YubiKey plugin for age clients

age-plugin-yubikey is a plugin for age clients like age and rage, which enables files to be encrypted to age identities stored on YubiKeys.

Installation

On Windows, Linux, and macOS, you can use the pre-built binaries.

Help from new packagers is very welcome.

Linux, BSD, etc.

On non-Windows, non-macOS systems, you need to ensure that the pcscd service is installed and running.

When installing via Cargo, you also need to ensure that the development headers for the pcsc-lite library are available, so that the pcsc-sys crate can be compiled.

Windows Subsystem for Linux (WSL)

WSL does not currently provide native support for USB devices. However, Windows binaries installed on the host can be run from inside a WSL environment. This means that you can encrypt or decrypt files inside a WSL environment with a YubiKey:

1. Install age-plugin-yubikey on the Windows host.
2. Install an age client inside the WSL environment.
3. Ensure that age-plugin-yubikey.exe is available in the WSL environment's PATH. For default WSL setups, the Windows host's PATH is automatically added to the WSL environment's PATH (see this Microsoft blog post for more details).

Configuration

age-plugin-yubikey identities have two parts:

• The secret key material, which is stored inside a YubiKey.
• An age identity file, which contains information that an age client can use to figure out which YubiKey secret key should be used.

There are two ways to configure a YubiKey as an age identity. You can run the plugin binary directly to use a simple text interface, which will create an age identity file:

$ age-plugin-yubikey

Or you can use command-line flags to programmatically generate an identity and print it to standard output:

$ age-plugin-yubikey --generate \
    [--serial SERIAL] \
    [--slot SLOT] \
    [--name NAME] \
    [--pin-policy PIN-POLICY] \
    [--touch-policy TOUCH-POLICY]

Once an identity has been created, you can regenerate it later:

$ age-plugin-yubikey --identity [--serial SERIAL] --slot SLOT

To use the identity with an age client, it needs to be stored in a file. When using the above programmatic flags, you can do this by redirecting standard output to a file. On a Unix system like macOS or Ubuntu:

$ age-plugin-yubikey --identity --slot SLOT > yubikey-identity.txt

Usage

The age recipients contained in all connected YubiKeys can be printed on standard output:

$ age-plugin-yubikey --list

To encrypt files to these YubiKey recipients, ensure that age-plugin-yubikey is accessible in your PATH, and then use the recipients with an age client as normal (e.g. rage -r age1yubikey1...).

The output of the --list command can also be used directly to encrypt files to all recipients (e.g. age -R filename.txt).

To decrypt files encrypted to a YubiKey identity, pass the identity file to the age client as normal (e.g. rage -d -i yubikey-identity.txt).

Advanced topics

Agent support

age-plugin-yubikey does not provide or interact with an agent for decryption. It does however attempt to preserve the PIN cache by not soft-resetting the YubiKey after a decryption or read-only operation, which enables YubiKey identities configured with a PIN policy of once to not prompt for the PIN on every decryption. This does not work for YubiKey 4 series.

The session that corresponds to the once policy can be ended in several ways, not all of which are necessarily intuitive:

• Unplugging the YubiKey (the obvious way).
• Using a different applet (e.g. FIDO2). This causes the PIV applet to be closed which clears its state.
  • This is why the YubiKey 4 series does not support PIN cache preservation: their serial can only be obtained by switching to the OTP applet.
• Generating a new age identity via age-plugin-yubikey --generate or the CLI interface. This is to avoid leaving the YubiKey authenticated with the management key.

If the current PIN UX proves to be insufficient, a decryption agent will most likely be implemented as a separate age plugin that interacts with yubikey-agent, enabling YubiKeys to be used simultaneously with age and SSH.

Manual setup and technical details

age-plugin-yubikey only officially supports the following YubiKey variants, set up either via the text interface or the --generate flag:

• YubiKey 4 series
• YubiKey 5 series

NOTE: Nano and USB-C variants of the above are also supported. The pre-YK4 YubiKey NEO series is NOT supported. The blue "Security Key by Yubico" will also not work (as it doesn't support PIV).

In practice, any PIV token with an ECDSA P-256 key and certificate in one of the 20 "retired" slots should work. You can list all age-compatible keys with:

$ age-plugin-yubikey --list-all

age-plugin-yubikey implements several automatic security management features:

• If it detects that the default PIN is being used, it will prompt the user to change the PIN. The PUK is then set to the same value as the PIN.
• If it detects that the default management key is being used, it generates a random management key and stores it in PIN-protected metadata. age-plugin-yubikey does not support custom management keys.

License

Licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.