpg-wsl2/

# Guide:
# Install GPG on windows & Unix
# Add "enable-putty-support" to gpg-agent.conf
# Download wsl-ssh-pageant and npiperelay and place the executables in "C:\Users\[USER]\AppData\Roaming\" under wsl-ssh-pageant & npiperelay
# https://github.com/benpye/wsl-ssh-pageant/releases/tag/20190513.14
# https://github.com/NZSmartie/npiperelay/releases/tag/v0.1
# Adjust relay() below if you alter those paths
# Place this script in WSL at ~/.local/bin/gpg-agent-relay
# Start it on login by calling it from your .bashrc: "$HOME/.local/bin/gpg-agent-relay start"

GNUPGHOME="$HOME/.gnupg"
PIDFILE="$GNUPGHOME/gpg-agent-relay.pid"

die() {
  # shellcheck disable=SC2059
  printf "$1\n" >&2
  exit 1
}

main() {
  checkdeps socat start-stop-daemon lsof timeout
  case $1 in
  start)
    if ! start-stop-daemon --pidfile "$PIDFILE" --background --notify-await --notify-timeout 5 --make-pidfile --exec "$0" --start -- foreground; then
      # shellcheck disable=SC2016
      die 'Failed to start. Run `gpg-agent-relay foreground` to see output.'
    fi
    ;;
  stop)
    start-stop-daemon --pidfile "$PIDFILE" --remove-pidfile --stop ;;
  status)
    start-stop-daemon --pidfile "$PIDFILE" --status
    local result=$?
    case $result in
      0) printf "gpg-agent-relay is running\n" ;;
      1 | 3) printf "gpg-agent-relay is not running\n" ;;
      4) printf "unable to determine status\n" ;;
    esac
    return $result
    ;;
  foreground)
    relay ;;
  *)
    die "Usage:\n  gpg-agent-relay start\n  gpg-agent-relay stop\n  gpg-agent-relay status\n  gpg-agent-relay foreground" ;;
  esac
}

relay() {
  set -e
  local winhome
  local wslwinhome
  winhome=$(cmd.exe /c "<nul set /p=%UserProfile%" 2>/dev/null || true)
  wslwinhome="$(wslpath -u "$winhome")"
  local npiperelay="$wslwinhome/AppData/Roaming/npiperelay/npiperelay.exe"
  local wslsshpageant="$wslwinhome/AppData/Roaming/wsl-ssh-pageant/wsl-ssh-pageant-amd64-gui.exe"
  local gpgconnectagent="/mnt/c/Program Files (x86)/GnuPG/bin/gpg-connect-agent.exe"
  local gpgagentsocket="$GNUPGHOME/S.gpg-agent"
  local sshagentsocket="$GNUPGHOME/S.gpg-agent.ssh"
  # backslash escaping in socat EXEC doesn't seem to work very well, use forward slashes instead
  # windows/npiperelay handle that just fine
  local wingpgagentpath="${winhome//\\/\/}/AppData/Roaming/gnupg/S.gpg-agent"

  killsocket "$gpgagentsocket"
  killsocket "$sshagentsocket"

  "$gpgconnectagent" /bye

  "$wslsshpageant" --systray --winssh ssh-pageant 2>/dev/null &
  # shellcheck disable=SC2034
  WSPPID=$!

  socat UNIX-LISTEN:"$gpgagentsocket,unlink-close,fork,umask=177" EXEC:"$npiperelay -ep -ei -s -a '$wingpgagentpath'",nofork &
  GNUPID=$!
  # shellcheck disable=SC2064
  trap "kill -TERM $GNUPID" EXIT

  socat UNIX-LISTEN:"$sshagentsocket,unlink-close,fork,umask=177" EXEC:"$npiperelay /\/\./\pipe/\ssh-pageant" &
  SSHPID=$!

  set +e
  # shellcheck disable=SC2064
  trap "kill -TERM $GNUPID; kill -TERM $SSHPID" EXIT

  systemd-notify --ready 2>/dev/null
  wait $GNUPID $SSHPID
  trap - EXIT
}

killsocket() {
  local socketpath=$1
  if [[ -e $socketpath ]]; then
    local socketpid
    if socketpid=$(lsof +E -taU -- "$socketpath"); then
      timeout .5s tail --pid=$socketpid -f /dev/null &
      local timeoutpid=$!
      kill "$socketpid"
      if ! wait $timeoutpid; then
        die "Timed out waiting for pid $socketpid listening at $socketpath"
      fi
    else
      rm "$socketpath"
    fi
  fi
}

checkdeps() {
  local deps=("$@")
  local dep
  local out
  local ret=0
  for dep in "${deps[@]}"; do
    if ! out=$(type "$dep" 2>&1); then
      printf -- "Dependency %s not found:\n%s\n" "$dep" "$out"
      ret=1
    fi
  done
  return $ret
}

main "$@"

@alanivey Did you try the symlink method? That was the only thing that worked for me.

@mew1033 yes, I symlinked the Gpg4Win exe in WSL2 Linux as $HOME/.local/bin/gpg with ln -sv /mnt/c/Program\ Files\ \(x86\)/GnuPG/bin/gpg.exe ~/.local/bin/gpg (in my WSL2 Linux environment, I have this directory at the front of $PATH). This binary does not attempt to use $HOME/.gnupg/S.gpg-agent when running gpg commands, nor does it use $HOME/.gnupg for its database, so it's not necessary to do any GPG setup in the WSL2 Linux environment. B/c I'm not looking to use the SSH agent integration, I ultimately find this easier; I maintain a single GPG database in Windows and can use it in multiple WSL2 Linux environments without replicating.

I'll caveat that I have not used this configuration for longer than a day so I might end up being wrong!

In my Debian-WSL2, I can't access cmd.exe (and I didn't investigate in this further, as there seems to be a cleaner way), so the script isn't working. I replaced line 55 and 56 with this (you have to install wslu for this):

username="$(wslvar USERPROFILE)"
wslwinhome="$(wslpath $username)"
winhome="$(wslpath -w $wslwinhome)"

additionally, I had to add export PATH=$PATH:/sbin to my .bashrc as otherwise start-stop-daemon isn't found in path.

@andsens I was having difficulty with this script and multiple shells (ie, a windows terminal and VSCode, which opens another terminal). It was failing because multiple PIDs were being included in the socket file, and kill was not super happy about that.

I'd end up with an error like;

gpg-agent-relay foreground
gpg-agent-relay: line 99: kill: 768
2650
8610
23065
26471
27892
30552: arguments must be process or job IDs
tail: cannot open '2650' for reading: No such file or directory
tail: cannot open '8610' for reading: No such file or directory
tail: cannot open '23065' for reading: No such file or directory
tail: cannot open '26471' for reading: No such file or directory
tail: cannot open '27892' for reading: No such file or directory
tail: cannot open '30552' for reading: No such file or directory
==> /dev/null <==

To correct it, I modified the killsocket function;

 killsocket() {
   local socketpath=$1
   if [[ -e $socketpath ]]; then
     # local socketpid
     lsof +E -taU -- "$socketpath" | awk 'length {print $1}' | xargs -rn1 kill
   fi
 }

This works and corrects my multiple-terminal issues, but the timeout is also removed. It could be added, but I am wondering if it is even necessary?

I've been using this for a couple of years now with few enough headaches that I never tried to look for a better way. But with systemd support now in WSL I decided to see if I could simplify this approach and I found that I could do it all just using systemd and npiperelay. Turns out I could so I packaged it all up in wsl-gpg-systemd if anyone is interested in giving it a shot.

@demonbane I tried it out on a fresh container this evening, and it works great! Thanks for simplifying it tenfold.

For anyone coming at systemd new; the socket set up by wsl-gpg-systemd is /run/user/1000/gnupg/S.gpg-agent.ssh. I had to pause for a second to figure out where the socket was being opened.

The installer takes care to always use gpgconfig to get any paths, and newer releases of gpg have been following the systemd spec and using /run for transient user session storage, like the sockets. And surprisingly, there doesn't appear to be an easy way to override that location. From the gpg-agent manpage:

       --use-standard-socket
       --no-use-standard-socket
       --use-standard-socket-p
              Since GnuPG 2.1 the standard socket is always used.  These options have no more  effect.   The  command
              gpg-agent --use-standard-socket-p will thus always return success.