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SSH Agent

Configuration

SSH requires no configuration, but you may put common command line options in ~/.ssh/agent.conf to avoid repeating them in every invocation.

See (trezor|keepkey|ledger|jade|onlykey)-agent -h for details on supported options and the configuration file format.

If you'd like a Trezor-style PIN entry program, follow these instructions.

Usage

Use the (trezor|keepkey|ledger|jade|onlykey)-agent program to work with SSH. It has three main modes of operation:

1. Export public keys

To get your public key so you can add it to authorized_hosts or allow ssh access to a service that supports it, run:

(trezor|keepkey|ledger|jade|onlykey)-agent identity@myhost

The identity (ex: identity@myhost) is used to derive the public key and is added as a comment to the exported key string.

2. Run a command with the agent's environment

Run

$ (trezor|keepkey|ledger|jade|onlykey)-agent identity@myhost -- COMMAND --WITH --ARGUMENTS

to start the agent in the background and execute the command with environment variables set up to use the SSH agent. The specified identity is used for all SSH connections. The agent will exit after the command completes. Note the -- separator, which is used to separate trezor-agent's arguments from the SSH command arguments.

Example:

 (trezor|keepkey|ledger|jade|onlykey)-agent -e ed25519 [email protected] -- rsync up/ [email protected]:/home/bob

As a shortcut you can run

$ (trezor|keepkey|ledger|jade|onlykey)-agent identity@myhost -s

to start a shell with the proper environment.

3. Connect to a server directly via (trezor|keepkey|ledger|jade|onlykey)-agent

If you just want to connect to a server this is the simplest way to do it:

$ (trezor|keepkey|ledger|jade|onlykey)-agent user@remotehost -c

The identity user@remotehost is used as both the destination user and host as well as for key derivation, so you must generate a separate key for each host you connect to.

Common Use Cases

Start a single SSH session

Demo

Start multiple SSH sessions from a sub-shell

This feature allows using regular SSH-related commands within a subprocess running user's shell. SSH_AUTH_SOCK environment variable is defined for the subprocess (pointing to the SSH agent, running as a parent process). This way the user can use SSH-related commands (e.g. ssh, ssh-add, sshfs, git, hg), while authenticating via the hardware device.

Subshell

Load different SSH identities from configuration file

Config

Implement passwordless login

Run:

/tmp $ trezor-agent [email protected] -v > hostname.pub
2015-09-02 15:03:18,929 INFO         getting "ssh://[email protected]" public key from Trezor...
2015-09-02 15:03:23,342 INFO         disconnected from Trezor
/tmp $ cat hostname.pub
ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBGSevcDwmT+QaZPUEWUUjTeZRBICChxMKuJ7dRpBSF8+qt+8S1GBK5Zj8Xicc8SHG/SE/EXKUL2UU3kcUzE7ADQ= ssh://[email protected]

Append hostname.pub contents to /home/user/.ssh/authorized_keys configuration file at ssh.hostname.com, so the remote server would allow you to login using the corresponding private key signature.

Access remote Git/Mercurial repositories

Export your public key and register it in your repository web interface (e.g. GitHub):

$ trezor-agent -v -e ed25519 [email protected] > ~/.ssh/github.pub

Add the following configuration to your ~/.ssh/config file:

Host github.com
	IdentityFile ~/.ssh/github.pub

Use the following Bash alias for convenient Git operations:

$ alias ssh-shell='trezor-agent ~/.ssh/github.pub -v --shell'

Now, you can use regular Git commands under the "SSH-enabled" sub-shell:

$ ssh-shell
$ git push origin master

The same works for Mercurial (e.g. on BitBucket):

$ ssh-shell
$ hg push

Git commit signing

For more details, see the following great blog post: https://calebhearth.com/sign-git-with-ssh

$ trezor-agent -e ed25519 user@host --shell
$ ssh-add -L
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDeAmtnhHlyg4dzGP3/OF4WHX7NoYhClS98EK22q/O5+ <ssh://user@host|ed25519>
$ git config --local user.signingkey "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDeAmtnhHlyg4dzGP3/OF4WHX7NoYhClS98EK22q/O5+"
$ git config --local gpg.format ssh
$ git config --local commit.gpgsign true

$ git config --local gpg.ssh.allowedSignersFile $PWD/.git/allowed-signers
$ echo "user@host ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDeAmtnhHlyg4dzGP3/OF4WHX7NoYhClS98EK22q/O5+" >> $PWD/.git/allowed-signers

$ git commit --allow-empty --message="Testing SSH signing"
[master 4a1f730] Testing SSH signing

$ git log --show-signature -1
commit 4a1f730d7f70fd31a0bda334734d0ac4dc9d97ad (HEAD -> master)
Good "git" signature for user@host with ED25519 key SHA256:aESFjLsydJHQg1vnAkq42jQDkCcn4Tde4J+v+0XFmwM
Author: Roman Zeyde <[email protected]>
Date:   Fri Oct 21 18:34:09 2022 +0300

    Testing SSH signing

$ cat .git/config
[user]
	signingkey = ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDeAmtnhHlyg4dzGP3/OF4WHX7NoYhClS98EK22q/O5+
[gpg]
	format = ssh
[commit]
	gpgsign = true
[gpg "ssh"]
	allowedSignersFile = /home/user/Code/test-git-ssh-sig/.git/allowed-signers

Start the agent as a systemd unit

1. Create these files in ~/.config/systemd/user

Replace trezor with keepkey or ledger or jade or onlykey as required.

trezor-ssh-agent.service
[Unit]
Description=trezor-agent SSH agent
Requires=trezor-ssh-agent.socket

[Service]
Type=simple
Restart=always
Environment="DISPLAY=:0"
Environment="PATH=/bin:/usr/bin:/usr/local/bin:%h/.local/bin"
ExecStart=/usr/bin/trezor-agent --foreground --sock-path %t/trezor-agent/S.ssh IDENTITY

If you've installed trezor-agent locally you may have to change the path in ExecStart=.

Replace IDENTITY with the identity you used when exporting the public key.

IDENTITY can be a path (starting with /) to a file containing a list of public keys generated by Trezor. I.e. /home/myUser/.ssh/trezor.conf with one public key per line. This is a more convenient way to have a systemd setup that has to handle multiple keys/hosts.

When updating the file, make sure to restart trezor-agent.

If you have multiple Trezors connected, you can select which one to use via a TREZOR_PATH environment variable. Use trezorctl list to find the correct path. Then add it to the agent with the following line:

Environment="TREZOR_PATH=<your path here>"

Note that USB paths depend on the USB port which you use.

trezor-ssh-agent.socket
[Unit]
Description=trezor-agent SSH agent socket

[Socket]
ListenStream=%t/trezor-agent/S.ssh
FileDescriptorName=ssh
Service=trezor-ssh-agent.service
SocketMode=0600
DirectoryMode=0700

[Install]
WantedBy=sockets.target
2. Run
systemctl --user start trezor-ssh-agent.service trezor-ssh-agent.socket
systemctl --user enable trezor-ssh-agent.socket
3. Add this line to your .bashrc or equivalent file:
export SSH_AUTH_SOCK=$(systemctl show --user --property=Listen trezor-ssh-agent.socket | grep -o "/run.*" | cut -d " " -f 1)

Make sure the SSH_AUTH_SOCK variable matches the location of the socket that trezor-agent is listening on: ps -x | grep trezor-agent. In this setup trezor-agent should start automatically when the socket is opened.

4. SSH will now automatically use your device key in all terminals.

SSH Signatures

SSH and ssh-keygen can make and verify signatures, see https://www.agwa.name/blog/post/ssh_signatures.

See here for more ssh protocol details:

generate SSH public key
$ trezor-agent -e ed25519 [email protected] | tee ~/.ssh/trezor-github.pub
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIvcbhXyaXXNytCLTDfEMlLuwEhtfo0XmPP1U5RsnOZ4 <ssh://[email protected]|ed25519>
sign the given file using TREZOR
$ trezor-agent -e ed25519 [email protected] -- ssh-keygen -Y sign -f ~/.ssh/trezor-github.pub -n file README.md
Signing file README.md
Write signature to README.md.sig
set allowed identities for verification (using the above public key)
$ cat allowed 
[email protected] ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIvcbhXyaXXNytCLTDfEMlLuwEhtfo0XmPP1U5RsnOZ4 <ssh://[email protected]|ed25519>
verify the above signature
$ ssh-keygen -Y verify -f allowed -I [email protected] -n file -s README.md.sig -vvv < README.md
debug1: sshsig_verify_fd: signature made with hash "sha512"
debug1: sshsig_wrap_verify: verify message length 64
debug1: Valid (unverified) signature from key SHA256:6UBhPb5SOoCUfasGC1/aCBegYov0/P3ajd6eNbYg77A
debug1: parse_principals_key_and_options: allowed:1: matched principal "[email protected]"
debug1: allowed:1: matched key and principal
Good "file" signature for [email protected] with ED25519 key SHA256:6UBhPb5SOoCUfasGC1/aCBegYov0/P3ajd6eNbYg77A

Troubleshooting

If SSH connection fails to work, please open an issue with a verbose log attached (by running trezor-agent -vv) .

IdentitiesOnly SSH option

Note that your local SSH configuration may ignore trezor-agent, if it has IdentitiesOnly option set to yes.

 IdentitiesOnly
         Specifies that ssh(1) should only use the authentication identity files configured in
         the ssh_config files, even if ssh-agent(1) or a PKCS11Provider offers more identities.
         The argument to this keyword must be “yes” or “no”.
         This option is intended for situations where ssh-agent offers many different identities.
         The default is “no”.

If you are failing to connect, save your public key using:

$ trezor-agent -vv [email protected] > ~/.ssh/hostname.pub

And add the following lines to ~/.ssh/config (providing the public key explicitly to SSH):

Host hostname.com
	User foobar
	IdentityFile ~/.ssh/hostname.pub

Then, the following commands should successfully command to the remote host:

$ trezor-agent -v [email protected] -s
$ ssh [email protected]

or,

$ trezor-agent -v [email protected] -c