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A Go YubiKey PIV implementation

YubiKeys implement the PIV specification for managing smart card certificates. This applet is a simpler alternative to GPG for managing asymmetric keys on a YubiKey.

This package is an alternative to Paul Tagliamonte's go-ykpiv, a wrapper for YubiKey's ykpiv.h C library. This package aims to provide:

Better error messages
Idiomatic Go APIs
Modern features such as PIN protected management keys

Signing

The piv-go package can be used to generate keys and store certificates on a YubiKey. This uses a management key to generate new keys on the applet, and a PIN for signing operations. The package provides default PIN values. If the PIV credentials on the YubiKey haven't been modified, the follow code generates a new EC key on the smartcard, and provides a signing interface:

// List all smartcards connected to the system.
cards, err := piv.Cards()
if err != nil {
	// ...
}

// Find a YubiKey and open the reader.
var yk *piv.YubiKey
for _, card := range cards {
	if strings.Contains(strings.ToLower(card), "yubikey") {
		if yk, err = piv.Open(card); err != nil {
			// ...
		}
		break
	}
}
if yk == nil {
	// ...
}

// Generate a private key on the YubiKey.
key := piv.Key{
	Algorithm:   piv.AlgorithmEC256,
	PINPolicy:   piv.PINPolicyAlways,
	TouchPolicy: piv.TouchPolicyAlways,
}
pub, err := yk.GenerateKey(piv.DefaultManagementKey, piv.SlotAuthentication, key)
if err != nil {
	// ...
}

auth := piv.KeyAuth{PIN: piv.DefaultPIN}
priv, err := yk.PrivateKey(piv.SlotAuthentication, pub, auth)
if err != nil {
	// ...
}
// Use private key to sign or decrypt.

PINs

The PIV applet has three unique credentials:

Management key (3DES key) used to generate new keys on the YubiKey.
PIN (up to 8 digits, usually 6) used to access signing operations.
PUK (up to 8 digits) used to unblock the PIN. Usually set once and thrown away or managed by an administrator.

piv-go implements PIN protected management keys to store the management key on the YubiKey. This allows users to only provide a PIN and still access management capabilities.

The following code generates new, random credentials for a YubiKey:

newPINInt, err := rand.Int(rand.Reader, big.NewInt(1_000_000))
if err != nil {
	// ...
}
newPUKInt, err := rand.Int(rand.Reader, big.NewInt(100_000_000))
if err != nil {
	// ...
}
var newKey [24]byte
if _, err := io.ReadFull(rand.Reader, newKey[:]); err != nil {
	// ...
}
// Format with leading zeros.
newPIN := fmt.Sprintf("%06d", newPINInt)
newPUK := fmt.Sprintf("%08d", newPUKInt)

// Set all values to a new value.
if err := yk.SetManagementKey(piv.DefaultManagementKey, newKey); err != nil {
	// ...
}
if err := yk.SetPUK(piv.DefaultPUK, newPUK); err != nil {
	// ...
}
if err := yk.SetPIN(piv.DefaultPIN, newPIN); err != nil {
	// ...
}
// Store management key on the YubiKey.
m := piv.Metadata{ManagementKey: &newKey}
if err := yk.SetMetadata(newKey, m); err != nil {
	// ...
}

fmt.Println("Credentials set. Your PIN is: %s", newPIN)

The user can user the PIN later to fetch the management key:

m, err := yk.Metadata(pin)
if err != nil {
	// ...
}
if m.ManagementKey == nil {
	// ...
}
key := *m.ManagementKey

Certificates

The PIV applet can also store X.509 certificates on the YubiKey:

cert, err := x509.ParseCertificate(certDER)
if err != nil {
	// ...
}
if err := yk.SetCertificate(managementKey, piv.SlotAuthentication, cert); err != nil {
	// ...
}

The certificate can later be used in combination with the private key. For example, to serve TLS traffic:

cert, err := yk.Certificate(piv.SlotAuthentication)
if err != nil {
	// ...
}
priv, err := yk.PrivateKey(piv.SlotAuthentication, cert.PublicKey, auth)
if err != nil {
	// ...
}
s := &http.Server{
	TLSConfig: &tls.Config{
		Certificates: []tls.Certificate{
			{
				Certificate: [][]byte{cert.Raw},
				PrivateKey:  priv,
			},
		},
	},
	Handler: myHandler,
}

Attestation

YubiKeys can attest that a particular key was generated on the smartcard, and that it was set with specific PIN and touch policies. The client generates a key, then asks the YubiKey to sign an attestation certificate:

// Get the YubiKey's attestation certificate, which is signed by Yubico.
yubiKeyAttestationCert, err := yk.AttestationCertificate()
if err != nil {
	// ...
}

// Generate a key on the YubiKey and generate an attestation certificate for
// that key. This will be signed by the YubiKey's attestation certificate.
key := piv.Key{
	Algorithm:   piv.AlgorithmEC256,
	PINPolicy:   piv.PINPolicyAlways,
	TouchPolicy: piv.TouchPolicyAlways,
}
if _, err := yk.GenerateKey(managementKey, piv.SlotAuthentication, key); err != nil {
	// ...
}
slotAttestationCertificate, err := yk.Attest(piv.SlotAuthentication)
if err != nil {
	// ...
}

// Send certificates to server.

A CA can then verify the attestation, proving a key was generated on the card and enforce policy:

// Server receives both certificates, then proves a key was generated on the
// YubiKey.
a, err := piv.Verify(yubiKeyAttestationCert, slotAttestationCertificate)
if err != nil {
	// ...
}
if a.TouchPolicy != piv.TouchPolicyAlways {
	// ...
}

// Record YubiKey's serial number and public key.
pub := slotAttestationCertificate.PublicKey
serial := a.Serial

Installation

On MacOS, piv-go doesn't require any additional packages.

To build on Linux, piv-go requires PCSC lite. To install on Debian-based distros, run:

sudo apt-get install libpcsclite-dev

On Fedora:

sudo yum install pcsc-lite-devel

On CentOS:

sudo yum install 'dnf-command(config-manager)'
sudo yum config-manager --set-enabled PowerTools
sudo yum install pcsc-lite-devel

On FreeBSD:

sudo pkg install pcsc-lite

On Windows:

No prerequisites are needed. The default driver by Microsoft supports all functionalities which get tested by unittests. However if you run into problems try the official YubiKey Smart Card Minidriver. Yubico states on their website the driver adds additional smart functionality.

Please notice the following:

Windows support is best effort due to lack of test hardware. This means the maintainers will take patches for Windows, but if you encounter a bug or the build is broken, you may be asked to fix it.

Testing

Tests automatically find connected available YubiKeys, but won't modify the smart card without the --wipe-yubikey flag. To let the tests modify your YubiKey's PIV applet, run:

go test -v ./piv --wipe-yubikey

Longer tests can be skipped with the --test.short flag.

go test -v --short ./piv --wipe-yubikey

Why?

YubiKey's C PIV library, ykpiv, is brittle. The error messages aren't terrific, and while it has debug options, plumbing them through isn't idiomatic or convenient.

ykpiv wraps PC/SC APIs available on Windows, Mac, and Linux. There's no requirement for it to be written in any particular langauge. As an alternative to pault.ag/go/ykpiv this package re-implements ykpiv in Go instead of calling it.

Alternatives

OpenSSH has experimental support for U2F keys (announcement) that directly use browser U2F challenges for smart cards.