ssh-keyselect

README

SSH KeySelect

Selective SSH Agent Proxy

SSH KeySelect is an interactive SSH agent proxy that lets you choose which keys an SSH client can offer from your existing agent. Private keys stay in your existing agent, and the proxy rejects signing requests for unselected keys. This helps prevent unintended key use and reduces the risk of reaching an SSH server's MaxAuthTries limit.

[!IMPORTANT] This project is experimental and under active development. Its interfaces, configuration options, and behavior may change as the project evolves.

Installation

Download an archive for Windows, macOS, or Linux from Releases. Each archive includes the standalone ssh-keyselect-gui application and ssh-keyselect for terminal use.

On RHEL-compatible Linux distributions, you can also download an RPM from Releases and install it directly, or configure the repository for DNF-managed installation.

To install a downloaded RPM:

dnf install ./ssh-keyselect*.rpm

To configure the repository, first install dnf-plugin-anyrepo, then run:

rpm --import https://raw.githubusercontent.com/jfut/ssh-keyselect/refs/heads/main/RPM-GPG-KEY-jfut-github
dnf-anyrepo add https://github.com/jfut/ssh-keyselect
dnf install ssh-keyselect

Quick start

GUI: ssh-keyselect-gui

Start the GUI first, then set SSH_AUTH_SOCK to the Listen endpoint shown in its main window. On Linux and macOS, the default endpoint is $HOME/.ssh/ssh-keyselect-agent.sock.

For Git Bash (Git for Windows), macOS, and Linux:

ssh-keyselect-gui &
export SSH_AUTH_SOCK="$HOME/.ssh/ssh-keyselect-agent.sock"
ssh user@example.org

For Windows OpenSSH, set the GUI's Listen mode to Named Pipe. Its default endpoint is \\.\pipe\ssh-keyselect-agent.socket.

For Windows PowerShell:

Start-Process .\ssh-keyselect-gui.exe
$env:SSH_AUTH_SOCK = "\\.\pipe\ssh-keyselect-agent.socket"
ssh user@example.org

For Windows Command Prompt:

start "" ssh-keyselect-gui.exe
set SSH_AUTH_SOCK=\\.\pipe\ssh-keyselect-agent.socket
ssh user@example.org

The GUI can start without an upstream agent. Open Settings from the gear button or File menu to configure the agent endpoints.

If you changed the Windows Listen endpoint in Settings, use the endpoint shown in the GUI instead of the default named pipe above.

The GUI's Auto Select control can temporarily expose all upstream keys without opening the picker. See Auto Select before enabling it.

CLI: ssh-keyselect

Run OpenSSH through a temporary agent proxy. The picker appears in the current terminal, and the temporary endpoint is removed when SSH exits.

ssh-keyselect ssh user@example.org
ssh-keyselect ssh -p 2222 user@example.org

To use the picker for terminal commands named ssh, add an alias:

alias ssh='ssh-keyselect ssh'

Git can use the same command:

GIT_SSH_COMMAND='ssh-keyselect ssh' git clone git@example.org:owner/repo.git
git config core.sshCommand 'ssh-keyselect ssh'

For scp and sftp, use an executable wrapper that runs ssh-keyselect ssh, then pass its path with -S.

Overview

SSH connections from your machine

Many SSH clients that support the SSH_AUTH_SOCK environment variable use it to request identities and signatures from SSH KeySelect when connecting to an SSH server. This includes direct use of ssh and OpenSSH launched by git, rsync, or the VS Code Remote - SSH extension.

sequenceDiagram
  participant Client as SSH client
  participant KeySelect as SSH KeySelect
  participant Agent as Existing SSH Agent
  participant Picker as TUI or GUI picker

  Client->>KeySelect: 1. Request identities via SSH_AUTH_SOCK
  KeySelect->>Agent: 2. List available identities
  Agent-->>KeySelect: Reply to 2: available identities
  KeySelect->>Picker: 3. Show request context and identities
  Picker-->>KeySelect: 4. Select identity or cancel
  KeySelect-->>Client: Reply to 1: selected identity or an empty list
  Client->>KeySelect: 5. Request signature for selected identity
  KeySelect->>Agent: 6. Sign with selected identity
  Agent-->>KeySelect: Reply to 6: signature
  KeySelect-->>Client: Reply to 5: signature

OpenSSH uses the returned signature to authenticate to the SSH server.

Through agent forwarding

With agent forwarding and Auto Select Off, a remote process such as Git or a nested SSH client uses its forwarded SSH_AUTH_SOCK. Its agent requests travel through the SSH connection to the local SSH client, then to SSH KeySelect.

sequenceDiagram
  participant Remote as Remote process
  participant Socket as Forwarded SSH_AUTH_SOCK
  participant Client as Local SSH client
  participant KeySelect as SSH KeySelect
  participant Picker as TUI or GUI picker
  participant Agent as Existing SSH Agent

  Remote->>Socket: 1. Request identities
  Socket->>Client: 2. Forward request over the SSH channel
  Client->>KeySelect: 3. Send request to local SSH_AUTH_SOCK
  KeySelect->>Agent: 4. List available identities
  Agent-->>KeySelect: Reply to 4: available identities
  KeySelect->>Picker: 5. Show request context and identities
  Picker-->>KeySelect: 6. Select identity or cancel
  KeySelect-->>Client: Reply to 3: selected identity or an empty list
  Client-->>Socket: Relay identity response over the SSH channel
  Socket-->>Remote: Return selected identity

Later signing requests and signature responses follow the same route. KeySelect forwards signing for the selected identity to the local agent; private keys stay on the local machine.

SSH agent forwarding keeps private keys on your machine, but lets a remote host request signatures from your agent. A malicious or compromised host can use forwarded keys to access SSH services and Git hosts reachable with your credentials, including reading or pushing to repositories. Forward your agent only to hosts you trust. See the OpenSSH manual for its warning about agent forwarding.

Shared behavior

Endpoint modes

--upstream-mode controls how KeySelect connects to the existing agent. --listen-mode controls how the SSH client connects to the proxy.

Mode	Endpoint
auto	Detect a compatible endpoint for the current shell and path.
unix	Unix-domain socket or Windows AF_UNIX socket.
cygwin	Cygwin and Git for Windows socket files.
wsl1	WSL1-compatible socket paths on Windows.
named-pipe	Named Pipe (Windows OpenSSH), such as \\.\pipe\openssh-ssh-agent.

Both modes default to auto in the CLI and configuration. GUI Settings resolves an automatic listener mode to a concrete mode for display. Set a mode explicitly when connecting across shells or when automatic detection does not match the endpoint.

Private key selection

The picker lets you choose a key available through the agent. The SSH agent protocol calls its selectable entry an identity: the public key and comment that identify the key the agent can use to sign. KeySelect forwards signing requests for the selected key to the upstream agent and never receives the private key material.

The picker shows each key's comment, key type, size in bits, and SHA256 fingerprint. Selection is scoped to an SSH session-binding chain:

• With Auto Select Off, a new connection starts a fresh selection. A forwarding hop that follows an authentication binding starts a new selection.
• The selected keys are cached for that chain. Signing requests for unselected keys are rejected.
• Canceling returns no keys to the SSH client.

See the developer notes for the request and connection flow.

Request context

Both pickers show request context in selectable, copyable text. For accepted session-bind@openssh.com messages, the context includes verified host key types and fingerprints, plus whether each connection is marked as forwarded. Forwarding hops and the current target are grouped in a tree so connection boundaries are clear.

The agent protocol does not provide a hostname. Names matched from the default local known_hosts files are hints, not verified hostnames. If no session binding is available or accepted, the picker says that no verified host key was provided. Press Esc to cancel a request you do not recognize.

Security and limitations

• The proxy supports listing identities, signing with selected identities, and verified OpenSSH session bindings. Key-management and unknown agent requests are always rejected.
• The proxy never stores private keys or signs data. It forwards signing requests for permitted identities to the upstream agent.
• The proxy cannot select keys based on a destination. Identity-listing and signing requests do not include the host, user, or port. A session binding can provide a signed host key, but not a hostname, user, or port.
• The proxy can only filter identities provided by the upstream agent.
• On Linux and macOS, Unix socket files use mode 0600, and a created parent directory uses mode 0700.
• Cygwin-compatible socket files use loopback TCP with a token stored in the socket file. Keep the file in a directory accessible only to your account.

CLI

Commands

Command	Purpose
ssh-keyselect ssh [options] [--] [SSH arguments...]	Run OpenSSH through the terminal picker.
ssh-keyselect list [options]	Print identities from the upstream agent.
ssh-keyselect test [options]	Check the upstream agent and summarize its identities.
ssh-keyselect version	Print the version and commit.
ssh-keyselect --help or ssh-keyselect <command> --help	Show commands and options.

Options and environment

The wrapper options go before --; arguments after it are passed to OpenSSH.

Command	Options
ssh-keyselect ssh	--upstream, --upstream-mode, --listen, --listen-mode, --log-level, --log-file
ssh-keyselect list and test	--upstream, --upstream-mode

The terminal commands ssh, list, and test do not read TOML configuration files. Unless --upstream is set, they use the first non-empty value from UPSTREAM_SSH_AUTH_SOCK, then SSH_AUTH_SOCK. Paths support environment-variable expansion and a leading ~.

Logging

Logging is off by default. Pass --log-level to use debug, info, warn, or error.

For an SSH session, pass --log-file FILE to write KeySelect diagnostic logs to a file instead of the terminal:

ssh-keyselect ssh --log-level debug --log-file ~/ssh-keyselect.log -- user@example.org

The log file is replaced on each run and uses mode 0600 on Unix-like systems. OpenSSH output remains in the terminal. Review logs before sharing them; they can include SSH host key fingerprints and request metadata.

Terminal picker

The terminal shows a blank line, [ssh-keyselect - YYYY-MM-DD HH:MM:SS TZ], request context, and a cyan Select an SSH key to allow: prompt above the key table. The empty filter line shows a gray Filter (If you don't recognize this request, press Esc to cancel.) hint with the cursor at its start; the hint disappears when you type. Host details are indented beneath their tree labels.

Key	Action
Type	Filter the list.
Enter	Select the highlighted identity.
Esc or Ctrl+C	Cancel.
Backspace or Delete	Edit the filter.
Ctrl+U	Clear the filter.

GUI

Linux runtime requirements

The Linux GUI requires X11 and OpenGL runtime libraries. Wayland sessions use XWayland.

Command options

Run ssh-keyselect-gui to start the GUI. Its options are:

Command	Options
ssh-keyselect-gui	--config, --listen, --listen-mode, --upstream, --upstream-mode, --log-level

Configuration

The GUI reads TOML from $XDG_CONFIG_HOME/ssh-keyselect/config.toml when XDG_CONFIG_HOME is set. Otherwise, it uses the platform's user configuration directory:

Platform	Default path
Windows	%APPDATA%\ssh-keyselect\config.toml (usually C:\Users\<user>\AppData\Roaming\ssh-keyselect\config.toml)
Linux	~/.config/ssh-keyselect/config.toml
macOS	~/Library/Application Support/ssh-keyselect/config.toml

Use --config FILE to select another file. Command-line values override file values. The following environment variables also affect endpoint paths:

• SSH_KEYSELECT_LISTEN overrides agent.listen.
• When agent.upstream is empty, UPSTREAM_SSH_AUTH_SOCK takes precedence over SSH_AUTH_SOCK.
• Environment variables and a leading ~ are expanded in configured paths using the GUI process environment.
• If a filesystem entry already exists at the configured Listen path when the GUI starts, the proxy starts as Not configured and leaves that entry untouched.

Remove the obsolete [security] section from older files. The configuration loader rejects unknown keys. Logging is off by default; set log.level in the TOML file or pass --log-level to enable it.

Example configuration for Git Bash on Windows, using a Cygwin-compatible listener:

[agent]
# Replace alice with your Windows user name:
upstream = 'C:\Users\alice\.ssh\ssh-upstream-agent.sock'
listen = 'C:\Users\alice\.ssh\ssh-keyselect-agent.sock'

# Linux/macOS:
# upstream = "$SSH_AUTH_SOCK"
# listen = "$HOME/.ssh/ssh-keyselect-agent.sock"

upstream_mode = "auto"
listen_mode = "cygwin"

[log]
level = "off"

Cross-shell setup

When using Git Bash with endpoints from different environments, set the modes explicitly:

ssh-keyselect-gui --upstream-mode cygwin --listen-mode cygwin --upstream ~/.ssh/ssh-upstream-agent.sock &
export SSH_AUTH_SOCK="$HOME/.ssh/ssh-keyselect-agent.sock"
ssh user@example.org

GUI controls

• Open Settings from the gear button or File > Settings. Apply activates endpoint changes immediately; use File > Save to store them.
• The File menu opens, saves, and saves as TOML configuration files. The GUI prompts before closing with unsaved changes.
• About shows the subtitle Selective SSH Agent Proxy, project URL, author Jun Futagawa (jfut), and third-party libraries and licenses in selectable text. Select text and press Ctrl+C (Command+C on macOS) to copy it.
• Release archives include full license notices in CREDITS.

Auto Select

Auto Select temporarily bypasses the per-connection picker. When On, the proxy returns every upstream identity and forwards signing requests for any of them. Any client that can access the proxy can then use every key in the upstream agent.

The GUI has Off and On controls beside Agent Proxy. On is red and requires confirmation. Use it only for trusted work, then turn it Off. The setting is temporary, is not saved in TOML, and applies to new identity requests. A client that already received all identities while Auto Select was On keeps that selection until its connection closes.

GUI picker

The window title combines the action, application name, and local display time. The request-context area begins with the same timestamp followed by host details. It lets you scroll to earlier hops. Use the arrow keys to move through matches, Enter to select, and Esc to cancel.

The picker and main-window key table support copying a key. Right-click a key and choose Copy, or press Ctrl+C (Command+C on macOS), to copy its Comment, Type, Size, and Fingerprint as tab-separated text.

License

Apache-2.0

Copyright contributors to the ssh-keyselect project.

Author

Jun Futagawa (jfut)