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 "$@"

This script is fantastic, thanks for putting it together!

I ran into some issue b/c winuser is detected as the user inside WSL (and they're different for me). However, changing that part to these lines works for me:

relay() {
  set -e
  local winhome="$(cmd.exe /c "<nul set /p=%UserProfile%" 2>/dev/null)"
  local wslhome="$(wslpath $winhome)"
  local wingnupghome="$winhome/AppData/Roaming/gnupg"
  local npiperelay="$wslhome/AppData/Roaming/npiperelay/npiperelay.exe"
  local wslsshpageant="$wslhome/AppData/Roaming/wsl-ssh-pageant/wsl-ssh-pageant-amd64-gui.exe"
  ...

@lekv thanks for the pointer. I have adjusted the script, I really like how your solution makes no assumption about the home dir location whatsoever :-)
There was an issue with the backslashes in the path returned by %UserProfile% though. I think socat or some other part of that chain re-interpretes the gpg-agent socket path argument and thereby makes it invalid. Instead of double escaping the backslashes I simply replaced them with forward slashes (local wingpgagentpath="${winhome//\\/\/}/AppData/Roaming/gnupg/S.gpg-agent").

Hi @andsens, thank you for the awesome script. I'm having a bit of trouble getting it to work. I've reached a stage where I had to slightly modify the script because my username contains a space and the script doesn't seem to be able to handle that. Now I just get a whole bunch of npiperelay.exe help messages. Here's my output of the foreground task when ran with bash -x

+ PIDFILE=/home/dannyverpoort/.gnupg/gpg-agent-relay.pid
+ main foreground
+ checkdeps socat start-stop-daemon lsof timeout
+ deps=("$@")
+ local deps
+ local dep
+ local out
+ local ret=0
+ for dep in "${deps[@]}"
++ type socat
+ out='socat is /usr/bin/socat'
+ for dep in "${deps[@]}"
++ type start-stop-daemon
+ out='start-stop-daemon is /usr/sbin/start-stop-daemon'
+ for dep in "${deps[@]}"
++ type lsof
+ out='lsof is /usr/bin/lsof'
+ for dep in "${deps[@]}"
++ type timeout
+ out='timeout is /usr/bin/timeout'
+ return 0
+ case $1 in
+ relay
+ set -e
+ local winhome
+ local wslwinhome
++ cmd.exe /c '<nul set /p=%UserProfile%'
++ true
+ winhome='C:\Users\Danny Verpoort'
++ wslpath -u 'C:\Users\Danny Verpoort'
+ wslwinhome='/mnt/c/Users/Danny Verpoort'
+ local npiperelay=/mnt/c/Users/Local/bin/npiperelay/npiperelay.exe
+ local wslsshpageant=/mnt/c/Users/Local/bin/wsl-ssh-pageant/wsl-ssh-pageant-amd64-gui.exe
+ local 'gpgconnectagent=/mnt/c/Program Files (x86)/GnuPG/bin/gpg-connect-agent.exe'
+ local gpgagentsocket=/home/dannyverpoort/.gnupg/S.gpg-agent
+ local sshagentsocket=/home/dannyverpoort/.gnupg/S.gpg-agent.ssh
+ local 'wingpgagentpath=C:/Users/Danny Verpoort/AppData/Roaming/gnupg/S.gpg-agent'
+ killsocket /home/dannyverpoort/.gnupg/S.gpg-agent
+ local socketpath=/home/dannyverpoort/.gnupg/S.gpg-agent
+ [[ -e /home/dannyverpoort/.gnupg/S.gpg-agent ]]
+ killsocket /home/dannyverpoort/.gnupg/S.gpg-agent.ssh
+ local socketpath=/home/dannyverpoort/.gnupg/S.gpg-agent.ssh
+ [[ -e /home/dannyverpoort/.gnupg/S.gpg-agent.ssh ]]
+ '/mnt/c/Program Files (x86)/GnuPG/bin/gpg-connect-agent.exe' /bye
+ WSPPID=4746
+ /mnt/c/Users/Local/bin/wsl-ssh-pageant/wsl-ssh-pageant-amd64-gui.exe --systray --winssh ssh-pageant
+ GNUPID=4747
+ trap 'kill -TERM 4747' EXIT
+ socat UNIX-LISTEN:/home/dannyverpoort/.gnupg/S.gpg-agent,unlink-close,fork,umask=177 'EXEC:/mnt/c/Users/Local/bin/npiperelay/npiperelay.exe -ep -ei -s -a '\''C:/Users/Danny Verpoort/AppData/Roaming/gnupg/S.gpg-agent'\'',nofork'
+ SSHPID=4748
+ socat UNIX-LISTEN:/home/dannyverpoort/.gnupg/S.gpg-agent.ssh,unlink-close,fork,umask=177 'EXEC:/mnt/c/Users/Local/bin/npiperelay/npiperelay.exe /\/\./\pipe/\ssh-pageant'
+ set +e
+ trap 'kill -TERM 4747; kill -TERM 4748' EXIT
+ systemd-notify --ready
+ wait 4747 4748

The agent-relay foreground command is silent until I try to use gpg --card-status in a second WSL window. Then the repeated help message shows up.

I'm running Ubuntu 20.04.1 on WSL, windows 10 as host and I'm trying to use a yubikey 5. If I try the card status in the power shell I can actually see the card. I have all the perquisites installed. Any suggestions on what's going on?

@dannyverp, socat EXEC is garbage at escaping the arguments for its invocations. I suggest fiddling around with different escaping mechanism and see what sticks (i.e. just run the socat command seperately).
The big question mark here is whether some of the argument parsing happens on the windows side of things when invoking .exe files, I have no clue about that.
To start off with try double-escaping wingpgagentpath like so wingpgagentpath=$(printf -- '%q' "$wingpgagentpath")

Hey, so I've found out that somehow sometimes a socket is open by multiple processes and thus the killsocket function fails as the socketpid variable is a multiline string. I think replacing it with something like echo "$socketpid" | xargs kill should do the trick.

@jmigual huh, curious. Of course, only on process can be listening on that socket. So the others must be clients. I'm not sure killing them is the best way to go, it would be better to have lsof filter properly.

This isn't working for me as is. I moved a few files around, but other than that I'm running the script exactly as you have here. The only thing that fixed gpg for me was to symlink /mnt/c/Program Files (x86)/GnuPG/bin/gpg.exe to /usr/local/sbin/gpg. (Found here: https://blog.nimamoh.net/yubi-key-gpg-wsl2/#isso-4)

Is there any way I can help figure out why it doesn't work with stock gpg?

P.S. I'm also going to work on modifying the script to only setup the GPG relay. I want to use the built in Windows OpenSSH client with its ssh-agent.

I'm also not able to get it working. When I run any gpg commands in WSL2 (Ubuntu 20.10 fwiw), it doesn't seem able to use the existing sockets. I added extra-socket /dev/null and browser-socket /dev/null to ~/.gnupg/gpg-agent.conf to keep it from creating any new sockets, and then any gpg commands return: gpg: can't connect to the agent: End of file

@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.