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gpg-bridge

A bridge connects openssh-portable and GnuPG on Windows.

Usage

  1. Install it on your System.

    cargo install -f --git https://github.com/busyjay/gpg-bridge

    It's recommanded to build the binary yourself. In case you don't want to setup Rust environment, you can also download a prebuilt binary from github action artifact. Here is an official guide on how to download artifacts. The artifact is packaged for every commit, make sure download the latest one.

  2. Make sure you have setup gpg agent forward following the guide.

  3. Directly using socket provided by GnuPG won't work on Windows, so change local socket to a TCP port instead.

    RemoteForward <socket_on_remote_box>  127.0.0.1:4321

    You are free to use any port that has not been taken, 4321 is just an example.

  4. Build a bridge between TCP port and GnuPG extra socket.

    ~/.cargo/bin/gpg-bridge --extra 127.0.0.1:4321

    If you have customized extra socket localtion, you set the path using --extra-socket.

Now you are all set, requests to gpg agent on remote should be able to forward to your local.

Why invent the wheel

There are several gotchas if not using bridge to forward gpg agent on Windows. See PowerShell/Win32-OpenSSH#1564.

  1. Specifying remote forward local socket path in openssh-portable can be tricky (for now).

    Path like C:/xxx, ~/xxx and %userprofile%/xxx will not work. You have to use form like /absolute/path/to/local/socket and execute ssh on the same driver path. See https://docs.microsoft.com/en-us/dotnet/standard/io/file-path-formats.

  2. Even path is correctly specified and accepted, forwarding will not work.

    Openssh-portable can't handle UDS(unix domain socket) on Windows correctly (for now).

  3. Even Openssh-portable handles UDS correctly, forwarding still can't work.

    Support for Unix domain sockets was introduced in Windows 10 Insider Build 17063. It became generally available in version 1809 (aka the October 2018 Update), and in Windows Server 1809/2019.

    GnuPG on Windows has not utilized native UDS support yet. It simulates a UDS using a TCP stream socket with customized connect step. So without extra tools, you can't really connect openssh-portable to GnuPG.

Using GnuPG Agent as ssh agent

GnuPG Agent supports OpenSSH Agent protocol. This tool also supports forwarding ssh queries by utilizing putty protocols.

  1. To forward it as ssh agent, you need to ensure --enable-putty-support is configured for gpg client. Or you can put it into the configuration files, homedir/gpg-agent.conf. homedir can be found by gpgconf --list-dir homedir.

    enable-putty-support

  2. Then pass --ssh \\.\pipe\gpg-bridge-ssh to gpg-bridge.

    ~/.cargo/bin/gpg-bridge --ssh \\.\pipe\gpg-bridge-ssh

    This can also be used with extra socket at the same time.

    ~/.cargo/bin/gpg-bridge --extra 127.0.0.1:4321 --ssh \\.\pipe\gpg-bridge-ssh

  3. Now let OpenSSH to use gpg agent by setting environment variable SSH_AUTH_SOCK to \\.\pipe\gpg-bridge-ssh.

The string "gpg-bridge-ssh" can be changed to anything you want, just make sure it's consistent everywhere.