PIV Agent

About

• piv-agent is an SSH and GPG agent providing simple integration of PIV hardware (e.g. a Yubikey) with ssh, and gpg workflows such as git signing, pass encryption, or keybase chat.
• piv-agent originated as a reimplementation of yubikey-agent because I needed some extra features, and also to gain a better understanding of the PIV applet on security key hardware.
• piv-agent makes heavy use of the Go standard library and supplementary crypto packages, as well as piv-go and pcsclite. Thanks for the great software!

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DISCLAIMER

I make no assertion about the security or otherwise of this software and I am not a cryptographer. If you are, please take a look at the code and send PRs or issues. 💚

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Features

• implements (a subset of) both ssh-agent and gpg-agent functionality
• support for multiple hardware security keys
• support for multiple slots in those keys
• support for multiple touch policies
• all cryptographic keys are generated on the hardware security key, rather than on your laptop
  • secret keys never touch your hard drive
• uses systemd (Linux) or launchd (macOS) socket activation
  • as a result, automatically drop the transaction on the security key and cached passphrases after some period of disuse
• provides "fall-back" to traditional SSH and OpenPGP keyfiles

Design philosophy

This agent should require no interaction and in general do the right thing when security keys are plugged/unplugged, laptop is power cycled, etc.

It is highly opinionated:

• Only supports 256-bit ECC keys on hardware tokens
• Only supports ed25519 SSH keys on disk (~/.ssh/id_ed25519)
• Requires socket activation

It makes some concession to practicality with OpenPGP:

• Supports RSA signing and decryption for OpenPGP keyfiles. RSA OpenPGP keys are widespread and Debian in particular only documents RSA keys.

It tries to strike a balance between security and usability:

• Takes a persistent transaction on the hardware token, effectively caching the PIN.
• Caches passphrases for on-disk keys (i.e. ~/.ssh/id_ed25519) in memory, so these only need to be provided once after the agent starts.
• After a period of inactivity (32 minutes by default) it exits, dropping both of these. Socket activation restarts it automatically as required.

Hardware support

Tested with:

• YubiKey 5C, firmware 5.2.4

Will be tested with (once PIV support is available):

• Solo V2

Any device implementing the SCard API (PC/SC), and supported by piv-go / pcsclite may work. If you have tested another device with piv-agent successfully, please send a PR adding it to this list.

Platform support

Currently tested on Linux with systemd and macOS with launchd.

Protocol / Encryption Algorithm support

Supported	Not Supported	Support Blocked (Curve25519)
✅	❌	⏳

Curve25519 algorithms are blocked on hardware support. Currently I'm only aware of Solo V2 which intends to implement this non-standard curve. Support is not yet available (see the link above).

ssh-agent

	Security Key	Keyfile
ecdsa-sha2-nistp256	✅	❌
ssh-ed25519	⏳	✅

gpg-agent

	Security Key	Keyfile
ECDSA Sign (NIST Curve P-256)	✅	✅
EDDSA Sign (Curve25519)	⏳	⏳
ECDH Decrypt	✅	✅
RSA Sign	❌	✅
RSA Decrypt	❌	✅

Install and Use

Please see the documentation.

Develop

Prerequisites

Install build dependencies:

# debian/ubuntu
sudo apt install libpcsclite-dev

Build and test

make

Build and test manually

This D-Bus variable is required for pinentry to use a graphical prompt:

go build ./cmd/piv-agent && systemd-socket-activate -l /tmp/piv-agent.sock -E DBUS_SESSION_BUS_ADDRESS ./piv-agent serve --debug

Then in another terminal:

export SSH_AUTH_SOCK=/tmp/piv-agent.sock
ssh ...

Build and test the documentation

cd docs && make serve