TKey SSH Agent

tkey-ssh-agent is an OpenSSH-compatible agent for use with the Tillitis TKey USB security token.

Warning: Please use tagged releases for any real use. Development on main might mean we change which version of the the signer device app we use which would cause the SSH key pair to change!

See Release notes.

Installing

tkey-ssh-agent might be available in your operating system's package system.

If not, see Tillitis' application page for the agent as well as instructions.

If there's no official package for your system the easiest way to install is probably to:

$ go install github.com/tillitis/tkey-ssh-agent/cmd/tkey-ssh-agent@latest

After this the tkey-ssh-agent command should be available in your $GOBIN directory.

Note that installing with go install doesn't set the version like building with other methods does. See building the agent below.

You will also have to install these manually if you use go install:

• Manual page system/tkey-ssh-agent.1.
• udev rules, see system/60-tkey.rules (Linux).
• systemd service unit, see system/tkey-ssh-agent.service.tmpl and change ##BINDIR## to where you installed tkey-ssh-agent (some Linux dists).

If you're building from source (see below) there is a make install target that installs the agent and the udev rules. Please see Makefile to see that everything ends up where you want to. Typically you will have to do:

$ sudo make install
$ sudo make reload-rules

Using tkey-ssh-agent

tkey-ssh-agent tries to auto-detect the TKey. If more than one is found, or if you're running on QEMU, then you'll need to use the --port flag:

$ ./tkey-ssh-agent -a ./agent.sock --port /dev/pts/1

This will start the SSH agent and tell it to listen on the specified socket ./agent.sock.

Nota bene: If the signer app binary, the USS, or the UDS in the physical USB stick change your key pair will change.

If you copy-paste the public key into your ~/.ssh/authorized_keys you can try to log onto your local computer (if sshd is running there). The socket path set/output above is also needed by SSH in SSH_AUTH_SOCK:

$ SSH_AUTH_SOCK=/path/to/agent.sock ssh -F /dev/null localhost

-F /dev/null is used to ignore your ~/.ssh/config which could interfere with this test.

The tkey-ssh-agent also supports the --uss and --uss-file flags to enter a User Supplied Secret.

You can use --show-pubkey (short flag: -p) to only output the pubkey. The pubkey is printed to stdout for easy redirection, but some messages are still present on stderr.

Building the agent

If you have Go and make installed, a simple:

$ make

or, for a Windows executable,

$ make tkey-ssh-agent.exe

should build the agent. A pre-compiled signer device app binary is included in the repo and will be automatically embedded.

Cross compiling the usual Go way with GOOS and GOARCH environment variables works for most targets but currently doesn't work for GOOS=darwin since the go.bug.st/serial package relies on macOS shared libraries for port enumeration.

Building agent with tkey-builder

If you want to use our tkey-builder image and you have make you can run:

$ podman pull ghcr.io/tillitis/tkey-builder:4
$ make podman

or run it directly with Podman:

$ podman run --rm --mount type=bind,source=$(CURDIR),target=/src --mount type=bind,source=$(CURDIR)/../tkey-libs,target=/tkey-libs -w /src -it ghcr.io/tillitis/tkey-builder:4 make -j

Note that building with Podman like this by default creates a Linux binary. Set GOOS and GOARCH with -e in the call to podman run to desired target. Again, this won't work with a macOS target.

Building with another signer

For convenience, and to be able to support go install, a precompiled signer device app binary is included under cmd/tkey-ssh-agent.

If you want to replace the signer used by the agent you have to:

1. Compile your own signer and place it in the cmd/tkey-ssh-agent directory.
2. Change the path to the embedded signer in cmd/tkey-ssh-agent/signer.go. Look for go:embed....
3. Change the appName directly under the go:embed to whatever your signer is called so the agent reports this correctly with --version.
4. Compute a new SHA-512 hash digest for your binary, typically by something like sha512sum cmd/tkey-ssh-agent/signer.bin-v0.0.7 and put the resulting output in the file signer.bin.sha512 at the top level.
5. make in the top level.

Disabling touch requirement

The signer device app normally requires the TKey to be physically touched to make a signature. For special purposes it can be compiled without this requirement by setting the environment variable TKEY_SIGNER_APP_NO_TOUCH to some value when building. Example: make TKEY_SIGNER_APP_NO_TOUCH=yesplease.

Note well: You have to do this when building both the signer and the client apps. tkey-ssh-agent will also stop displaying notifications about touch if the variable is set.

Warning: Of course changing the code also changes the signer binary and as a consequence the SSH key pair will also change.

Building the signer

1. See the Devoloper Handbook for setup of development tools. We recommend you use tkey-builder.
2. See the instructions in the tkey-device-signer repo.
3. Copy its signer/app.bin to cmd/tkey-sign/signer.bin-${signer_version} and run make.

To help prevent unpleasant surprises we keep a digest of the signer in cmd/tkey-ssh-agent/signer.bin.sha512. The compilation will fail if this is not the expected binary. If you really intended to build with another signer, see Building with another signer above.

Windows support

tkey-ssh-agent can be built for Windows. The Makefile has a windows target that produces tkey-ssh-agent.exe and tkey-ssh-agent-tray.exe. The former is a regular command-line application, suitable for use in environments like PowerShell. The latter is a small application built for the windowsgui subsystem, meaning it operates without a console. Its primary function is to create a tray icon and initiate tkey-ssh-agent.exe with the identical arguments it received. They are assumed to be located in the same directory. For automatically starting the SSH agent when logging onto the computer, a shortcut to tkey-ssh-agent-tray.exe, with the required arguments, can be added in your user's Startup folder.

When using the --uss option the Windows build by default uses the pinentry program from Gpg4win for requesting the User-Supplied Secret. This package can be installed using: winget install GnuPG.Gpg4win.

The SSH Agent supports being used by the native OpenSSH client ssh.exe (part of Windows Optional Features and installable using winget). The environment variable SSH_AUTH_SOCK should be set to the complete path of the Named Pipe that tkey-ssh-agent listens on.

For example, if it is started using ./tkey-ssh-agent.exe -a tkey-ssh-agent the environment variable could be set for the current PowerShell like this:

$env:SSH_AUTH_SOCK = '\\.\pipe\tkey-ssh-agent'

Setting this environment variable persistently, for future PowerShell terminals, Visual Studio Code, and other programs can be done through the System Control Panel. Or using PowerShell:

[Environment]::SetEnvironmentVariable('SSH_AUTH_SOCK', '\\.\pipe\tkey-ssh-agent', 'User')

You can learn more about environment variables on Windows in Microsoft's article.

The SSH Agent can also be used with the Git-for-Windows client (winget install Git.Git). By default, it uses its own bundled ssh-client. Run the following PowerShell commands to make git.exe use the system's native ssh.exe:

$sshpath = (get-command ssh.exe).path -replace '\\','/'
git config --global core.sshCommand $sshpath
git config --global --get core.sshCommand

The last command should output something like C:/Windows/System32/OpenSSH/ssh.exe.

For details on how we package and build an MSI installer, see system/windows/README.md.

Licenses and SPDX tags

Unless otherwise noted, the project sources are licensed under the terms and conditions of the "GNU General Public License v2.0 only":

Copyright Tillitis AB.

These programs are free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2 only.

These programs are distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see:

https://www.gnu.org/licenses

See LICENSE for the full GPLv2-only license text.

External source code we have imported are isolated in their own directories. They may be released under other licenses. This is noted with a similar LICENSE file in every directory containing imported sources.

The project uses single-line references to Unique License Identifiers as defined by the Linux Foundation's SPDX project on its own source files, but not necessarily imported files. The line in each individual source file identifies the license applicable to that file.

The current set of valid, predefined SPDX identifiers can be found on the SPDX License List at:

https://spdx.org/licenses/

All contributors must adhere to the Developer Certificate of Origin.