YubiKey-Guide/README.md

This is a guide to using [YubiKey](https://www.yubico.com/products/) as a [smart card](https://security.stackexchange.com/questions/38924/how-does-storing-gpg-ssh-private-keys-on-smart-cards-compare-to-plain-usb-drives) for cryptographic encryption, signing and authentication operations.

Keys stored on YubiKey are [non-exportable](https://web.archive.org/web/20201125172759/https://support.yubico.com/hc/en-us/articles/360016614880-Can-I-Duplicate-or-Back-Up-a-YubiKey-), unlike filesystem-based credentials, while remaining convenient for daily use. Instead of having to remember and enter complicated passphrases to unlock SSH/GnuPG keys, YubiKey needs only a physical touch after being unlocked with a PIN.

**Important** If you followed this guide before Jan 2021, *PIN* and *Admin PIN* may be set to default values of `123456` and `12345678`. See [Change PIN](#change-pin) to change PINs.

To suggest an improvement, please send a pull request or open an [issue](https://github.com/drduh/YubiKey-Guide/issues).

**Tip** [drduh/Purse](https://github.com/drduh/Purse) is a password manager which uses GnuPG and YubiKey to securely store and use credentials.

- [Purchase](#purchase)
- [Prepare environment](#prepare-environment)
- [Required software](#required-software)
   * [Debian and Ubuntu](#debian-and-ubuntu)
   * [Fedora](#fedora)
   * [Arch](#arch)
   * [RHEL7](#rhel7)
   * [NixOS](#nixos)
   * [OpenBSD](#openbsd)
   * [macOS](#macos)
   * [Windows](#windows)
- [Entropy](#entropy)
   * [YubiKey](#yubikey)
   * [OneRNG](#onerng)
- [Generate keys](#generate-keys)
   * [Temporary working directory](#temporary-working-directory)
   * [Harden configuration](#harden-configuration)
- [Certify key](#certify-key)
- [Sign with existing key](#sign-with-existing-key)
- [Subkeys](#subkeys)
   * [Signing](#signing)
   * [Encryption](#encryption)
   * [Authentication](#authentication)
   * [Add extra identities](#add-extra-identities)
- [Verify](#verify)
- [Export secret keys](#export-secret-keys)
- [Revocation certificate](#revocation-certificate)
- [Backup](#backup)
- [Export public keys](#export-public-keys)
- [Configure Smartcard](#configure-smartcard)
   * [Enable KDF](#enable-kdf)
   * [Change PIN](#change-pin)
   * [Set information](#set-information)
- [Transfer keys](#transfer-keys)
   * [Signing](#signing-1)
   * [Encryption](#encryption-1)
   * [Authentication](#authentication-1)
- [Verify card](#verify-card)
- [Multiple YubiKeys](#multiple-yubikeys)
   * [Switching between YubiKeys](#switching-between-yubikeys)
- [Multiple Hosts](#multiple-hosts)
- [Finish](#finish)
- [Using keys](#using-keys)
- [Rotating keys](#rotating-keys)
   * [Setup environment](#setup-environment)
   * [Renewing Subkeys](#renewing-subkeys)
   * [Rotating keys](#rotating-keys-1)
- [Adding notations](#adding-notations)
- [SSH](#ssh)
   * [Create configuration](#create-configuration)
   * [Replace agents](#replace-agents)
   * [Copy public key](#copy-public-key)
   * [(Optional) Save public key for identity file configuration](#optional-save-public-key-for-identity-file-configuration)
   * [Connect with public key authentication](#connect-with-public-key-authentication)
   * [Import SSH keys](#import-ssh-keys)
   * [Remote Machines (SSH Agent Forwarding)](#remote-machines-ssh-agent-forwarding)
      + [Use ssh-agent ](#use-ssh-agent)
      + [Use S.gpg-agent.ssh](#use-sgpg-agentssh)
      + [Chained SSH Agent Forwarding](#chained-ssh-agent-forwarding)
   * [GitHub](#github)
   * [OpenBSD](#openbsd-1)
   * [Windows](#windows-1)
      + [WSL](#wsl)
         - [Use ssh-agent or use S.weasel-pageant](#use-ssh-agent-or-use-sweasel-pageant)
         - [Prerequisites](#prerequisites)
         - [WSL configuration](#wsl-configuration)
         - [Remote host configuration](#remote-host-configuration)
   * [macOS](#macos-1)
- [Remote Machines (GPG Agent Forwarding)](#remote-machines-gpg-agent-forwarding)
   * [Steps for older distributions](#steps-for-older-distributions)
   * [Chained GPG Agent Forwarding](#chained-gpg-agent-forwarding)
- [Using Multiple Keys](#using-multiple-keys)
- [Adding an identity](#adding-an-identity)
   * [Updating YubiKey](#updating-yubikey)
- [Require touch](#require-touch)
- [Email](#email)
   * [Mailvelope](#mailvelope)
   * [Mutt](#mutt)
- [Reset](#reset)
   * [Recovery after reset](#recovery-after-reset)
- [Notes](#notes)
- [Troubleshooting](#troubleshooting)
- [Alternatives](#alternatives)
   * [Create keys with batch](#create-keys-with-batch)
- [Additional resources](#additional-resources)

# Purchase

All YubiKeys except the blue "security key" model and the "Bio Series - FIDO Edition" are compatible with this guide. NEO models are limited to 2048-bit RSA keys. Compare YubiKeys [here](https://www.yubico.com/products/yubikey-hardware/compare-products-series/). A list of the YubiKeys compatible with OpenPGP is available [here](https://support.yubico.com/hc/en-us/articles/360013790259-Using-Your-YubiKey-with-OpenPGP). In May 2021, Yubico also released a press release and blog post about supporting resident ssh keys on their YubiKeys including blue "security key 5 NFC" with OpenSSH 8.2 or later, see [here](https://www.yubico.com/blog/github-now-supports-ssh-security-keys/) for details.

To verify a YubiKey is genuine, open a [browser with U2F support](https://support.yubico.com/hc/en-us/articles/360013723419-How-to-Confirm-Your-Yubico-Device-is-Genuine) to [https://www.yubico.com/genuine/](https://www.yubico.com/genuine/). Insert a Yubico device, and select *Verify Device* to begin the process. Touch the YubiKey when prompted, and if asked, allow it to see the make and model of the device. If you see *Verification complete*, the device is authentic.

This website verifies YubiKey device attestation certificates signed by a set of Yubico certificate authorities, and helps mitigate [supply chain attacks](https://media.defcon.org/DEF%20CON%2025/DEF%20CON%2025%20presentations/DEF%20CON%2025%20-%20r00killah-and-securelyfitz-Secure-Tokin-and-Doobiekeys.pdf).

You will also need several portable storage devices (microSD cards work well) for storing encrypted backups of your keys.

# Prepare environment

To create cryptographic keys, a secure environment that can be reasonably assured to be free of adversarial control is recommended.

The following is a general ranking of environments most to least likely to be compromised:

1. Daily-use system with full Internet access
1. Virtual machine on daily-use host OS (using [virt-manager](https://virt-manager.org/), VirtualBox or VMware)
1. Separate hardened [Debian](https://www.debian.org/) or [OpenBSD](https://www.openbsd.org/) installation which can be dual booted
1. Live image, such as [Debian Live](https://www.debian.org/CD/live/) or [Tails](https://tails.boum.org/index.en.html)
1. Secure hardware/firmware ([Coreboot](https://www.coreboot.org/), [Intel ME removed](https://github.com/corna/me_cleaner))
1. Dedicated air-gapped system without networking capabilities (ARM-based Raspberry Pi or other architecturally diverse equivalent)

A Debian Linux live image is recommended to balance usability and security.

Download the latest image and signature files:

```console
curl -fLO "https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/SHA512SUMS"

curl -fLO "https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/SHA512SUMS.sign"

curl -fLO "https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/$(awk '/xfce.iso$/ {print $2}' SHA512SUMS)"
```

Obtain the Debian signing key:

```console
gpg --keyserver hkps://keyring.debian.org --recv DF9B9C49EAA9298432589D76DA87E80D6294BE9B
```

If the public key cannot be received, use a different keyserver or DNS server:

```console
gpg --keyserver hkps://keyserver.ubuntu.com:443 --recv DF9B9C49EAA9298432589D76DA87E80D6294BE9B
```

Verify the signature:

```console
gpg --verify SHA512SUMS.sign SHA512SUMS
```

`gpg: Good signature from "Debian CD signing key <debian-cd@lists.debian.org>"` must appear in the output.

Ensure the cryptographic hash of the image file matches the one in the signed file:

```console
grep $(sha512sum debian-live-*-amd64-xfce.iso) SHA512SUMS
```

See [Verifying authenticity of Debian CDs](https://www.debian.org/CD/verify) for more information.

Mount a portable storage device and copy the image:

**Linux**

```console
$ sudo dmesg | tail
usb-storage 3-2:1.0: USB Mass Storage device detected
scsi host2: usb-storage 3-2:1.0
scsi 2:0:0:0: Direct-Access     TS-RDF5  SD  Transcend    TS3A PQ: 0 ANSI: 6
sd 2:0:0:0: Attached scsi generic sg1 type 0
sd 2:0:0:0: [sdb] 31116288 512-byte logical blocks: (15.9 GB/14.8 GiB)
sd 2:0:0:0: [sdb] Write Protect is off
sd 2:0:0:0: [sdb] Mode Sense: 23 00 00 00
sd 2:0:0:0: [sdb] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
sdb: sdb1 sdb2
sd 2:0:0:0: [sdb] Attached SCSI removable disk

$ sudo dd if=debian-live-*-amd64-xfce.iso of=/dev/sdb bs=4M status=progress ; sync
465+1 records in
465+1 records out
1951432704 bytes (2.0 GB, 1.8 GiB) copied, 42.8543 s, 45.5 MB/s
```

**OpenBSD**

```console
$ dmesg | tail -n2
sd2 at scsibus4 targ 1 lun 0: <TS-RDF5, SD Transcend, TS3A> SCSI4 0/direct removable serial.0000000000000
sd2: 15193MB, 512 bytes/sector, 31116288 sectors

$ doas dd if=debian-live-*-amd64-xfce.iso of=/dev/rsd2c bs=4m
465+1 records in
465+1 records out
1951432704 bytes transferred in 139.125 secs (14026448 bytes/sec)
```

Power off, then disconnect internal hard drives and all unnecessary devices, such as the wireless card.

# Required software

Boot the live image and configure networking.

**Note** If the screen locks, unlock with `user` / `live`

Open terminal and install required software packages.

## Debian and Ubuntu

```console
sudo apt update

sudo apt -y upgrade

sudo apt -y install \
    wget gnupg2 gnupg-agent dirmngr \
    cryptsetup scdaemon pcscd secure-delete \
    yubikey-personalization
```

**Note** `hopenpgp-tools` is no longer part of the latest Debian stable package repositories. To install it, go to [https://packages.debian.org/sid/hopenpgp-tools](https://packages.debian.org/sid/hopenpgp-tools) to select your architecture (likely `amd64`) and then an ftp server.

Edit `/etc/apt/sources.list` and add the ftp server:

```
deb http://ftp.debian.org/debian sid main
```

Then add this to `/etc/apt/preferences` (or a fragment, e.g. `/etc/apt/preferences.d/00-sid`) so that APT still prioritizes packages from the stable repository over sid.

```
Package: *
Pin: release n=sid
Pin-Priority: 10
```

**Note** Live Ubuntu images [may require modification](https://github.com/drduh/YubiKey-Guide/issues/116) to `/etc/apt/sources.list` and may need additional packages:

```console
sudo apt -y install libssl-dev swig libpcsclite-dev
```

**Optional** Install the `ykman` utility, which will allow you to enable touch policies (requires admin PIN):

```console
sudo apt -y install python3-pip python3-pyscard

pip3 install PyOpenSSL

pip3 install yubikey-manager

sudo service pcscd start

~/.local/bin/ykman openpgp info
```

**Note** Debian 12 does not recommend installing non-Debian packaged Python applications globally. But fortunately, it is not necessary as `yubikey-manager` is available in the stable main repository:

```console
sudo apt install yubikey-manager
```

## Fedora

```console
sudo dnf install wget

wget https://github.com/rpmsphere/noarch/raw/master/r/rpmsphere-release-38-1.noarch.rpm

sudo rpm -Uvh rpmsphere-release*rpm

sudo dnf install \
    gnupg2 dirmngr cryptsetup gnupg2-smime \
    pcsc-tools opensc pcsc-lite secure-delete \
    pgp-tools yubikey-personalization-gui
```

## Arch

```console
sudo pacman -Syu gnupg pcsclite ccid hopenpgp-tools yubikey-personalization
```

## RHEL7

```console
sudo yum install -y gnupg2 pinentry-curses pcsc-lite pcsc-lite-libs gnupg2-smime
```

## NixOS

Build an air-gapped NixOS LiveCD image:

```console
ref=$(git ls-remote https://github.com/drduh/Yubikey-Guide refs/heads/master | awk '{print $1}')

nix build --experimental-features "nix-command flakes" github:drduh/YubiKey-Guide/$ref#nixosConfigurations.yubikeyLive.x86_64-linux.config.system.build.isoImage
```

If you have this repository checked out:

Recommended, but optional: update `nixpkgs` and `drduh/config`:

```console
nix flake update --commit-lock-file
```

Build the ISO:

```console
nix build --experimental-features "nix-command flakes" .#nixosConfigurations.yubikeyLive.x86_64-linux.config.system.build.isoImage
```

Copy it to a USB drive:

```console
sudo cp -v result/iso/yubikeyLive.iso /dev/sdb; sync
```

With this image, you won't need to create a [temporary working directory](#temporary-working-directory) or [harden the configuration](#harden-configuration), as it was done when creating the image.

## OpenBSD

```console
doas pkg_add gnupg pcsc-tools
```

## macOS

Download and install [Homebrew](https://brew.sh/) and the following packages:

```console
brew install gnupg yubikey-personalization hopenpgp-tools ykman pinentry-mac wget
```

**Note** An additional Python package dependency may need to be installed to use [`ykman`](https://support.yubico.com/support/solutions/articles/15000012643-yubikey-manager-cli-ykman-user-guide) - `pip install yubikey-manager`

## Windows

Download and install [Gpg4Win](https://www.gpg4win.org/) and [PuTTY](https://putty.org).

You may also need more recent versions of [yubikey-personalization](https://developers.yubico.com/yubikey-personalization/Releases/) and [yubico-c](https://developers.yubico.com/yubico-c/Releases/).

# Entropy

Generating cryptographic keys requires high-quality [randomness](https://www.random.org/randomness/), measured as entropy.

Most operating systems use software-based pseudorandom number generators or CPU-based hardware random number generators (HRNG).

**Optional** A hardware device such as [OneRNG](https://onerng.info/onerng/) may be used to [increase the speed](https://lwn.net/Articles/648550/) and possibly the quality of available entropy.

## YubiKey

YubiKey firmware version 5.2.3 introduced "Enhancements to OpenPGP 3.4 Support" - which can gather additional entropy from YubiKey via the SmartCard interface.

To seed the kernel's PRNG with additional 512 bytes retrieved from the YubiKey:

```console
echo "SCD RANDOM 512" | gpg-connect-agent | sudo tee /dev/random | hexdump -C
```

## OneRNG

Configure [rng-tools](https://wiki.archlinux.org/index.php/Rng-tools) software:

```console
sudo apt -y install at rng-tools python3-gnupg openssl

wget https://github.com/OneRNG/onerng.github.io/raw/master/sw/onerng_3.7-1_all.deb
```

Verify the package:

```console
$ sha256sum onerng_3.7-1_all.deb
b7cda2fe07dce219a95dfeabeb5ee0f662f64ba1474f6b9dddacc3e8734d8f57  onerng_3.7-1_all.deb
```

Install the package:

```console
sudo dpkg -i onerng_3.7-1_all.deb

echo "HRNGDEVICE=/dev/ttyACM0" | sudo tee /etc/default/rng-tools
```

Plug in the device and restart rng-tools:

```console
sudo atd

sudo service rng-tools restart
```

# Generate keys

## Temporary working directory

Create a temporary directory which will be cleared on [reboot](https://en.wikipedia.org/wiki/Tmpfs) and set it as the GnuPG directory:

```console
export GNUPGHOME=$(mktemp -d -t gnupg_$(date +%Y%m%d%H%M)_XXX)
```

## Harden configuration

Import or create a hardened configuration for GnuPG:

```console
wget -O $GNUPGHOME/gpg.conf https://raw.githubusercontent.com/drduh/config/master/gpg.conf
```

The options will look similar to:

```console
$ grep -ve "^#" $GNUPGHOME/gpg.conf
personal-cipher-preferences AES256 AES192 AES
personal-digest-preferences SHA512 SHA384 SHA256
personal-compress-preferences ZLIB BZIP2 ZIP Uncompressed
default-preference-list SHA512 SHA384 SHA256 AES256 AES192 AES ZLIB BZIP2 ZIP Uncompressed
cert-digest-algo SHA512
s2k-digest-algo SHA512
s2k-cipher-algo AES256
charset utf-8
fixed-list-mode
no-comments
no-emit-version
no-greeting
keyid-format 0xlong
list-options show-uid-validity
verify-options show-uid-validity
with-fingerprint
require-cross-certification
no-symkey-cache
use-agent
throw-keyids
```

**Tip** Networking can be disabled for the remainder of the setup.

# Certify key

The primary key to generate is the Certify key, which will be used to issue Subkeys for Encrypt, Sign and Authenticate operations.

**Important** The Certify key should be kept offline at all times and only accessed from a secure environment to revoke or issue new Subkeys. Keys can also be generated on the YubiKey itself to ensure copies do not exist, however for usability and durability reasons this guide recommends against doing so.

Generate a passphrase which will be needed throughout the guide to create and export Subkeys.

**Important** The passphrase should be memorized or written down in a secure place separate from the backup storage disk.

The passphrase should consist of only upper case letters and numbers for improved readability.

The following command should generate strong passphrases while avoiding ambiguous characters:

```console
LC_ALL=C tr -dc 'A-Z1-9' < /dev/urandom | \
    tr -d "1IOS5U" | fold -w 30 | head -n10 | \
    sed "-es/./ /"{1..26..5} | cut -c2- | tr " " "-"
```

Such as:

```console
A4ZK-YRRJ-8WPM-82NY-CX9T-AGKT
PH9Z-HFDX-QDB9-YMMC-GQZB-Z3EV
EC3H-C42G-8E9K-VF7F-ZWT7-BTL6
B3CA-QCCE-JMNE-VAZG-ZEYD-J3XP
YKP4-M42X-4WWE-WEKR-C3J7-GZYF
ZQWC-E7MN-M7CT-4Y4Z-9QFV-44VY
KY4F-C83Q-BTYQ-V8EM-WGCR-DPZN
GYWQ-WNAC-ERWM-XGAD-6XVD-ZCLD
L8JL-EK8H-Z4ZF-MA93-NND8-FPKA
WM2J-XF7L-QV6D-AWLY-Y2D8-4TQQ
```

**Tip** On Linux or OpenBSD, select the password using the mouse or by double-clicking on it to copy to clipboard. Paste using the middle mouse button or `Shift`-`Insert`

Generate a new key with GnuPG, selecting `(8) RSA (set your own capabilities)`, `Certify` capability only and `4096` bit key size.

Do **not** set the Certify key to expire - see [Note #3](#notes).

```console
$ gpg --expert --full-generate-key
Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
   (9) ECC and ECC
  (10) ECC (sign only)
  (11) ECC (set your own capabilities)
  (13) Existing key
  (14) Existing key from card
Your selection? 8

Possible actions for a RSA key: Sign Certify Encrypt Authenticate
Current allowed actions: Sign Certify Encrypt

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? E

Possible actions for a RSA key: Sign Certify Encrypt Authenticate
Current allowed actions: Sign Certify

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? S

Possible actions for a RSA key: Sign Certify Encrypt Authenticate
Current allowed actions: Certify

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? Q
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 0
Key does not expire at all
Is this correct? (y/N) y
```

Input any name and email address (it doesn't have to be valid):

```console
GnuPG needs to construct a user ID to identify your key.

Real name: Dr Duh
Email address: doc@duh.to
Comment: [Optional - leave blank]
You selected this USER-ID:
    "Dr Duh <doc@duh.to>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.

gpg: /tmp.FLZC0xcM/trustdb.gpg: trustdb created
gpg: key 0xFF3E7D88647EBCDB marked as ultimately trusted
gpg: directory '/tmp.FLZC0xcM/openpgp-revocs.d' created
gpg: revocation certificate stored as '/tmp.FLZC0xcM/openpgp-revocs.d/011CE16BD45B27A55BA8776DFF3E7D88647EBCDB.rev'
public and secret key created and signed.

pub   rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [C]
      Key fingerprint = 011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB
uid                              Dr Duh <doc@duh.to>
```

Export the key ID as a [variable](https://stackoverflow.com/questions/1158091/defining-a-variable-with-or-without-export/1158231#1158231) (`KEYID`) for use later:

```console
export KEYID=0xFF3E7D88647EBCDB
```

# Sign with existing key

(Optional) If you already have a PGP key, you may want to sign the new key with the old one to prove that the new key is controlled by you.

Export your existing key to move it to the working keyring:

```console
gpg --export-secret-keys --armor --output /tmp/new.sec
```

Then sign the new key:

```console
gpg  --default-key $OLDKEY --sign-key $KEYID
```

# Subkeys

Edit the identity to add Subkeys:

```console
$ gpg --expert --edit-key $KEYID

Secret key is available.

sec  rsa4096/0xEA5DE91459B80592
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
[ultimate] (1). Dr Duh <doc@duh.to>
```

Use of 4096-bit RSA keys is recommended.

Subkeys are recommended to have one or several year expirations. They must be renewed using the Certify key. See [rotating keys](#rotating-keys).

## Signing

Create a [signing key](https://stackoverflow.com/questions/5421107/can-rsa-be-both-used-as-encryption-and-signature/5432623#5432623) by selecting `addkey` then `(4) RSA (sign only)`:

```console
gpg> addkey
Key is protected.

You need a passphrase to unlock the secret key for
user: "Dr Duh <doc@duh.to>"
4096-bit RSA key, ID 0xFF3E7D88647EBCDB, created 2016-05-24

Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
Your selection? 4
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 1y
Key expires at Mon 10 Sep 2018 00:00:00 PM UTC
Is this correct? (y/N) y
Really create? (y/N) y
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09       usage: S
[ultimate] (1). Dr Duh <doc@duh.to>
```

## Encryption

Next, create an [encryption key](https://www.cs.cornell.edu/courses/cs5430/2015sp/notes/rsa_sign_vs_dec.php) by selecting `(6) RSA (encrypt only)`:

```console
gpg> addkey
Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
  (10) ECC (sign only)
  (11) ECC (set your own capabilities)
  (12) ECC (encrypt only)
  (13) Existing key
Your selection? 6
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 1y
Key expires at Mon 10 Sep 2018 00:00:00 PM UTC
Is this correct? (y/N) y
Really create? (y/N) y
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09       usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: 2018-10-09       usage: E
[ultimate] (1). Dr Duh <doc@duh.to>
```

## Authentication

Finally, create an [authentication key](https://superuser.com/questions/390265/what-is-a-gpg-with-authenticate-capability-used-for).

GnuPG doesn't provide an authenticate-only key type, so select `(8) RSA (set your own capabilities)` and toggle the required capabilities until the only allowed action is `Authenticate`:

```console
gpg> addkey
Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
  (10) ECC (sign only)
  (11) ECC (set your own capabilities)
  (12) ECC (encrypt only)
  (13) Existing key
Your selection? 8

Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Sign Encrypt

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? S

Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Encrypt

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? E

Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions:

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? A

Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Authenticate

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? Q
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 1y
Key expires at Mon 10 Sep 2018 00:00:00 PM UTC
Is this correct? (y/N) y
Really create? (y/N) y
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09       usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: 2018-10-09       usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: 2018-10-09       usage: A
[ultimate] (1). Dr Duh <doc@duh.to>
```

Finish by saving the keys.

```console
gpg> save
```

## Add extra identities

**Optional** To add additional email addresses or identities, use `adduid`.

Edit the identity:

```console
gpg --expert --edit-key $KEYID
```

Then add the new identity:
```console
gpg> adduid
Real name: Dr Duh
Email address: DrDuh@other.org
Comment:
You selected this USER-ID:
    "Dr Duh <DrDuh@other.org>"

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: never       usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: never       usage: A
[ultimate] (1). Dr Duh <doc@duh.to>
[ unknown] (2). Dr Duh <DrDuh@other.org>

gpg> trust
sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: never       usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: never       usage: A
[ultimate] (1). Dr Duh <doc@duh.to>
[ unknown] (2). Dr Duh <DrDuh@other.org>

Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)

  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu

Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: never       usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: never       usage: A
[ultimate] (1). Dr Duh <doc@duh.to>
[ unknown] (2). Dr Duh <DrDuh@other.org>

gpg> uid 1

sec  rsa4096/0xFF3E7D88647EBCDB
created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: never       usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: never       usage: A
[ultimate] (1)* Dr Duh <doc@duh.to>
[ unknown] (2). Dr Duh <DrDuh@other.org>

gpg> primary

sec  rsa4096/0xFF3E7D88647EBCDB
created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: never       usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: never       usage: A
[ultimate] (1)* Dr Duh <doc@duh.to>
[ unknown] (2)  Dr Duh <DrDuh@other.org>

gpg> save
```

By default, the last identity added will be the primary user ID - use `primary` to change that.

# Verify

List available secret keys:

```console
gpg -K
```

Verify output:

```console
/tmp.FLZC0xcM/pubring.kbx
-------------------------------------------------------------------------
sec   rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [C]
      Key fingerprint = 011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB
uid                            Dr Duh <doc@duh.to>
ssb   rsa4096/0xBECFA3C1AE191D15 2017-10-09 [S] [expires: 2018-10-09]
ssb   rsa4096/0x5912A795E90DD2CF 2017-10-09 [E] [expires: 2018-10-09]
ssb   rsa4096/0x3F29127E79649A3D 2017-10-09 [A] [expires: 2018-10-09]
```

Add any additional identities or email addresses as needed using the `adduid` command.

**Tip** Verify with a OpenPGP [key best practice checker](https://riseup.net/en/security/message-security/openpgp/best-practices#openpgp-key-checks):

```console
gpg --export $KEYID | hokey lint
```

The output will display any problems with your key in red text. If everything is green, your key passes each of the tests. If it is red, your key has failed one of the tests.

> hokey may warn (orange text) about cross certification for the authentication key. GnuPG [Signing Subkey Cross-Certification](https://gnupg.org/faq/subkey-cross-certify.html) documentation has more detail on cross certification, and gpg v2.2.1 notes "subkey <keyid> does not sign and so does not need to be cross-certified". hokey may also indicate a problem (red text) with `Key expiration times: []` on the primary key (see [Note #3](#notes) about not setting an expiry for the primary key).

# Export secret keys

The Certify key and Subkeys will be encrypted with your passphrase when exported.

Save a copy of all keys:

```console
gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/certify.key

gpg --armor --export-secret-subkeys $KEYID > $GNUPGHOME/subkeys.key
```

On Windows, note that using any extension other than `.gpg` or attempting IO redirection to a file will garble the secret key, making it impossible to import it again at a later date:

```console
gpg -o \path\to\dir\certify.gpg --armor --export-secret-keys $KEYID

gpg -o \path\to\dir\subkeys.gpg --armor --export-secret-subkeys $KEYID
```

# Revocation certificate

Although we will backup and store the Certify key in a safe place, it is best practice to never rule out the possibility of losing it or having the backup fail. Without the Certify key, it will be impossible to renew or rotate Subkeys or generate a revocation certificate; the PGP identity will be useless.

Even worse, we cannot advertise this fact in any way to those that are using our keys. It is reasonable to assume this *will* occur at some point and the only remaining way to deprecate orphaned keys is a revocation certificate.

To create the revocation certificate:

``` console
gpg --output $GNUPGHOME/revoke.asc --gen-revoke $KEYID
```

The `revoke.asc` certificate file should be stored (or printed) in a (secondary) place that allows retrieval in case the main backup fails.

# Backup

Create an **encrypted** backup on portable storage to be kept offline in a secure and durable location.
	
**Tip** The ext2 filesystem (without encryption) can be mounted on both Linux and OpenBSD. Consider using a FAT32/NTFS filesystem for MacOS/Windows compatibility instead.

As an additional backup measure, consider using a [paper copy](https://www.jabberwocky.com/software/paperkey/) of the keys. The [Linux Kernel Maintainer PGP Guide](https://www.kernel.org/doc/html/latest/process/maintainer-pgp-guide.html#back-up-your-master-key-for-disaster-recovery) points out that such printouts *are still password-protected*. It recommends to *write the password on the paper*, since it will be unlikely that you remember the original key password that was used when the paper backup was created. Obviously, you need a really good place to keep such a printout.

It is strongly recommended to keep even encrypted OpenPGP private key material offline to deter [key overwriting attacks](https://www.kopenpgp.com/), for example.

**Linux**

Attach another portable storage device and check its label:

```console
$ sudo dmesg | tail
mmc0: new high speed SDHC card at address a001
mmcblk0: mmc0:a001 SS16G 14.8 GiB

$ sudo fdisk -l /dev/mmcblk0
Disk /dev/mmcblk0: 14.9 GiB, 15931539456 bytes, 31116288 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
```

Write it with random data to prepare for encryption:

```console
sudo dd if=/dev/urandom of=/dev/mmcblk0 bs=4M status=progress
```

Erase and create a new partition table:

```console
$ sudo fdisk /dev/mmcblk0

Welcome to fdisk (util-linux 2.33.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x3c1ad14a.

Command (m for help): g
Created a new GPT disklabel (GUID: 4E7495FD-85A3-3E48-97FC-2DD8D41516C3).

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

```

Create a new partition with a 25 Megabyte size:

```console
$ sudo fdisk /dev/mmcblk0

Welcome to fdisk (util-linux 2.36.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): n
Partition number (1-128, default 1):
First sector (2048-30261214, default 2048):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-30261214, default 30261214): +25M

Created a new partition 1 of type 'Linux filesystem' and of size 25 MiB.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.
```

Use [LUKS](https://askubuntu.com/questions/97196/how-secure-is-an-encrypted-luks-filesystem) to encrypt the new partition. Generate a different password which will be used to protect the filesystem:

```console
sudo cryptsetup luksFormat /dev/mmcblk0p1
```

Mount the partition:

```console
sudo cryptsetup luksOpen /dev/mmcblk0p1 secret
```

Create an ext2 filesystem:

```console
sudo mkfs.ext2 /dev/mapper/secret -L gpg-$(date +%F)
```

Mount the filesystem and copy the temporary GnuPG directory with keyring:

```console
sudo mkdir /mnt/encrypted-storage

sudo mount /dev/mapper/secret /mnt/encrypted-storage

sudo cp -avi $GNUPGHOME /mnt/encrypted-storage/
```

**Optional** Backup the OneRNG package:

```console
sudo cp onerng_3.7-1_all.deb /mnt/encrypted-storage/
```

**Note** If you plan on setting up multiple keys, keep the backup mounted or remember to terminate the gpg process before [saving](https://lists.gnupg.org/pipermail/gnupg-users/2016-July/056353.html).

Unmount, close and disconnect the encrypted volume:

```console
sudo umount /mnt/encrypted-storage/

sudo cryptsetup luksClose secret
```

**OpenBSD**

Attach a USB disk and determine its label:

```console
$ dmesg | grep sd.\ at
sd2 at scsibus5 targ 1 lun 0: <TS-RDF5, SD Transcend, TS37> SCSI4 0/direct removable serial.00000000000000000000
```

Print the existing partitions to make sure it's the right device:

```console
doas disklabel -h sd2
```

Initialize the disk by creating an `a` partition with FS type `RAID` and size of 25 Megabytes:

```console
$ doas fdisk -giy sd2
Writing MBR at offset 0.
Writing GPT.

$ doas disklabel -E sd2
Label editor (enter '?' for help at any prompt)
sd2> a a
offset: [64]
size: [31101776] 25M
FS type: [4.2BSD] RAID
sd2*> w
sd2> q
No label changes
```

Encrypt with bioctl:

```console
$ doas bioctl -c C -l sd2a softraid0
New passphrase:
Re-type passphrase:
softraid0: CRYPTO volume attached as sd3
```

Create an `i` partition on the new crypto volume and the filesystem:

```console
$ doas fdisk -giy sd3
Writing MBR at offset 0.
Writing GPT.

$ doas disklabel -E sd3
Label editor (enter '?' for help at any prompt)
sd3> a i
offset: [64]
size: [16001]
FS type: [4.2BSD]
sd3*> w
sd3> q
No label changes.

$ doas newfs sd3i
```

Mount the filesystem and copy the temporary directory with the keyring:

```console
doas mkdir /mnt/encrypted-storage

doas mount /dev/sd3i /mnt/encrypted-storage

doas cp -avi $GNUPGHOME /mnt/encrypted-storage
```

**Note** If you plan on setting up multiple keys, keep the backup mounted or remember to terminate the gpg process before [saving](https://lists.gnupg.org/pipermail/gnupg-users/2016-July/056353.html).

Otherwise, unmount and disconnect the encrypted volume:

```console
doas umount /mnt/encrypted-storage

doas bioctl -d sd3
```

See [OpenBSD FAQ#14](https://www.openbsd.org/faq/faq14.html#softraidCrypto) for more information.

# Export public keys

**Important** Without the *public* key, you will **not** be able to use GnuPG to encrypt, decrypt, nor sign messages. However, you will still be able to use YubiKey for SSH authentication.

Create another partition on the portable storage device to store the public key, or reconnect networking and upload to a key server.

**Linux**

```console
$ sudo fdisk /dev/mmcblk0

Welcome to fdisk (util-linux 2.36.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): n
Partition number (2-128, default 2):
First sector (53248-30261214, default 53248):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (53248-30261214, default 30261214): +25M

Created a new partition 2 of type 'Linux filesystem' and of size 25 MiB.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

$ sudo mkfs.ext2 /dev/mmcblk0p2

$ sudo mkdir /mnt/public

$ sudo mount /dev/mmcblk0p2 /mnt/public/

$ gpg --armor --export $KEYID | sudo tee /mnt/public/gpg-$KEYID-$(date +%F).asc
```

**OpenBSD**

```console
$ doas disklabel -E sd2
Label editor (enter '?' for help at any prompt)
sd2> a b
offset: [32130]
size: [31069710] 25M
FS type: [swap] 4.2BSD
sd2*> w
sd2> q
No label changes.

$ doas newfs sd2b

$ doas mkdir /mnt/public

$ doas mount /dev/sd2b /mnt/public

$ gpg --armor --export $KEYID | doas tee /mnt/public/gpg-$KEYID-$(date +%F).asc
```

**Windows**

```console
gpg -o \path\to\dir\pubkey.gpg --armor --export $KEYID
```

**Keyserver**

(Optional) Upload the public key to a [public keyserver](https://debian-administration.org/article/451/Submitting_your_GPG_key_to_a_keyserver):

```console
gpg --send-key $KEYID

gpg --keyserver keys.gnupg.net --send-key $KEYID

gpg --keyserver hkps://keyserver.ubuntu.com:443 --send-key $KEYID
```

Or if [uploading to keys.openpgp.org](https://keys.openpgp.org/about/usage):

```console
gpg --send-key $KEYID | curl -T - https://keys.openpgp.org
```

# Configure Smartcard

Plug in the YubiKey and use GnuPG to configure it as a smartcard:

```console
$ gpg --card-edit

Reader ...........: Yubico Yubikey 4 OTP U2F CCID
Application ID ...: D2760001240102010006055532110000
Application type .: OpenPGP
Version ..........: 3.4
Manufacturer .....: Yubico
Serial number ....: 05553211
Name of cardholder: [not set]
Language prefs ...: [not set]
Salutation .......:
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: not forced
Key attributes ...: rsa2048 rsa2048 rsa2048
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 0
KDF setting ......: off
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]
```

Enter administrative mode:

```console
gpg/card> admin
Admin commands are allowed
```

**Note** If the card is locked, see [Reset](#reset).

**Windows**

Use the [YubiKey Manager](https://developers.yubico.com/yubikey-manager) application (note, this is not the similarly named older YubiKey NEO Manager) to enable CCID functionality.

## Enable KDF

Key Derived Function (KDF) enables YubiKey to store the hash of PIN, preventing the PIN from being passed as plain text.

**Note** This feature may not be compatible with older GnuPG versions, especially mobile clients. These incompatible clients will not function because the PIN will always be rejected.

```console
gpg/card> kdf-setup
```

## Change PIN

The [PGP interface](https://developers.yubico.com/PGP/) is separate from other modules on YubiKey, such as the [PIV interface](https://developers.yubico.com/PIV/Introduction/YubiKey_and_PIV.html) - the PGP interface has its own *PIN*, *Admin PIN*, and *Reset Code* which must be changed from default values.

Entering the *PIN* incorrectly three times will cause the PIN to become blocked. It can be unblocked with either the *Admin PIN* or *Reset Code*.

Entering the *Admin PIN* or *Reset Code* incorrectly three times destroys all GnuPG data on the card.

Name       | Default Value | Use
-----------|---------------|-------------------------------------------------------------
PIN        | `123456`      | cryptographic operations (decrypt, sign, authenticate) PIN
Admin PIN  | `12345678`    | reset PIN, change Reset Code, add keys and owner information
Reset Code | None          | reset PIN ([more information](https://forum.yubico.com/viewtopicd01c.html?p=9055#p9055))

Values are valid up to 127 ASCII characters and must be at least 6 (*PIN*) or 8 (*Admin PIN*, *Reset Code*) characters. See the GnuPG documentation on [Managing PINs](https://www.gnupg.org/howtos/card-howto/en/ch03s02.html) for details.

To update PINs:

```console
gpg/card> passwd
gpg: OpenPGP card no. D2760001240102010006055532110000 detected

1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit

Your selection? 3
PIN changed.

1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit

Your selection? 1
PIN changed.

1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit

Your selection? q
```

**Note** The number of retry attempts can be changed later with the following command, documented [here](https://docs.yubico.com/software/yubikey/tools/ykman/OpenPGP_Commands.html#ykman-openpgp-access-set-retries-options-pin-retries-reset-code-retries-admin-pin-retries):

```bash
ykman openpgp access set-retries 5 5 5 -f -a YOUR_ADMIN_PIN
```

## Set information

Some fields are optional.

```console
gpg/card> name
Cardholder's surname: Duh
Cardholder's given name: Dr

gpg/card> lang
Language preferences: en

gpg/card> login
Login data (account name): doc@duh.to

gpg/card> list

Application ID ...: D2760001240102010006055532110000
Version ..........: 3.4
Manufacturer .....: unknown
Serial number ....: 05553211
Name of cardholder: Dr Duh
Language prefs ...: en
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: doc@duh.to
Private DO 4 .....: [not set]
Signature PIN ....: not forced
Key attributes ...: rsa2048 rsa2048 rsa2048
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 0
KDF setting ......: on
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]

gpg/card> quit
```

# Transfer keys

**Important** Transferring keys to YubiKey using `keytocard` is a one-way/destructive operation. Make sure a backup was made before proceeding. `keytocard` converts the local, on-disk key into a stub, which means the on-disk copy is no longer usable to transfer to subsequent YubiKeys.

Previous GnuPG versions required the `toggle` command before selecting keys. The currently selected key(s) are indicated with an `*`. When moving keys only one key should be selected at a time.

```console
$ gpg --edit-key $KEYID

Secret key is available.

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09  usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: 2018-10-09  usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>
```

## Signing

**Important** You will be prompted for the Certify key passphrase and Admin PIN.

Select and transfer the signature key.

```console
gpg> key 1

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb* rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09  usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: 2018-10-09  usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>

gpg> keytocard
Please select where to store the key:
   (1) Signature key
   (3) Authentication key
Your selection? 1

You need a passphrase to unlock the secret key for
user: "Dr Duh <doc@duh.to>"
4096-bit RSA key, ID 0xBECFA3C1AE191D15, created 2016-05-24
```

## Encryption

Type `key 1` again to de-select and `key 2` to select the next key:

```console
gpg> key 1

gpg> key 2

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09  usage: S
ssb* rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: 2018-10-09  usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>

gpg> keytocard
Please select where to store the key:
   (2) Encryption key
Your selection? 2

[...]
```

## Authentication

Type `key 2` again to deselect and `key 3` to select the last key:

```console
gpg> key 2

gpg> key 3

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09  usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: 2018-10-09  usage: E
ssb* rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>

gpg> keytocard
Please select where to store the key:
   (3) Authentication key
Your selection? 3
```

Save and quit:

```console
gpg> save
```

# Verify card

Verify the Subkeys have been moved to YubiKey as indicated by `ssb>`

```console
$ gpg -K
/tmp.FLZC0xcM/pubring.kbx
-------------------------------------------------------------------------
sec   rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [C]
      Key fingerprint = 011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB
uid                            Dr Duh <doc@duh.to>
ssb>  rsa4096/0xBECFA3C1AE191D15 2017-10-09 [S] [expires: 2018-10-09]
ssb>  rsa4096/0x5912A795E90DD2CF 2017-10-09 [E] [expires: 2018-10-09]
ssb>  rsa4096/0x3F29127E79649A3D 2017-10-09 [A] [expires: 2018-10-09]
```

# Multiple YubiKeys

To provision additional YubiKeys, restore the Certify key backup and repeat [Configure Smartcard](#configure-smartcard).

```console
$ mv -vi $GNUPGHOME $GNUPGHOME.1
renamed '/tmp.FLZC0xcM' -> '/tmp.FLZC0xcM.1'

$ cp -avi /mnt/encrypted-storage/tmp.XXX $GNUPGHOME
'/mnt/encrypted-storage/tmp.FLZC0xcM' -> '/tmp.FLZC0xcM'

$ cd $GNUPGHOME
```

## Switching between YubiKeys

When you add a GnuPG key to a YubiKey using the *keytocard* command, the key is deleted from the keyring and a *stub* is added, pointing to the YubiKey. The stub identifies the GnuPG key ID and YubiKey serial number.

However, when the operation is repeated for an additional YubiKey, the stub is overwritten by the *keytocard* operation and now will point to the latest YubiKey.

When using key operations with YubiKey, GnuPG will request a specific YubiKey by serial number (as referenced by the stub). GnuPG will not recognize another YubiKey with a different serial number without manual intervention.

Insert the first YubiKey (which has a different serial number) and run the following command:

```console
gpg-connect-agent "scd serialno" "learn --force" /bye
```

GnuPG will scan the first YubiKey for keys and recreate the stubs to point to the key ID and YubiKey serial number of the first YubiKey.

To use the second YubiKey, repeat the command.

# Multiple Hosts

Export the public key and trust setting from the current host:

```console
gpg --armor --export $KEYID > gpg-public-key-$KEYID.asc

gpg --export-ownertrust > gpg-owner-trust.txt
```

Move both files to the second host, then define the key ID:

```console
export KEYID=0xFF3E7D88647EBCDB
```

Import the public key:

```console
gpg --import gpg-public-key-$KEYID.asc
```

Import the trust setting:

```console
gpg --import-ownertrust < gpg-owner-trust.txt
```

Insert YubiKey and import key stubs:

```console
gpg --card-status
```

Or obtain from a public key server:

```console
gpg --keyserver hkps://keyserver.ubuntu.com:443 --recv $KEYID
```

Configure trust:

```console
$ gpg --edit-key $KEYID
gpg> trust
Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y
gpg> quit
```

The public key URL can also be added to YubiKey (based on [Shaw 2003](https://datatracker.ietf.org/doc/html/draft-shaw-openpgp-hkp-00)):

```console
[[ ! "$KEYID" =~ ^"0x" ]] && KEYID="0x${KEYID}"
URL="hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=${KEYID}"
```

Edit the YubiKey using the Admin PIN:

```console
$ gpg --edit-card
gpg/card> admin
gpg/card> url
URL to retrieve public key: hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF3E7D88647EBCDB
gpg/card> quit
```

With the URL on YubiKey, the `fetch` command can be used to retrieve the public key:

```console
$ gpg --edit-card

gpg/card> fetch
gpg: requesting key from 'hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF3E7D88647EBCDB'
gpg: /home/pi/.gnupg/trustdb.gpg: trustdb created
gpg: key FF3E7D88647EBCDB: public key "Dr Duh <doc@duh.to>" imported
gpg: Total number processed: 1
gpg:               imported: 1

gpg/card> quit
```

# Finish

Before completing setup, verify the following:

- [ ] Saved encryption, signing and authentication Subkeys to YubiKey (`gpg -K` will show `ssb>` for Subkeys)
- [ ] Saved YubiKey user and admin PINs, which are unique and were changed from default values
- [ ] Saved Certify key passphrase to a secure and durable location
- [ ] Saved Certify key, Subkeys and revocation certificate on encrypted portable storage, to be kept offline
- [ ] Saved password to encrypted volume on portable storage
- [ ] Saved copy of public key where is can be easily accessed later

Reboot to finish.

If an ephemeral environment was not used for setup, delete secret keys from the keyring and [securely delete](https://srm.sourceforge.net/) `$GNUPGHOME`.

```console
gpg --delete-secret-key $KEYID

sudo srm -r $GNUPGHOME || sudo rm -rf $GNUPGHOME

unset GNUPGHOME
```

# Using keys

Initialize GnuPG:

```console
gpg -k
```

Change the working directory:

```console
cd ~/.gnupg
```

Download [drduh/config/gpg.conf](https://github.com/drduh/config/blob/master/gpg.conf):

```console
wget https://raw.githubusercontent.com/drduh/config/master/gpg.conf

chmod 600 gpg.conf
```

Set the following option. This avoids the problem where GnuPG will prompt, repeatedly, for the insertion of an already-inserted YubiKey:

```console
touch scdaemon.conf

echo "disable-ccid" >>scdaemon.conf
```

> The `disable-ccid` option is only required for GnuPG versions 2.3 or later. However, setting this option does not appear to interfere with the operation of earlier versions of GnuPG so it is recommended for all installations.

Install the required packages and mount the non-encrypted volume created earlier:

**Linux**

```console
sudo apt update

sudo apt install -y gnupg2 gnupg-agent gnupg-curl scdaemon pcscd

sudo mount /dev/mmcblk0p2 /mnt
```

**OpenBSD**

```console
doas pkg_add gnupg pcsc-tools

doas mount /dev/sd2b /mnt
```

Import the public key file:

```console
gpg --import /mnt/gpg-0x*.asc
```

Or download the public key from a keyserver:

```console
gpg --recv $KEYID
```

Edit the Certify key:

```console
export KEYID=0xFF3E7D88647EBCDB

gpg --edit-key $KEYID
```

Assign ultimate trust by selecting `trust` and `5`:

```console
gpg> trust
pub  4096R/0xFF3E7D88647EBCDB  created: 2016-05-24  expires: never       usage: C
                               trust: unknown       validity: unknown
sub  4096R/0xBECFA3C1AE191D15  created: 2017-10-09  expires: 2018-10-09  usage: S
sub  4096R/0x5912A795E90DD2CF  created: 2017-10-09  expires: 2018-10-09  usage: E
sub  4096R/0x3F29127E79649A3D  created: 2017-10-09  expires: 2018-10-09  usage: A
[ unknown] (1). Dr Duh <doc@duh.to>

Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)

  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu

Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y

pub  4096R/0xFF3E7D88647EBCDB  created: 2016-05-24  expires: never       usage: C
                               trust: ultimate      validity: unknown
sub  4096R/0xBECFA3C1AE191D15  created: 2017-10-09  expires: 2018-10-09  usage: S
sub  4096R/0x5912A795E90DD2CF  created: 2017-10-09  expires: 2018-10-09  usage: E
sub  4096R/0x3F29127E79649A3D  created: 2017-10-09  expires: 2018-10-09  usage: A
[ unknown] (1). Dr Duh <doc@duh.to>

gpg> quit
```

Remove and re-insert YubiKey and verify the status:

```console
$ gpg --card-status
Reader ...........: Yubico YubiKey OTP FIDO CCID 00 00
Application ID ...: D2760001240102010006055532110000
Version ..........: 3.4
Manufacturer .....: Yubico
Serial number ....: 05553211
Name of cardholder: Dr Duh
Language prefs ...: en
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: doc@duh.to
Signature PIN ....: not forced
Key attributes ...: rsa4096 rsa4096 rsa4096
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 0
KDF setting ......: on
Signature key ....: 07AA 7735 E502 C5EB E09E  B8B0 BECF A3C1 AE19 1D15
      created ....: 2016-05-24 23:22:01
Encryption key....: 6F26 6F46 845B BEB8 BDF3  7E9B 5912 A795 E90D D2CF
      created ....: 2016-05-24 23:29:03
Authentication key: 82BE 7837 6A3F 2E7B E556  5E35 3F29 127E 7964 9A3D
      created ....: 2016-05-24 23:36:40
General key info..: pub  4096R/0xBECFA3C1AE191D15 2016-05-24 Dr Duh <doc@duh.to>
sec#  4096R/0xFF3E7D88647EBCDB  created: 2016-05-24  expires: never
ssb>  4096R/0xBECFA3C1AE191D15  created: 2017-10-09  expires: 2018-10-09
                      card-no: 0006 05553211
ssb>  4096R/0x5912A795E90DD2CF  created: 2017-10-09  expires: 2018-10-09
                      card-no: 0006 05553211
ssb>  4096R/0x3F29127E79649A3D  created: 2017-10-09  expires: 2018-10-09
                      card-no: 0006 05553211
```

`sec#` indicates the corresponding key is not available.

**Note** If you see `General key info..: [none]` in the output instead - go back and import the public key using the previous step.

Encrypt a message to yourself (useful for storing password credentials and other data):

```console
echo "test message string" | gpg --encrypt --armor --recipient $KEYID -o encrypted.txt
```

To encrypt to multiple recipients or keys (the preferred key ID should be last):

```console
echo "test message string" | \
    gpg --encrypt --armor \
    --recipient $KEYID_0 --recipient $KEYID_1 --recipient $KEYID_2 \
    -o encrypted.txt
```

Decrypt the message:

```console
$ gpg --decrypt --armor encrypted.txt
gpg: anonymous recipient; trying secret key 0x0000000000000000 ...
gpg: okay, we are the anonymous recipient.
gpg: encrypted with RSA key, ID 0x0000000000000000
test message string
```

Sign a message:

```console
echo "test message string" | gpg --armor --clearsign > signed.txt
```

Verify the signature:

```console
$ gpg --verify signed.txt
gpg: Signature made Wed 25 May 2016 00:00:00 AM UTC
gpg:                using RSA key 0xBECFA3C1AE191D15
gpg: Good signature from "Dr Duh <doc@duh.to>" [ultimate]
Primary key fingerprint: 011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB
     Subkey fingerprint: 07AA 7735 E502 C5EB E09E  B8B0 BECF A3C1 AE19 1D15
```

Use a [shell function](https://github.com/drduh/config/blob/master/zshrc) to make encrypting files easier:

```
secret () {
        output=~/"${1}".$(date +%s).enc
        gpg --encrypt --armor --output ${output} -r 0x0000 -r 0x0001 -r 0x0002 "${1}" && echo "${1} -> ${output}"
}

reveal () {
        output=$(echo "${1}" | rev | cut -c16- | rev)
        gpg --decrypt --output ${output} "${1}" && echo "${1} -> ${output}"
}
```

```console
$ secret document.pdf
document.pdf -> document.pdf.1580000000.enc

$ reveal document.pdf.1580000000.enc
gpg: anonymous recipient; trying secret key 0xFF3E7D88647EBCDB ...
gpg: okay, we are the anonymous recipient.
gpg: encrypted with RSA key, ID 0x0000000000000000
document.pdf.1580000000.enc -> document.pdf
```

# Rotating keys

PGP does not provide [forward secrecy](https://en.wikipedia.org/wiki/Forward_secrecy) - a compromised key may be used to decrypt all past messages. Although keys stored on YubiKey are difficult to steal, it is not impossible; the key and PIN could be taken, or a vulnerability may be discovered in key hardware or the random number generator used to create them, for example. Therefore, it is good practice to occassionally rotate Subkeys.

When a Subkey expires, it can either be renewed or replaced. Both actions require access to the Certify key.

- Renewing Subkeys by updating expiration dates indicates you are still in possession of the Certify key and is more convenient.

- Replacing Subkeys is less convenient but potentially more secure: the new Subkeys will **not** be able to decrypt previous messages, authenticate with SSH, etc. Contacts will need to receive the updated public key and any encrypted secrets need to be decrypted and re-encrypted to new Subkeys to be usable. This process is functionally equivalent to losing the YubiKey and provisioning a new one.

Neither rotation method is superior and it is up to personal philosophy on identity management and individual threat modeling to decide which one to use, or whether to expire Subkeys at all. Ideally, Subkeys would be ephemeral: used only once for each unique encryption, signing and authentication event, however in practice that is not really practical nor worthwhile with YubiKey. Advanced users may dedicate an air-gapped machine for frequent credential rotation.

## Setup environment

To renew or rotate Subkeys, follow the same process as generating keys: boot to a secure environment, install required software and disconnect networking.

Connect the portable storage device with the Certify key and identify the disk label:

```console
$ sudo dmesg | tail
mmc0: new high speed SDHC card at address a001
mmcblk0: mmc0:a001 SS16G 14.8 GiB (ro)
mmcblk0: p1 p2
```

Decrypt and mount the encrypted volume:

```console
sudo cryptsetup luksOpen /dev/mmcblk0p1 secret

sudo mount /dev/mapper/secret /mnt/encrypted-storage
```

Import the Certify key and configuration to a temporary working directory.

Note that Windows users should import certify.gpg:

```console
export GNUPGHOME=$(mktemp -d -t gnupg_$(date +%Y%m%d%H%M)_XXX)

gpg --import /mnt/encrypted-storage/tmp.XXX/certify.key

cp -v /mnt/encrypted-storage/tmp.XXX/gpg.conf $GNUPGHOME
```

Edit the Certify key:

```console
export KEYID=0xFF3E7D88647EBCDB

gpg --expert --edit-key $KEYID
```

## Renewing Subkeys

To renew Subkeys, the expiry time associated with the corresponding public key will need to be updated, which will require access to the Certify key.

Start by selecting all keys:

```console
$ gpg --edit-key $KEYID

Secret key is available.

sec  rsa4096/0xFF3E7D88647EBCDB
    created: 2017-10-09  expires: never       usage: C
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: 2018-10-09  usage: S
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: 2018-10-09  usage: E
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>

gpg> key 1

Secret key is available.

sec  rsa4096/0xFF3E7D88647EBCDB
     created: 2017-10-09  expires: never       usage: C
     trust: ultimate      validity: ultimate
ssb* rsa4096/0xBECFA3C1AE191D15
     created: 2017-10-09  expires: 2018-10-09  usage: S
ssb  rsa4096/0x5912A795E90DD2CF
     created: 2017-10-09  expires: 2018-10-09  usage: E
ssb  rsa4096/0x3F29127E79649A3D
     created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>

gpg> key 2

Secret key is available.

sec  rsa4096/0xFF3E7D88647EBCDB
     created: 2017-10-09  expires: never       usage: C
     trust: ultimate      validity: ultimate
ssb* rsa4096/0xBECFA3C1AE191D15
     created: 2017-10-09  expires: 2018-10-09  usage: S
ssb* rsa4096/0x5912A795E90DD2CF
     created: 2017-10-09  expires: 2018-10-09  usage: E
ssb  rsa4096/0x3F29127E79649A3D
     created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>

gpg> key 3

Secret key is available.

sec   rsa4096/0xFF3E7D88647EBCDB
      created: 2017-10-09  expires: never       usage: C
      trust: ultimate      validity: ultimate
ssb*  rsa4096/0xBECFA3C1AE191D15
      created: 2017-10-09  expires: 2018-10-09  usage: S
ssb*  rsa4096/0x5912A795E90DD2CF
      created: 2017-10-09  expires: 2018-10-09  usage: E
ssb*  rsa4096/0x3F29127E79649A3D
      created: 2017-10-09  expires: 2018-10-09  usage: A
[ultimate] (1). Dr Duh <doc@duh.to>
```

Use `expire` to configure the expiration date. This will **not** expire valid keys.

```console
gpg> expire
Changing expiration time for a subkey.
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0)
```

Follow the prompt to set the expiration date, then `save`

Next, [Export public keys](#export-public-keys):

```console
gpg --armor --export $KEYID > gpg-$KEYID-$(date +%F).asc
```

Transfer the public key to the destination host, and then import it:

```console
gpg --import gpg-0x*.asc
```

Alternatively, publish to a public key server to update the expiration:

```console
gpg --send-key $KEYID
```

Download the public key with updated expiration:

```console
gpg --recv $KEYID
```

The validitiy of the GPG identity will be extended, allowing it to be used for encryption, signing and authentication operations. The SSH public key does **not** need to be updated on remote hosts.

## Rotating keys

Follow the original steps to generate and add each Subkey.

Previous Subkeys may be kept or deleted from the identity.

Finish by exporting new keys:

```console
gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/certify.key

gpg --armor --export-secret-subkeys $KEYID > $GNUPGHOME/subkeys.key
```

Copy the **new** temporary working directory to encrypted storage, which should still be mounted:

```console
sudo cp -avi $GNUPGHOME /mnt/encrypted-storage
```

There should now be at least two versions of the Certify and Subkeys:

```console
$ ls /mnt/encrypted-storage
lost+found  tmp.ykhTOGjR36  tmp.2gyGnyCiHs
```

Unmount and close the encrypted volume:

```console
sudo umount /mnt/encrypted-storage

sudo cryptsetup luksClose /dev/mapper/secret
```

Export the updated public key:

```console
sudo mkdir /mnt/public

sudo mount /dev/mmcblk0p2 /mnt/public

gpg --armor --export $KEYID | sudo tee /mnt/public/$KEYID-$(date +%F).asc

sudo umount /mnt/public
```

Disconnect the storage device and follow the original steps to transfer new keys (4, 5 and 6) to the YubiKey, replacing existing ones. Reboot or securely erase the GPG temporary working directory.

# Adding notations

Notations can be added to user ID(s) and can be used in conjunction with [Keyoxide](https://keyoxide.org) to create [OpenPGP identity proofs](https://docs.keyoxide.org/wiki/identity-proof-formats/).

Adding notations requires access to the Certify key so we can follow the setup instructions taken from this [section](#setup-environment) of this guide.

Please note that there is no need to connect the YubiKey to the setup environment and that we do not need to generate new keys, move keys to the YubiKey, or update any SSH public keys linked to the GPG key.

After having completed the environment setup, it is possible to follow any of the guides listed under "Adding proofs" in the Keyoxide ["Guides"](https://keyoxide.org/guides/) page up until the notation is saved using the `save` command.

At this point the public key can be exported:

```console
gpg --export $KEYID > pubkey.asc
```

The public key can now be transferred to the computer where the GPG key is used and it is imported with:

```console
gpg --import pubkey.asc
```

N.B.: The `showpref` command can be issued to ensure that the notions were correctly added.

# SSH

**Tip** YubiKey can be used for SSH only, without PGP/GPG features, starting in [OpenSSH v8.2](https://www.openssh.com/txt/release-8.2). For more information, see [ed25519-sk.md](https://github.com/vorburger/vorburger.ch-Notes/blob/develop/security/ed25519-sk.md) and [Yubico - GitHub now supports SSH security keys](https://www.yubico.com/blog/github-now-supports-ssh-security-keys/).

[gpg-agent](https://wiki.archlinux.org/title/GnuPG#SSH_agent) supports the OpenSSH ssh-agent protocol (`enable-ssh-support`) as well as PuTTy's Pageant on Windows (`enable-putty-support`). This means it can be used instead of the traditional ssh-agent / pageant. There are some differences from ssh-agent, notably that gpg-agent does not _cache_ keys rather it converts, encrypts and stores them - persistently - as GPG keys and then makes them available to ssh clients. Any existing ssh private keys that you'd like to keep in `gpg-agent` should be deleted after they've been imported to the GPG agent.

When importing the key to `gpg-agent`, a passphrase will be required to encrypt it within the GPG key store. GPG can cache both passphrases with `cache-ttl` options. Note than when removing the old private key after importing to `gpg-agent`, keep the `.pub` key file around for use in specifying ssh identities (e.g. `ssh -i /path/to/identity.pub`).

Probably the biggest thing missing from `gpg-agent`'s ssh agent support is being able to remove keys. `ssh-add -d/-D` have no effect. Instead, you need to use the `gpg-connect-agent` utility to lookup a key's keygrip, match that with the desired ssh key fingerprint (as an MD5) and then delete that keygrip. The [gnupg-users mailing list](https://lists.gnupg.org/pipermail/gnupg-users/2016-August/056499.html) has more information.

## Create configuration

Create a hardened configuration for gpg-agent by downloading [drduh/config/gpg-agent.conf](https://github.com/drduh/config/blob/master/gpg-agent.conf):

```console
cd ~/.gnupg

wget https://raw.githubusercontent.com/drduh/config/master/gpg-agent.conf
```

**Important** The `cache-ttl` options do **NOT** apply when using a YubiKey as a smartcard as the PIN is [cached by the smartcard itself](https://dev.gnupg.org/T3362). Therefore, in order to clear the PIN from cache (smartcard equivalent to `default-cache-ttl` and `max-cache-ttl`), you need to unplug the YubiKey, or set the `forcesig` flag when editing the card to be prompted for the PIN each time.

**Tip** Set `pinentry-program /usr/bin/pinentry-gnome3` for a GUI-based prompt. If the _pinentry_ graphical dialog doesn't show and you get this error: `sign_and_send_pubkey: signing failed: agent refused operation`, you may need to install the `dbus-user-session` package and restart the computer for the `dbus` user session to be fully inherited; this is because behind the scenes, `pinentry` complains about `No $DBUS_SESSION_BUS_ADDRESS found`, falls back to `curses` but doesn't find the expected `tty`.

On macOS, use `brew install pinentry-mac` and set the program path to `pinentry-program /usr/local/bin/pinentry-mac` for Intel Macs, `/opt/homebrew/bin/pinentry-mac` for ARM/Apple Silicon Macs or `pinentry-program /usr/local/MacGPG2/libexec/pinentry-mac.app/Contents/MacOS/pinentry-mac` if using MacGPG Suite. For the configuration to take effect you have to run `gpgconf --kill gpg-agent`.

## Replace agents

To launch `gpg-agent` for use by SSH, use the `gpg-connect-agent /bye` or `gpgconf --launch gpg-agent` commands.

Add these to the shell `rc` file:

```console
export GPG_TTY="$(tty)"
export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"
gpg-connect-agent updatestartuptty /bye > /dev/null
```

On modern systems, `gpgconf --list-dirs agent-ssh-socket` will automatically set `SSH_AUTH_SOCK` to the correct value and is better than hard-coding to `run/user/$UID/gnupg/S.gpg-agent.ssh`, if available:

```console
export GPG_TTY="$(tty)"
export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
gpgconf --launch gpg-agent
```

For fish, `config.fish` should look like this (consider putting them into the `is-interactive` block):

```fish
set -x GPG_TTY (tty)
set -x SSH_AUTH_SOCK (gpgconf --list-dirs agent-ssh-socket)
gpgconf --launch gpg-agent
```

Note that if you use `ForwardAgent` for ssh-agent forwarding, `SSH_AUTH_SOCK` only needs to be set on the *local* laptop (workstation), where the YubiKey is plugged in. On the *remote* server, `ssh` will automatically set `SSH_AUTH_SOCK` to something like `/tmp/ssh-mXzCzYT2Np/agent.7541` upon connection. Do **not** set `SSH_AUTH_SOCK` on the server - doing so will break [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding).

If you use `S.gpg-agent.ssh` (see [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding) for more info), `SSH_AUTH_SOCK` should also be set on the *remote*. However, `GPG_TTY` should not be set on the *remote*, explanation specified in that section.

## Copy public key

**Note** It is **not** necessary to import the corresponding GPG public key in order to use SSH.

Copy and paste the output from `ssh-add` to the server's `authorized_keys` file:

```console
$ ssh-add -L
ssh-rsa AAAAB4NzaC1yc2EAAAADAQABAAACAz[...]zreOKM+HwpkHzcy9DQcVG2Nw== cardno:000605553211
```

## (Optional) Save public key for identity file configuration

By default, SSH attempts to use all the identities available via the agent. It's often a good idea to manage exactly which keys SSH will use to connect to a server, for example to separate different roles or [to avoid being fingerprinted by untrusted ssh servers](https://blog.filippo.io/ssh-whoami-filippo-io/). To do this you'll need to use the command line argument `-i [identity_file]` or the `IdentityFile` and `IdentitiesOnly` options in `.ssh/config`.

The argument provided to `IdentityFile` is traditionally the path to the _private_ key file (for example `IdentityFile ~/.ssh/id_rsa`). For the YubiKey - indeed, in general for keys stored in an ssh agent - `IdentityFile` should point to the _public_ key file, `ssh` will select the appropriate private key from those available via the ssh agent. To prevent `ssh` from trying all keys in the agent use the `IdentitiesOnly yes` option along with one or more `-i` or `IdentityFile` options for the target host.

To reiterate, with `IdentitiesOnly yes`, `ssh` will not automatically enumerate public keys loaded into `ssh-agent` or `gpg-agent`. This means `publickey` authentication will not proceed unless explicitly named by `ssh -i [identity_file]` or in `.ssh/config` on a per-host basis.

In the case of YubiKey usage, to extract the public key from the ssh agent:

```console
ssh-add -L | grep "cardno:000605553211" > ~/.ssh/id_rsa_yubikey.pub
```

Then you can explicitly associate this YubiKey-stored key for used with a host, `github.com` for example, as follows:

```console
$ cat << EOF >> ~/.ssh/config
Host github.com
    IdentitiesOnly yes
    IdentityFile ~/.ssh/id_rsa_yubikey.pub
EOF
```

## Connect with public key authentication

```console
$ ssh git@github.com -vvv
[...]
debug2: key: cardno:000605553211 (0x1234567890),
debug1: Authentications that can continue: publickey
debug3: start over, passed a different list publickey
debug3: preferred gssapi-keyex,gssapi-with-mic,publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: cardno:000605553211
debug3: send_pubkey_test
debug2: we sent a publickey packet, wait for reply
debug1: Server accepts key: pkalg ssh-rsa blen 535
debug2: input_userauth_pk_ok: fp e5:de:a5:74:b1:3e:96:9b:85:46:e7:28:53:b4:82:c3
debug3: sign_and_send_pubkey: RSA e5:de:a5:74:b1:3e:96:9b:85:46:e7:28:53:b4:82:c3
debug1: Authentication succeeded (publickey).
[...]
```

**Tip** To make multiple connections or securely transfer many files, consider using the [ControlMaster](https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing) ssh option. Also see [drduh/config/ssh_config](https://github.com/drduh/config/blob/master/ssh_config).

## Import SSH keys

If there are existing SSH keys to make available via `gpg-agent`, they will need to be imported. Then, remove the original private keys. When importing the key, `gpg-agent` uses the key filename as the label - this makes it easier to follow where the key originated from. In this example, we're starting with just the YubiKey in place and importing `~/.ssh/id_rsa`:

```console
$ ssh-add -l
4096 SHA256:... cardno:00060123456 (RSA)

$ ssh-add ~/.ssh/id_rsa && rm ~/.ssh/id_rsa
```

When invoking `ssh-add`, a prompt for the SSH key passphrase will appear, then the `pinentry` program will prompt and confirm a new passphrase to encrypt the converted key within the GPG key store.

The migrated key will be listed in `ssh-add -l`:

```console
$ ssh-add -l
4096 SHA256:... cardno:00060123456 (RSA)
2048 SHA256:... /Users/username/.ssh/id_rsa (RSA)
```

To show the keys with MD5 fingerprints, as used by `gpg-connect-agent`'s `KEYINFO` and `DELETE_KEY` commands:

```console
$ ssh-add -E md5 -l
4096 MD5:... cardno:00060123456 (RSA)
2048 MD5:... /Users/username/.ssh/id_rsa (RSA)
```

When using the key `pinentry` will be invoked to request the key passphrase. The passphrase will be cached for up to 10 idle minutes between uses, up to a maximum of 2 hours.

## Remote Machines (SSH Agent Forwarding)

**Note** SSH Agent Forwarding can [add additional risk](https://matrix.org/blog/2019/05/08/post-mortem-and-remediations-for-apr-11-security-incident/#ssh-agent-forwarding-should-be-disabled) - proceed with caution!

There are two methods for ssh-agent forwarding, one is provided by OpenSSH and the other is provided by GnuPG.

The latter one may be more insecure as raw socket is just forwarded (not like `S.gpg-agent.extra` with only limited functionality; if `ForwardAgent` implemented by OpenSSH is just forwarding the raw socket, then they are insecure to the same degree). But for the latter one, one convenience is that one may forward once and use this agent everywhere in the remote. So again, proceed with caution!

For example, `tmux` does not have some environment variables like `$SSH_AUTH_SOCK` when you ssh into remote and attach an old `tmux` session. In this case if you use `ForwardAgent`, you need to find the socket and `export SSH_AUTH_SOCK=/tmp/ssh-agent-xxx/xxxx.socket` for each shell. But with `S.gpg-agent.ssh` in fixed place, one can just use it as ssh-agent in their shell rc file.

### Use ssh-agent 

In the above steps, you have successfully configured a local ssh-agent.

You should now be able to use `ssh -A remote` on the _local_ machine to log into _remote_, and should then be able to use YubiKey as if it were connected to the remote machine. For example, using e.g. `ssh-add -l` on that remote machine should show the public key from the YubiKey (note `cardno:`).  (If you don't want to have to remember to use `ssh -A`, you can use `ForwardAgent yes` in `~/.ssh/config`.  As a security best practice, always use `ForwardAgent yes` only for a single `Hostname`, never for all servers.)

### Use S.gpg-agent.ssh

First you need to go through [Remote Machines (GPG Agent Forwarding)](#remote-machines-gpg-agent-forwarding), know the conditions for gpg-agent forwarding and know the location of `S.gpg-agent.ssh` on both the local and the remote.

You may use the command:

```console
$ gpgconf --list-dirs agent-ssh-socket
```

Edit `.ssh/config` to add the remote host:

```
Host
  Hostname remote-host.tld
  StreamLocalBindUnlink yes
  RemoteForward /run/user/1000/gnupg/S.gpg-agent.ssh /run/user/1000/gnupg/S.gpg-agent.ssh
  # RemoteForward [remote socket] [local socket]
  # Note that ForwardAgent is not wanted here!
```

After successfully ssh into the remote host, confirm `/run/user/1000/gnupg/S.gpg-agent.ssh` exists.

Then in the *remote* you can type in command line or configure in the shell rc file with:

```console
export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"
```

After sourcing the shell rc file, `ssh-add -l` will return the correct public key.

**Note** In this process no gpg-agent in the remote is involved, hence `gpg-agent.conf` in the remote is of no use. Also pinentry is invoked locally.

### Chained SSH Agent Forwarding

If you use `ssh-agent` provided by OpenSSH and want to forward it into a *third* box, you can just `ssh -A third` on the *remote*.

Meanwhile, if you use `S.gpg-agent.ssh`, assume you have gone through the steps above and have `S.gpg-agent.ssh` on the *remote*, and you would like to forward this agent into a *third* box, first you may need to configure `sshd_config` and `SSH_AUTH_SOCK` of *third* in the same way as *remote*, then in the ssh config of *remote*, add the following lines

```console
Host third
  Hostname third-host.tld
  StreamLocalBindUnlink yes
  RemoteForward /run/user/1000/gnupg/S.gpg-agent.ssh /run/user/1000/gnupg/S.gpg-agent.ssh
  # RemoteForward [remote socket] [local socket]
  # Note that ForwardAgent is not wanted here!
```

The path must be set according to `gpgconf --list-dirs agent-ssh-socket` on *remote* and *third* hosts.

## GitHub

YubiKey can be used to sign commits and tags, and authenticate SSH to GitHub.

Manage SSH and PGP keys in [Settings](https://github.com/settings/keys).

Configure a signing key:

```console
git config --global user.signingkey $KEYID
```

The `user.email` option must match the email address associated with the PGP identity.

To sign commits or tags, use the `-S` option.

**Windows**

To configure authentication:

```console
git config --global core.sshcommand "plink -agent"

git config --global gpg.program 'C:\Program Files (x86)\GnuPG\bin\gpg.exe'
```

Update the repository URL to `git@github.com:USERNAME/repository` and any authenticated commands will be authorized by YubiKey.

**Note** If you encounter the error `gpg: signing failed: No secret key` - run `gpg --card-status` with YubiKey plugged in and try the git command again.

## OpenBSD

Install and enable tools for use with PC/SC drivers, cards, readers, then reboot to recognize YubiKey:

```console
doas pkg_add pcsc-tools

doas rcctl enable pcscd

doas reboot
```

## Windows

Windows can already have some virtual smartcard readers installed, like the one provided for Windows Hello. To ensure YubiKey is the correct one used by scdaemon, add it to its configuration.

Find the YubiKey label using PowerShell:

```powershell
PS C:\WINDOWS\system32> Get-PnpDevice -Class SoftwareDevice | Where-Object {$_.FriendlyName -like "*YubiKey*"} | Select-Object -ExpandProperty FriendlyName
Yubico YubiKey OTP+FIDO+CCID 0
```

See [How to setup Signed Git Commits with a YubiKey NEO and GPG and Keybase on Windows (2018)](https://www.hanselman.com/blog/HowToSetupSignedGitCommitsWithAYubiKeyNEOAndGPGAndKeybaseOnWindows.aspx) for more information.

Edit `%APPDATA%/gnupg/scdaemon.conf` to add:

```console
reader-port <device name, e.g. Yubico YubiKey OTP+FIDO+CCID 0>
```

Edit `%APPDATA%/gnupg/gpg-agent.conf` to add:

```console
enable-ssh-support
enable-putty-support
```

Restart the agent:

```console
gpg-connect-agent killagent /bye
gpg-connect-agent /bye
```

Verify YubiKey details:

```console
gpg --card-status
```

Import the public key and set ultimate trust:

```console
gpg --import <path to public key file>
```

Retrieve the public key id

```console
gpg --list-public-keys
```

Export the SSH public key:

```console
gpg --export-ssh-key <public key id>
```

Copy the public SSH key to a file - it corresponds to the secret key on YubiKey and can be copied to SSH destination hosts.

Create a shortcut that points to `gpg-connect-agent /bye` and place it in the startup folder `shell:startup` to make sure the agent starts after reboot. Modify the shortcut properties so it starts in a "Minimized" window.

PuTTY can now be used for public-key SSH authentication. When the server asks for public-key verification, PuTTY will forward the request to GPG, which will prompt for a PIN and authorize the operation.

### WSL

The goal here is to make the SSH client inside WSL work together with the Windows agent you are using (gpg-agent.exe in our case). Here is what we are going to achieve:
![WSL agent architecture](media/schema_gpg.png)

**Note** this works only for SSH agent forwarding. Real GPG forwarding (encryption/decryption) is actually not supported. See [weasel-pageant](https://github.com/vuori/weasel-pageant) for further information or consider using [wsl2-ssh-pageant](https://github.com/BlackReloaded/wsl2-ssh-pageant) which supports both SSH and GPG agent forwarding.

#### Use ssh-agent or use S.weasel-pageant

One way to forward is just `ssh -A` (still need to eval weasel to setup local ssh-agent), and only relies on OpenSSH. In this track, `ForwardAgent` and `AllowAgentForwarding` in ssh/sshd config may be involved. However, when using gpg ssh socket forwarding, do not enable `ForwardAgent` in ssh config. See [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding) for more information.

Another way is to forward the gpg ssh socket, as described below.

#### Prerequisites

* Ubuntu 16.04 or newer for WSL
* Kleopatra
* [Windows configuration](#windows)

#### WSL configuration

Download or clone [weasel-pageant](https://github.com/vuori/weasel-pageant).

Add `eval $(/mnt/c/<path of extraction>/weasel-pageant -r -a /tmp/S.weasel-pageant)` to shell rc file. Use a named socket here so it can be used in the `RemoteForward` directive of `~/.ssh/config`. Source it with `source ~/.bashrc`.

Display the SSH key with `$ ssh-add -l`

Edit `~/.ssh/config` to add the following for each host you want to use agent forwarding:

```
RemoteForward <remote SSH socket path> /tmp/S.weasel-pageant
```

**Note** The remote SSH socket path can be found with `gpgconf --list-dirs agent-ssh-socket`

#### Remote host configuration

You may have to add the following to the shell rc file. 

```
export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
```

Add the following to `/etc/ssh/sshd_config`:

```
StreamLocalBindUnlink yes
```

And reload the SSH daemon (e.g., `sudo service sshd reload`).

Unplug YubiKey, disconnect or reboot. Log back into Windows, open a WSL console and enter `ssh-add -l` - you should see nothing.

Plug in YubiKey, enter the same command to display the ssh key.

Log into the remote host, you should have the pinentry dialog asking for the YubiKey pin.

On the remote host, type `ssh-add -l` - if you see the ssh key, that means forwarding works!

**Note** Agent forwarding may be chained through multiple hosts - just follow the same [protocol](#remote-host-configuration) to configure each host. You may also read this part on [chained ssh agent forwarding](#chained-ssh-agent-forwarding).

## macOS

To use gui applications on macOS, [a little bit more setup is needed](https://jms1.net/yubikey/make-ssh-use-gpg-agent.md).

Create `$HOME/Library/LaunchAgents/gnupg.gpg-agent.plist` with the following contents:

```
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
  "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
    <dict>
        <key>Label</key>
        <string>gnupg.gpg-agent</string>
        <key>RunAtLoad</key>
        <true/>
        <key>KeepAlive</key>
        <false/>
        <key>ProgramArguments</key>
        <array>
            <string>/usr/local/MacGPG2/bin/gpg-connect-agent</string>
            <string>/bye</string>
        </array>
    </dict>
</plist>
```

```console
launchctl load $HOME/Library/LaunchAgents/gnupg.gpg-agent.plist
```

Create `$HOME/Library/LaunchAgents/gnupg.gpg-agent-symlink.plist` with the following contens:

```
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/ProperyList-1.0/dtd">
<plist version="1.0">
    <dict>
        <key>Label</key>
        <string>gnupg.gpg-agent-symlink</string>
        <key>ProgramArguments</key>
        <array>
            <string>/bin/sh</string>
            <string>-c</string>
            <string>/bin/ln -sf $HOME/.gnupg/S.gpg-agent.ssh $SSH_AUTH_SOCK</string>
        </array>
        <key>RunAtLoad</key>
        <true/>
    </dict>
</plist>
```

```console
launchctl load $HOME/Library/LaunchAgents/gnupg.gpg-agent-symlink.plist
```

You will need to either reboot, or log out and log back in, in order to activate these changes.

# Remote Machines (GPG Agent Forwarding)

This section is different from ssh-agent forwarding in [SSH](#ssh) as gpg-agent forwarding has a broader usage, not only limited to ssh.

To use YubiKey to sign a git commit on a remote host, or signing email/decrypt files on a remote host, configure and use GPG Agent Forwarding. To ssh through another network, especially to push to/pull from GitHub using ssh, see [Remote Machines (SSH Agent forwarding)](#remote-machines-ssh-agent-forwarding) for more information.

To do this, you need access to the remote machine and the YubiKey has to be set up on the host machine.

After gpg-agent forwarding, it is nearly the same as if YubiKey was inserted in the remote. Hence configurations except `gpg-agent.conf` for the remote can be the same as those for the local.

**Important** `gpg-agent.conf` for the remote is of no use, hence `$GPG_TTY` is of no use too for the remote. The mechanism is that after forwarding, remote `gpg` directly communicates with `S.gpg-agent` without *starting* `gpg-agent` on the remote.

On the remote machine, edit `/etc/ssh/sshd_config` to set `StreamLocalBindUnlink yes`

**Optional** If you do not have root access to the remote machine to edit `/etc/ssh/sshd_config`, you will need to remove the socket (located at `gpgconf --list-dir agent-socket`) on the remote machine before forwarding works. For example, `rm /run/user/1000/gnupg/S.gpg-agent`. Further information can be found on the [AgentForwarding GNUPG wiki page](https://wiki.gnupg.org/AgentForwarding).

Import public keys to the remote machine. This can be done by fetching from a keyserver. On the local machine, copy the public keyring to the remote machine:

```console
scp ~/.gnupg/pubring.kbx remote:~/.gnupg/
```

On modern distributions, such as Fedora 30, there is typically no need to also set `RemoteForward` in `~/.ssh/config` as detailed in the next chapter, because the right thing happens automatically.

If any error happens (or there is no `gpg-agent.socket` in the remote) for modern distributions, you may go through the configuration steps in the next section.

## Steps for older distributions

On the local machine, run:

```console
gpgconf --list-dirs agent-extra-socket
```

This should return a path to agent-extra-socket - `/run/user/1000/gnupg/S.gpg-agent.extra` - though on older Linux distros (and macOS) it may be `/home/<user>/.gnupg/S/gpg-agent.extra`

Find the agent socket on the **remote** machine:

```console
gpgconf --list-dirs agent-socket
```

This should return a path such as `/run/user/1000/gnupg/S.gpg-agent`

Finally, enable agent forwarding for a given machine by adding the following to the local machine's ssh config file `~/.ssh/config` (your agent sockets may be different):

```
Host
  Hostname remote-host.tld
  StreamLocalBindUnlink yes
  RemoteForward /run/user/1000/gnupg/S.gpg-agent /run/user/1000/gnupg/S.gpg-agent.extra
  # RemoteForward [remote socket] [local socket]
```

If you're still having problems, it may be necessary to edit `gpg-agent.conf` file on the *local* machines to add the following information:

```
pinentry-program /usr/bin/pinentry-gtk-2
extra-socket /run/user/1000/gnupg/S.gpg-agent.extra
```

**Note** The pinentry program starts on *local* machine, not remote. Hence when there are needs to enter the pin you need to find the prompt on the local machine.

**Important** Any pinentry program except `pinentry-tty` or `pinentry-curses` may be used. This is because local `gpg-agent` may start headlessly (By systemd without `$GPG_TTY` set locally telling which tty it is on), thus failed to obtain the pin. Errors on the remote may be misleading saying that there is *IO Error*. (Yes, internally there is actually an *IO Error* since it happens when writing to/reading from tty while finding no tty to use, but for end users this is not friendly.)

See [Issue #85](https://github.com/drduh/YubiKey-Guide/issues/85) for more information and troubleshooting.

## Chained GPG Agent Forwarding

Assume you have gone through the steps above and have `S.gpg-agent` on the *remote*, and you would like to forward this agent into a *third* box, first you may need to configure `sshd_config` of *third* in the same way as *remote*, then in the ssh config of *remote*, add the following lines:

```console
Host third
  Hostname third-host.tld
  StreamLocalBindUnlink yes
  RemoteForward /run/user/1000/gnupg/S.gpg-agent /run/user/1000/gnupg/S.gpg-agent
  # RemoteForward [remote socket] [local socket]
```

You should change the path according to `gpgconf --list-dirs agent-socket` on *remote* and *third*.

**Note** On *local* you have `S.gpg-agent.extra` whereas on *remote* and *third*, you only have `S.gpg-agent`

# Using Multiple Keys

To use a single identity with multiple YubiKeys - or to replace a lost card with another - issue this command to switch keys:

```console
gpg-connect-agent "scd serialno" "learn --force" /bye
```

Alternatively, use a script to delete the GnuPG shadowed key, where the card serial number is stored (see [GnuPG #T2291](https://dev.gnupg.org/T2291)):

```console
cat >> ~/scripts/remove-keygrips.sh <<EOF
#!/usr/bin/env bash
(( $# )) || { echo "Specify a key." >&2; exit 1; }
KEYGRIPS=$(gpg --with-keygrip --list-secret-keys "$@" | awk '/Keygrip/ { print $3 }')
for keygrip in $KEYGRIPS
do
    rm "$HOME/.gnupg/private-keys-v1.d/$keygrip.key" 2> /dev/null
done

gpg --card-status
EOF

chmod +x ~/scripts/remove-keygrips.sh

~/scripts/remove-keygrips.sh $KEYID
```

See discussion in Issues [#19](https://github.com/drduh/YubiKey-Guide/issues/19) and [#112](https://github.com/drduh/YubiKey-Guide/issues/112) for more information and troubleshooting steps.

# Adding an identity

To add an identity after creating and backing up a YubiKey, first add the identity to the Certify key, and then reset YubiKey and use `keytocard` to move the Subkeys to the card again.

Follow the same process as generating keys: boot to a secure environment, install required software and disconnect networking.

Connect the portable storage device with the Certify key and identify the disk label:

```console
$ sudo dmesg | tail
mmc0: new high speed SDHC card at address a001
mmcblk0: mmc0:a001 SS16G 14.8 GiB (ro)
mmcblk0: p1 p2
```

Decrypt and mount the encrypted volume:

```console
sudo cryptsetup luksOpen /dev/mmcblk0p1 secret

sudo mount /dev/mapper/secret /mnt/encrypted-storage
```

Restore the backup to a temporary directory:

```console
export GNUPGHOME=$(mktemp -d -t gnupg_$(date +%Y%m%d%H%M)_XXX)

cp -avi /mnt/encrypted-storage/tmp.XXX/* $GNUPGHOME
```

Edit the Certify key to add the new identity:

```console
$ KEYID=<your keyID>

$ gpg --expert --edit-key $KEYID

gpg> adduid
Real name:
Email address:
Comment:

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O

gpg> trust

Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)

  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu

Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y

gpg> save
```

Export Certify and Subkeys again:

```console
gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/certify.key

gpg --armor --export-secret-subkeys $KEYID > $GNUPGHOME/subkeys.key
```

Export the public key:

```console
gpg --armor --export $KEYID | sudo tee /mnt/public/gpg-$KEYID-$(date +%F).asc
```

As before, on Windows, note that using any extension other than `.gpg` or attempting IO redirection to a file will garble the secret key, making it impossible to import it again at a later date:

```console
gpg -o \path\to\dir\certify.gpg --armor --export-secret-keys $KEYID

gpg -o \path\to\dir\subkeys.gpg --armor --export-secret-subkeys $KEYID

gpg -o \path\to\dir\pubkey.gpg --armor --export $KEYID
```

Copy the **new** temporary working directory to encrypted storage, which should still be mounted:

```console
sudo cp -avi $GNUPGHOME /mnt/encrypted-storage
```

Unmount and close the encrypted volume:

```console
sudo umount /mnt/encrypted-storage

sudo cryptsetup luksClose /dev/mapper/secret
```

## Updating YubiKey

Now that keys have been updated with the new identity, they will need to be loaded to YubiKey.

First, [Reset](#reset) the OpenPGP applet, then follow the steps to [Configure Smartcard](#configure-smartcard) again.

Next, [Transfer Keys](#transfer-keys) and reboot or securely erase the GPG temporary working directory.

Finally, re-import the public key, as described in [Using Keys](#using-keys).

Use `gpg -K` to verify the identity is listed.

# Require touch

**Note** This is not possible on YubiKey NEO.

By default, YubiKey will perform encryption, signing and authentication operations without requiring any action from the user after the key is plugged in and unlocked once with the PIN.

To require a touch for each key operation, install [YubiKey Manager](https://developers.yubico.com/yubikey-manager/) and recall the Admin PIN:

Authentication:

```console
ykman openpgp keys set-touch aut on
```

Signing:

```console
ykman openpgp keys set-touch sig on
```

Encryption:

```console
ykman openpgp keys set-touch dec on
```

**Note** Versions of YubiKey Manager before 5.1.0 use `enc` instead of `dec` for encryption.

**Note** Older versions of YubiKey Manager use `touch` instead of `set-touch`

Depending on how the YubiKey is going to be used, you may want to look at the policy options for each of these and adjust the above commands accordingly. They can be viewed with the following command:

```
$ ykman openpgp keys set-touch -h
Usage: ykman openpgp keys set-touch [OPTIONS] KEY POLICY

  Set the touch policy for OpenPGP keys.

  The touch policy is used to require user interaction for all operations using the private key on the YubiKey. The touch policy is set
  individually for each key slot. To see the current touch policy, run the "openpgp info" subcommand.

  Touch policies:

  Off (default)   no touch required
  On              touch required
  Fixed           touch required, can't be disabled without deleting the private key
  Cached          touch required, cached for 15s after use
  Cached-Fixed    touch required, cached for 15s after use, can't be disabled
                  without deleting the private key

  KEY     key slot to set (sig, dec, aut or att)
  POLICY  touch policy to set (on, off, fixed, cached or cached-fixed)

Options:
  -a, --admin-pin TEXT  Admin PIN for OpenPGP
  -f, --force           confirm the action without prompting
  -h, --help            show this message and exit
```

If the YubiKey is going to be used within an email client that opens and verifies encrypted mail, `Cached` or `Cached-Fixed` may be desirable.

YubiKey will blink when it is waiting for a touch. On Linux you can also use [yubikey-touch-detector](https://github.com/maximbaz/yubikey-touch-detector) to have an indicator or notification that YubiKey is waiting for a touch.

# Email

GPG keys on YubiKey can be used with ease to encrypt and/or sign emails and attachments using [Thunderbird](https://www.thunderbird.net/), [Enigmail](https://www.enigmail.net) and [Mutt](http://www.mutt.org/). Thunderbird supports OAuth 2 authentication and can be used with Gmail. See [this guide](https://ssd.eff.org/en/module/how-use-pgp-linux) from EFF for detailed instructions. Mutt has OAuth 2 support since version 2.0.

## Mailvelope

[Mailvelope](https://www.mailvelope.com/en) allows GPG keys on YubiKey to be used with Gmail and others.

**Important** Mailvelope [does not work](https://github.com/drduh/YubiKey-Guide/issues/178) with the `throw-keyids` option set in `gpg.conf`.

On macOS, install gpgme using Homebrew:

```console
brew install gpgme
```

To allow Chrome to run gpgme, edit `~/Library/Application\ Support/Google/Chrome/NativeMessagingHosts/gpgmejson.json` and add:

```json
{
    "name": "gpgmejson",
    "description": "Integration with GnuPG",
    "path": "/usr/local/bin/gpgme-json",
    "type": "stdio",
    "allowed_origins": [
        "chrome-extension://kajibbejlbohfaggdiogboambcijhkke/"
    ]
}
```

Edit the default path to allow Chrome to find GPG:

```console
sudo launchctl config user path /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
```

Finally, install the [Mailvelope extension](https://chrome.google.com/webstore/detail/mailvelope/kajibbejlbohfaggdiogboambcijhkke) from the Chrome app store.

## Mutt

Mutt has both CLI and TUI interfaces - the latter provides powerful functions for processing email. In addition, PGP can be integrated such that cryptographic operations can be done without leaving TUI.

To enable GnuPG support, copy `/usr/share/doc/mutt/samples/gpg.rc`

Edit the file to enable options `pgp_default_key`, `pgp_sign_as` and `pgp_autosign`

`source` the file in `muttrc`

**Important** `pinentry-tty` set as the pinentry program in `gpg-agent.conf` is reported to cause problems with Mutt TUI, because it uses curses. It is recommended to use `pinentry-curses` or other graphic pinentry program instead.

# Reset

If PIN attempts are exceeded, the YubiKey is locked and must be [Reset](https://developers.yubico.com/ykneo-openpgp/ResetApplet.html) and set up again using the encrypted backup.

Copy the following script to a file and run `gpg-connect-agent -r $file` to lock and terminate the card. Then re-insert YubiKey to reset.

```console
/hex
scd serialno
scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40
scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40
scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40
scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40
scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40
scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40
scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40
scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40
scd apdu 00 e6 00 00
scd apdu 00 44 00 00
/echo Card has been successfully reset.
```

Or use `ykman` (sometimes in `~/.local/bin/`):

```console
$ ykman openpgp reset
WARNING! This will delete all stored OpenPGP keys and data and restore factory settings? [y/N]: y
Resetting OpenPGP data, don't remove your YubiKey...
Success! All data has been cleared and default PINs are set.
PIN:         123456
Reset code:  NOT SET
Admin PIN:   12345678
```

## Recovery after reset

To reinstate YubiKey from the Certify key backup (such as the one on encrypted portable storage described in [Backup](#backup)), follow [Rotating keys](#rotating-keys) to setup the environment, then [Configure Smartcard](#configure-smartcard).

# Notes

- YubiKey has two configurations, invoked with either a short or long press. By default, the short-press mode is configured for HID OTP; a brief touch will emit an OTP string starting with `cccccccc`. If the OTP mode is not used, swap it to the second configuration via the YubiKey Personalization tool or disable it entirely using [YubiKey Manager](https://developers.yubico.com/yubikey-manager). The command to disable OTP with ykman is `ykman config usb -d OTP`

- Programming YubiKey for GPG keys still lets allows use of other configurations - [U2F](https://en.wikipedia.org/wiki/Universal_2nd_Factor), [OTP](https://www.yubico.com/faq/what-is-a-one-time-password-otp/) and [static password](https://www.yubico.com/products/services-software/personalization-tools/static-password/) modes, for example.

- Setting an expiry essentially forces lifecycle management of Subkeys. Setting an expiry on a primary key is ineffective for protecting the key from loss, because whomever has the primary key can simply extend its expiry period. Revocation certificates are [better suited](https://security.stackexchange.com/questions/14718/does-openpgp-key-expiration-add-to-security/79386#79386) for this purpose. It may be appropriate for your use case to set expiry dates on subkeys.

- To switch between two or more identities on different YubiKeys, unplug the first YubiKey and restart gpg-agent, ssh-agent and pinentry with `pkill gpg-agent ; pkill ssh-agent ; pkill pinentry ; eval $(gpg-agent --daemon --enable-ssh-support)`, then plug in the other key and run `gpg-connect-agent updatestartuptty /bye`

- To use YubiKey on multiple computers, import the corresponding public keys on them. Confirm gpg can see the card via `gpg --card-status`, then trust the import public keys ultimately. `gpg --list-secret-keys` should show the correct and trusted key.

# Troubleshooting

- Use `man gpg` to understand GPG options and command-line flags.

- To get more information on potential errors, restart the `gpg-agent` process with debug output to the console with `pkill gpg-agent; gpg-agent --daemon --no-detach -v -v --debug-level advanced --homedir ~/.gnupg`.

- If you encounter problems connecting to YubiKey with GPG - try unplugging and re-inserting YubiKey, and restarting the `gpg-agent` process.

- If you receive the error, `gpg: decryption failed: secret key not available` - you likely need to install GnuPG version 2.x. Another possibility is that there is a problem with the PIN, e.g. it is too short or blocked.

- If you receive the error, `Yubikey core error: no yubikey present` - make sure the YubiKey is inserted correctly. It should blink once when plugged in.

- If you still receive the error, `Yubikey core error: no yubikey present` - you likely need to install newer versions of yubikey-personalize as outlined in [Required software](#required-software).

- If you receive the error, `Yubikey core error: write error` - YubiKey is likely locked. Install and run yubikey-personalization-gui to unlock it.

- If you receive the error, `Key does not match the card's capability` - you likely need to use 2048 bit RSA key sizes.

- If you receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - make sure you replaced `ssh-agent` with `gpg-agent` as noted above.

- If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - [run the command](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=835394) `gpg-connect-agent updatestartuptty /bye`

- If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - edit `~/.gnupg/gpg-agent.conf` to set a valid `pinentry` program path. `gpg: decryption failed: No secret key` could also indicate an invalid `pinentry` path

- If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - it is a [known issue](https://bbs.archlinux.org/viewtopic.php?id=274571) that openssh 8.9p1 and higher has issues with YubiKey. Adding `KexAlgorithms -sntrup761x25519-sha512@openssh.com` to `/etc/ssh/ssh_config` often resolves the issue.

- If you receive the error, `The agent has no identities` from `ssh-add -L`, make sure you have installed and started `scdaemon`.

- If you receive the error, `Error connecting to agent: No such file or directory` from `ssh-add -L`, the UNIX file socket that the agent uses for communication with other processes may not be set up correctly. On Debian, try `export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"`. Also see that `gpgconf --list-dirs agent-ssh-socket` is returning single path, to existing `S.gpg-agent.ssh` socket.

- If you receive the error, `Permission denied (publickey)`, increase ssh verbosity with the `-v` flag and ensure the public key from the card is being offered: `Offering public key: RSA SHA256:abcdefg... cardno:00060123456`. If it is, ensure you are connecting as the right user on the target system, rather than as the user on the local system. Otherwise, be sure `IdentitiesOnly` is not [enabled](https://github.com/FiloSottile/whosthere#how-do-i-stop-it) for this host.

- If SSH authentication still fails - add up to 3 `-v` flags to the `ssh` client to increase verbosity.

- If it still fails, it may be useful to stop the background `sshd` daemon process service on the server (e.g. using `sudo systemctl stop sshd`) and instead start it in the foreground with extensive debugging output, using `/usr/sbin/sshd -eddd`. Note that the server will not fork and will only process one connection, therefore has to be re-started after every `ssh` test.

- If you receive the error, `Please insert the card with serial number` see [Using Multiple Keys](#using-multiple-keys).

- If you receive the error, `There is no assurance this key belongs to the named user` or `encryption failed: Unusable public key` use `gpg --edit-key` to set `trust` to `5 = I trust ultimately`.

- If, when you try the above command, you get the error `Need the secret key to do this` - specify trust for the key in `~/.gnupg/gpg.conf` by using the `trust-key [key ID]` directive.

- If, when using a previously provisioned YubiKey on a new computer with `pass`, you see the following error on `pass insert`, you need to adjust the trust associated with the key. See the note above.

```
gpg: 0x0000000000000000: There is no assurance this key belongs to the named user
gpg: [stdin]: encryption failed: Unusable public key
```

- If you receive the error, `gpg: 0x0000000000000000: skipped: Unusable public key`, `signing failed: Unusable secret key`, or `encryption failed: Unusable public key` the Subkey may be expired and can no longer be used to encrypt nor sign messages. It can still be used to decrypt and authenticate, however.

- If you lost your GPG public key, follow [this guide](https://www.nicksherlock.com/2021/08/recovering-lost-gpg-public-keys-from-your-yubikey/) to recover it from YubiKey.

- Refer to Yubico article [Troubleshooting Issues with GPG](https://support.yubico.com/hc/en-us/articles/360013714479-Troubleshooting-Issues-with-GPG) for additional guidance.

- If, when you try the above `--card-status` command, you get receive the error, `gpg: selecting card failed: No such device` or `gpg: OpenPGP card not available: No such device`, it's possible that the latest release of pcscd is now requires polkit rules to operate properly. Create the following file to allow users in the `wheel` group to use the card. Be sure to restart pcscd when you're done to allow the new rules to take effect.
```
cat << EOF >  /etc/polkit-1/rules.d/99-pcscd.rules
polkit.addRule(function(action, subject) {
        if (action.id == "org.debian.pcsc-lite.access_card" &&
                subject.isInGroup("wheel")) {
                return polkit.Result.YES;
        }
});
polkit.addRule(function(action, subject) {
        if (action.id == "org.debian.pcsc-lite.access_pcsc" &&
                subject.isInGroup("wheel")) {
                return polkit.Result.YES;
        }
});
EOF
```

# Alternatives

* [`smlx/piv-agent`](https://github.com/smlx/piv-agent) is an SSH and GPG agent which you can use with your PIV hardware security device (e.g. a Yubikey).
* [`keytotpm`](https://www.gnupg.org/documentation/manuals/gnupg/OpenPGP-Key-Management.html) is an option to use GnuPG with TPM systems.

## Create keys with batch

Keys can also be generated using template files and the `batch` parameter - see [GnuPG documentation](https://www.gnupg.org/documentation/manuals/gnupg/Unattended-GPG-key-generation.html).

Use the example [gen-params-rsa4096](contrib/gen-params-rsa4096) or [gen-params-ed25519](contrib/gen-params-ed25519) template (the latter requires GnuPG v2.1.7).

Generate the Certify key:

```console
$ gpg --batch --generate-key gen-params-rsa4096
gpg: Generating a basic OpenPGP key
gpg: key 0xEA5DE91459B80592 marked as ultimately trusted
gpg: revocation certificate stored as '/tmp.FLZC0xcM/openpgp-revocs.d/D6F924841F78D62C65ABB9588B461860159FFB7B.rev'
gpg: done
```

Verify the result:

```console
$ gpg --list-key
gpg: checking the trustdb
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
/tmp.FLZC0xcM/pubring.kbx
-------------------------------
pub   rsa4096/0xFF3E7D88647EBCDB 2021-08-22 [C]
       Key fingerprint = 011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB
uid                   [ultimate] Dr Duh <doc@duh.to>
```

The fingerprint (`011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB`) is used to create the three Subkeys for signing, authentication and encryption.

Use a one or several year expiration for Subkeys - they can be renewed using the Certify key, see [rotating keys](#rotating-keys).

Create a [signing subkey](https://stackoverflow.com/questions/5421107/can-rsa-be-both-used-as-encryption-and-signature/5432623#5432623):

```console
gpg --quick-add-key "011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB" \
  rsa4096 sign 1y
```

Now create an [encryption subkey](https://www.cs.cornell.edu/courses/cs5430/2015sp/notes/rsa_sign_vs_dec.php):

```console
gpg --quick-add-key "011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB" \
  rsa4096 encrypt 1y
```

Finally, create an [authentication subkey](https://superuser.com/questions/390265/what-is-a-gpg-with-authenticate-capability-used-for):

```console
gpg --quick-add-key "011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB" \
  rsa4096 auth 1y
```

Continue with the Verify section of this guide.

# Additional resources

* [Yubico - PGP](https://developers.yubico.com/PGP/)
* [Yubico - PGP Card edit](https://developers.yubico.com/PGP/Card_edit.html)
* [Yubico - Yubikey Personalization](https://developers.yubico.com/yubikey-personalization/)
* [A Visual Explanation of GPG Subkeys (2022)](https://rgoulter.com/blog/posts/programming/2022-06-10-a-visual-explanation-of-gpg-subkeys.html)
* [dhess/nixos-yubikey](https://github.com/dhess/nixos-yubikey)
* [lsasolutions/makegpg](https://gitlab.com/lsasolutions/makegpg)
* [Trammell Hudson - Yubikey (2020)](https://trmm.net/Yubikey)
* [Yubikey forwarding SSH keys (2019)](https://blog.onefellow.com/post/180065697833/yubikey-forwarding-ssh-keys)
* [GPG Agent Forwarding (2018)](https://mlohr.com/gpg-agent-forwarding/)
* [Stick with security: YubiKey, SSH, GnuPG, macOS (2018)](https://evilmartians.com/chronicles/stick-with-security-yubikey-ssh-gnupg-macos)
* [PGP and SSH keys on a Yubikey NEO (2015)](https://www.esev.com/blog/post/2015-01-pgp-ssh-key-on-yubikey-neo/)
* [Offline GnuPG Master Key and Subkeys on YubiKey NEO Smartcard (2014)](https://blog.josefsson.org/2014/06/23/offline-gnupg-master-key-and-subkeys-on-yubikey-neo-smartcard/)
* [Creating the perfect GPG keypair (2013)](https://alexcabal.com/creating-the-perfect-gpg-keypair/)
* [GPG and SSH with Yubikey NEO (2013)](https://blog.habets.se/2013/02/GPG-and-SSH-with-Yubikey-NEO)
* [Riseup - OpenPGP Best Practices](https://help.riseup.net/en/security/message-security/openpgp/best-practices)