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 secure 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. YubiKey can be configured to require a physical touch for cryptographic operations, reducing the risk of credential compromise. **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) * [OneRNG](#onerng) - [Generate keys](#generate-keys) * [Temporary working directory](#temporary-working-directory) * [Hardened configuration](#hardened-configuration) - [Certify key](#certify-key) - [Subkeys](#subkeys) * [Signature key](#signature-key) * [Encryption key](#encryption-key) * [Authentication key](#authentication-key) - [Verify](#verify) - [Export secret keys](#export-secret-keys) - [Revocation certificate](#revocation-certificate) - [Backup](#backup) - [Export public key](#export-public-key) - [Configure YubiKey](#configure-yubikey) * [Enable KDF](#enable-kdf) * [Change PIN](#change-pin) * [Set information](#set-information) - [Transfer keys](#transfer-keys) * [Signature key](#signature-key-1) * [Encryption key](#encryption-key-1) * [Authentication key](#authentication-key-1) - [Verify transfer](#verify-transfer) - [Multiple YubiKeys](#multiple-yubikeys) * [Switching between YubiKeys](#switching-between-yubikeys) - [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) * [macOS](#macos-1) - [Remote Machines (GPG Agent Forwarding)](#remote-machines-gpg-agent-forwarding) * [Steps for older distributions](#steps-for-older-distributions) * [Chained GnuPG agent forwarding](#chained-gnupg-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 more information. To [verify a YubiKey](https://support.yubico.com/hc/en-us/articles/360013723419-How-to-Confirm-Your-Yubico-Device-is-Genuine), visit [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 the site to see the make and model of the device. This device attestation may help 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). Several portable storage devices (such as microSD cards) for storing encrypted backups are also recommended. # Prepare environment To generate cryptographic keys, creating a dedicated secure environment is recommended. The following is a general ranking of environments most to least likely to be compromised: 1. Daily-use system with unrestricted network access 1. Virtual machine on daily-use host OS (using [virt-manager](https://virt-manager.org/), VirtualBox or VMware) 1. Dedicated and hardened [Debian](https://www.debian.org/) or [OpenBSD](https://www.openbsd.org/) system 1. Live image, such as [Debian Live](https://www.debian.org/CD/live/) or [Tails](https://tails.boum.org/index.en.html) 1. Hardened hardware and firmware ([Coreboot](https://www.coreboot.org/), [Intel ME removed](https://github.com/corna/me_cleaner)) 1. Dedicated air-gapped system without network 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)" ``` Download the Debian signing public 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 "` must appear in the output. Verify 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: 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 the correct 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](#hardened-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 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. ## OneRNG Configure [rng-tools](https://wiki.archlinux.org/title/Rng-tools): ```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 ``` Insert 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) ``` ## Hardened 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. The Certify key should be kept offline at all times and only accessed from a secure environment to revoke or issue Subkeys. Keys can also be generated on the YubiKey itself to avoid duplication, however for usability and durability reasons this guide recommends against doing so. Generate a passphrase which will be needed throughout the guide to create Subkeys. The passphrase should be memorized or written down in a secure location, ideally separate from the portable storage device used for key material. The passphrase is recommended to consist of only upper case letters and numbers for improved readability. The following command will 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 " " "-" ``` Example output: ```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 passphrase using the mouse or by double-clicking on it to copy to clipboard. Paste using the middle mouse button or `Shift`-`Insert` Generate the Certify key with GnuPG: ```console gpg --expert --full-generate-key ``` Select `(8) RSA (set your own capabilities)`, then type `E` and `S` to deselect **Encrypt** and **Sign** actions, so the only **Current allowed actions** remaining is **Certify**: ```console 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 ``` Type `Q` then `4096` as the requested keysize: ```console 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 ``` Type `0` for key validity - there is no reason to expire the Certify key (see [Note #3](#notes)) - then type `y` to confirm. ```console Please specify how long the key should be valid. 0 = key does not expire = key expires in n days w = key expires in n weeks m = key expires in n months 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 value for **Real name** and **Email address**; **Comment** is optional, then type `O` to confirm: ```console GnuPG needs to construct a user ID to identify your key. Real name: YubiKey User Email address: yubikey@example Comment: You selected this USER-ID: "YubiKey User " 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/gnupg_202401011200_TnL/trustdb.gpg: trustdb created gpg: directory '/tmp/gnupg_202401011200_TnL/openpgp-revocs.d' created gpg: revocation certificate stored as '/tmp/gnupg_202401011200_TnL/openpgp-revocs.d/4E2C1FA3372CBA96A06AC34AF0F2CFEB04341FB5.rev' public and secret key created and signed. pub rsa4096/0xF0F2CFEB04341FB5 2024-01-01 [C] Key fingerprint = 4E2C 1FA3 372C BA96 A06A C34A F0F2 CFEB 0434 1FB5 uid YubiKey User ``` Copy the Certify key identifier beginning with `0x` from the output and export it as a [variable](https://stackoverflow.com/questions/1158091/defining-a-variable-with-or-without-export/1158231#1158231) (`KEYID`): ```console export KEYID=0xF0F2CFEB04341FB5 ``` **Optional** Existing keys may be used to sign new ones to prove ownership. Export the existing key to the working keyring and sign the new key: ```console gpg --export-secret-keys --armor --output /tmp/new.sec gpg --default-key $OLDKEY --sign-key $KEYID ``` # Subkeys Edit the identity to add Subkeys: ```console gpg --expert --edit-key $KEYID ``` RSA with 4096-bit key length is recommended. Subkeys are recommended to have one or several year expirations. They must be renewed or replaced using the Certify key - see [Rotating keys](#rotating-keys). ## Signature key Generate a Signature key by typing `addkey` then `4` to select the `(4) RSA (sign only)` option: ```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 (14) Existing key from card Your selection? 4 RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (3072) 4096 Requested keysize is 4096 bits Please specify how long the key should be valid. 0 = key does not expire = key expires in n days w = key expires in n weeks m = key expires in n months y = key expires in n years Key is valid for? (0) 2y 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 rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S [ultimate] (1). YubiKey User ``` ## Encryption key Generate an Encryption key by typing `addkey` then `6` to select the `(6) RSA (encrypt only)` option: ```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 (14) Existing key from card Your selection? 6 RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (3072) 4096 Requested keysize is 4096 bits Please specify how long the key should be valid. 0 = key does not expire = key expires in n days w = key expires in n weeks m = key expires in n months y = key expires in n years Key is valid for? (0) 2y 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 rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S ssb rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 usage: E [ultimate] (1). YubiKey User ``` ## Authentication key Generate an Authentication key by typing `addkey` then `8` to select the `(8) RSA (set your own capabilities)` option. Toggle the required capabilities with `S`, `E` and `A` until **Authenticate** is the only allowed action: ```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 (14) Existing key from card 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 = key expires in n days w = key expires in n weeks m = key expires in n months y = key expires in n years Key is valid for? (0) 2y 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 rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S ssb rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 usage: E ssb rsa4096/0xAD9E24E1B8CB9600 created: 2024-01-01 expires: 2026-01-01 usage: A [ultimate] (1). YubiKey User ``` Finish by saving Subkeys: ```console gpg> save gpg> quit ``` **Optional** To add additional email addresses or identities, use `adduid` Edit the identity: ```console gpg --expert --edit-key $KEYID ``` Add the new identity: ```console gpg> adduid Real name: YubiKey User Email address: yubikey@somewhere Comment: You selected this USER-ID: "YubiKey User " Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o sec rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S ssb rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 usage: E ssb rsa4096/0xAD9E24E1B8CB9600 created: 2024-01-01 expires: 2026-01-01 usage: A [ultimate] (1) YubiKey User [ unknown] (2). YubiKey User ``` Then configure ultimate trust for the new identity: ```console gpg> trust [...] Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y [...] gpg> save ``` By default, the latest identity added will be the primary user ID. Select `uid 2` or equivalent and `primary` to change it. # Verify List available secret keys: ```console gpg -K ``` The output will display Certify, Signature, Encryption and Authentication keys, for example: ```console sec rsa4096/0xF0F2CFEB04341FB5 2024-01-01 [C] Key fingerprint = 4E2C 1FA3 372C BA96 A06A C34A F0F2 CFEB 0434 1FB5 uid [ultimate] YubiKey User ssb rsa4096/0xB3CD10E502E19637 2024-01-01 [S] [expires: 2026-01-01] ssb rsa4096/0x30CBE8C4B085B9F7 2024-01-01 [E] [expires: 2026-01-01] ssb rsa4096/0xAD9E24E1B8CB9600 2024-01-01 [A] [expires: 2026-01-01] ``` **Optional** 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 ``` 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 version 2.2.1 notes "subkey 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). # Export secret keys 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 the Certify key may be backed up to a secure place, the possibility of losing it cannot be ruled out. Without the Certify key, it will be impossible to renew or rotate Subkeys or generate a revocation certificate; the PGP identity will be useless. To mitigate this risk, deprecate an orphaned identity with a revocation certificate. To create one: ``` console gpg --output $GNUPGHOME/revoke.asc --gen-revoke $KEYID ``` The `revoke.asc` file should be stored in a secondary location to the primary backup. # Backup Create an **encrypted** backup on portable storage to be kept offline in a secure and durable location. **Tip** The [ext2](https://en.wikipedia.org/wiki/Ext2) filesystem without encryption can be mounted on Linux and OpenBSD. Use [FAT32](https://en.wikipedia.org/wiki/Fat32) or [NTFS](https://en.wikipedia.org/wiki/Ntfs) filesystem for macOS and Windows compatibility instead. As an additional backup measure, use [Paperkey](https://www.jabberwocky.com/software/paperkey/) to make a physical copy of materials. See [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) for more information. 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). 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). 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 unique passphrase 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** To set up multiple keys, keep the backup mounted or remember to terminate the GnuPG 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: 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** To set up multiple YubiKeys, keep the backup mounted or remember to terminate GnuPG 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 key **Important** Without the public key, it will **not** be possible to use GnuPG to decrypt and sign messages. However, YubiKey can still be used 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** Provision the portable storage device: ```console $ sudo fdisk /dev/mmcblk0 Welcome to fdisk (util-linux 2.36.1). 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. ``` Create a filesystem and export the public key to it: ```console sudo mkfs.ext2 /dev/mmcblk0p2 sudo mkdir /mnt/public sudo mount /dev/mmcblk0p2 /mnt/public/ gpg --armor --export $KEYID | sudo tee /mnt/public/$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. ``` Create a filesystem and export the public key to it: ```console doas newfs sd2b doas mkdir /mnt/public doas mount /dev/sd2b /mnt/public gpg --armor --export $KEYID | doas tee /mnt/public/$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: ```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 $KEYID | curl -T - https://keys.openpgp.org ``` The public key URL can also be added to YubiKey (based on [Shaw 2003](https://datatracker.ietf.org/doc/html/draft-shaw-openpgp-hkp-00)): ```console URL="hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=${KEYID}" ``` Edit YubiKey with `gpg --edit-card` and the Admin PIN: ```console gpg/card> admin gpg/card> url URL to retrieve public key: hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF00000000000000 gpg/card> quit ``` # Configure YubiKey Insert YubiKey and use GnuPG to configure it: ```console gpg --card-edit ``` 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 ``` This step must be completed before changing PINs or moving keys or an error will occur: `gpg: error for setup KDF: Conditions of use not satisfied` ## 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)) *PIN* values must be at least 6 characters. *Admin PIN* values must be at least 8 characters. A maximum of 127 ASCII characters are allowed. See the GnuPG documentation on [Managing PINs](https://www.gnupg.org/howtos/card-howto/en/ch03s02.html) for more information. 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 While still in administrative mode: ```console gpg/card> list gpg/card> name Cardholder's surname: User Cardholder's given name: YubiKey gpg/card> lang Language preferences: en gpg/card> login Login data (account name): yubikey@example gpg/card> quit ``` # Transfer keys **Important** Transferring keys to YubiKey is a one-way operation. Verify backups were 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. The currently selected key(s) are indicated with an `*`. When transferring keys, only one subkey should be selected at a time. ```console gpg --edit-key $KEYID ``` The Certify key passphrase and Admin PIN are required to transfer keys. ## Signature key Type `key 1` to select the first key and `keytocard` to transfer it, then `1` as the destination: ```console gpg> key 1 sec rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb* rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S ssb rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 usage: E ssb rsa4096/0xAD9E24E1B8CB9600 created: 2024-01-01 expires: 2026-01-01 usage: A [ultimate] (1). YubiKey User gpg> keytocard Please select where to store the key: (1) Signature key (3) Authentication key Your selection? 1 ``` ## Encryption key Type `key 1` again to deselect the first key and `key 2` to select the next key, then `keytocard` to transfer it, then `2` as the destination: ```console gpg> key 1 gpg> key 2 sec rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S ssb* rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 usage: E ssb rsa4096/0xAD9E24E1B8CB9600 created: 2024-01-01 expires: 2026-01-01 usage: A [ultimate] (1). YubiKey User gpg> keytocard Please select where to store the key: (2) Encryption key Your selection? 2 ``` ## Authentication key Type `key 2` again to deselect the second key and `key 3` to select the third key, then `keytocard` to transfer it, then `3` as the destination: ```console gpg> key 2 gpg> key 3 sec rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S ssb rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 usage: E ssb* rsa4096/0xAD9E24E1B8CB9600 created: 2024-01-01 expires: 2026-01-01 usage: A [ultimate] (1). YubiKey User gpg> keytocard Please select where to store the key: (3) Authentication key Your selection? 3 ``` Save and quit: ```console gpg> save ``` # Verify transfer To Verify Subkeys have been moved to YubiKey, look for `ssb>` with `gpg -K`, for example: ```console sec rsa4096/0xF0F2CFEB04341FB5 2024-01-01 [C] Key fingerprint = 4E2C 1FA3 372C BA96 A06A C34A F0F2 CFEB 0434 1FB5 uid [ultimate] YubiKey User ssb> rsa4096/0xB3CD10E502E19637 2024-01-01 [S] [expires: 2026-01-01] ssb> rsa4096/0x30CBE8C4B085B9F7 2024-01-01 [E] [expires: 2026-01-01] ssb> rsa4096/0xAD9E24E1B8CB9600 2024-01-01 [A] [expires: 2026-01-01] ``` A `>` after a tag indicates the key is stored on a smart card. # Multiple YubiKeys To provision additional YubiKeys, restore the Certify key backup and repeat [Configure YubiKey](#configure-yubikey). ```console mv -vi $GNUPGHOME $GNUPGHOME.1 cp -avi /mnt/encrypted-storage/tmp.XXX $GNUPGHOME cd $GNUPGHOME ``` ## Switching between YubiKeys When a GnuPG key is added to YubiKey using `keytocard`, the key is deleted from the keyring and a **stub** is added, pointing to the YubiKey. The stub identifies the GnuPG key ID and YubiKey serial number. When the operation is repeated for an additional YubiKey, the stub is overwritten `keytocard` and now will point to the latest YubiKey. GnuPG will request a specific YubiKey by serial number, as referenced by the stub, and will not recognize another YubiKey with a different serial number without manual intervention. Insert the first YubiKey (which has a different serial number) and run the following command: ```console gpg-connect-agent "scd serialno" "learn --force" /bye ``` GnuPG will scan the first YubiKey for keys and recreate the stubs to point to the key ID and YubiKey serial number of the first YubiKey. To use the second YubiKey, repeat the command. # Finish Before completing setup, verify the following: - [ ] Saved Encryption, Signature and Authentication Subkeys to YubiKey (`gpg -K` will show `ssb>` for 3 Subkeys) - [ ] Saved YubiKey user and admin PINs, which are unique and were changed from default values - [ ] Saved Certify key passphrase to a secure and durable location - [ ] Saved Certify key, Subkeys and revocation certificate on encrypted portable storage, to be kept offline - [ ] Saved passphrase to encrypted volume on portable storage - [ ] Saved copy of public key where is can be easily accessed later Reboot to finish. If an ephemeral environment was not used for setup, delete secret keys from the keyring and [securely delete](https://srm.sourceforge.net/) `$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 ``` Or with the URL on YubiKey, retrieve the public key: ```console gpg/card> fetch gpg/card> quit ``` Edit the Certify key: ```console export KEYID=0xF0F2CFEB04341FB5 gpg --edit-key $KEYID ``` Assign ultimate trust by typing `trust` and selecting option `5`: ```console gpg> trust Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y gpg> quit ``` Remove and re-insert YubiKey. Verify the status with `gpg --card-status` which should be similar to: ```console Reader ...........: Yubico YubiKey OTP FIDO CCID 00 00 Application ID ...: D2760001240102010006055532110000 Application type .: OpenPGP Version ..........: 3.4 Manufacturer .....: Yubico Serial number ....: 05553211 Name of cardholder: YubiKey User Language prefs ...: en Salutation .......: URL of public key : [not set] Login data .......: yubikey@example 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 ....: CF5A 305B 808B 7A0F 230D A064 B3CD 10E5 02E1 9637 created ....: 2024-01-01 12:00:00 Encryption key....: A5FA A005 5BED 4DC9 889D 38BC 30CB E8C4 B085 B9F7 created ....: 2024-01-01 12:00:00 Authentication key: 570E 1355 6D01 4C04 8B6D E2A3 AD9E 24E1 B8CB 9600 created ....: 2024-01-01 12:00:00 General key info..: sub rsa4096/0xB3CD10E502E19637 2024-01-01 YubiKey User sec# rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never ssb> rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 card-no: 0006 05553211 ssb> rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 card-no: 0006 05553211 ssb> rsa4096/0xAD9E24E1B8CB9600 created: 2024-01-01 expires: 2026-01-01 card-no: 0006 05553211 ``` `sec#` indicates the corresponding key is not available (the Certify key is offline). **Note** If `General key info..: [none]` appears in the output instead - go back and import the public key using the previous step. Encrypt a message to yourself (useful for storing credentials): ```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 Mon 01 Jan 2024 12:00:00 PM UTC gpg: using RSA key CF5A305B808B7A0F230DA064B3CD10E502E19637 gpg: Good signature from "YubiKey User " [ultimate] Primary key fingerprint: 4E2C 1FA3 372C BA96 A06A C34A F0F2 CFEB 0434 1FB5 Subkey fingerprint: CF5A 305B 808B 7A0F 230D A064 B3CD 10E5 02E1 9637 ``` 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 0xF0F2CFEB04341FB5 ... 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 exploit, it is not impossible; the key and PIN could be physically compromised, or a vulnerability may be discovered in firmware or in the random number generator used to create keys, for example. Therefore, it is good practice to rotate Subkeys periodically. When a Subkey expires, it can either be renewed or replaced. Both actions require access to the Certify key. - Renewing Subkeys by updating expiration indicates continued 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, signature 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. ```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=0xF0F2CFEB04341FB5 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 editing the identity: ```console gpg --edit-key $KEYID ``` Select all expired keys: ```console gpg> key 1 gpg> key 2 gpg> key 3 sec rsa4096/0xF0F2CFEB04341FB5 created: 2024-01-01 expires: never usage: C trust: ultimate validity: ultimate ssb* rsa4096/0xB3CD10E502E19637 created: 2024-01-01 expires: 2026-01-01 usage: S ssb* rsa4096/0x30CBE8C4B085B9F7 created: 2024-01-01 expires: 2026-01-01 usage: E ssb* rsa4096/0xAD9E24E1B8CB9600 created: 2024-01-01 expires: 2026-01-01 usage: A [ultimate] (1). YubiKey User ``` 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 = key expires in n days w = key expires in n weeks m = key expires in n months y = key expires in n years Key is valid for? (0) ``` Set the expiration date, then `save` Next, [Export public keys](#export-public-keys): ```console gpg --armor --export $KEYID > $KEYID-$(date +%F).asc ``` Transfer the public key to the destination host, and then import it: ```console gpg --import 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 validity of the GnuPG identity will be extended, allowing it to be used again for encryption, signature and authentication operations. The SSH public key does **not** need to be updated on remote hosts. ## Rotating keys Follow the original procedure to generate and add [Subkeys](#subkeys). 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 ``` 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 Subkeys (4, 5 and 6) to YubiKey, replacing existing ones. Reboot or securely erase the GnuPG 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. After configuring the environment, 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. Export the public key: ```console gpg --export $KEYID > pubkey.asc ``` Transfer the public key and import it: ```console gpg --import pubkey.asc ``` Use `showpref` to verify notions were correctly added. # SSH **Tip** YubiKey can be used directly for SSH only, without GnuPG features, starting in [OpenSSH v8.2](https://www.openssh.com/txt/release-8.2). For more information, see [vorburger/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 keys, then makes them available to ssh clients. Any existing ssh private keys should be deleted after importing to GnuPG agent. When importing the key to `gpg-agent`, a passphrase will be required to encrypt within the key store. GnuPG 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`). Missing from `gpg-agent` ssh agent support is the ability to remove keys. `ssh-add -d/-D` have no effect. Instead, use the `gpg-connect-agent` utility to lookup a keygrip, match it 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 YubiKey as a smart card, because the PIN is [cached by the smart card itself](https://dev.gnupg.org/T3362). To clear the PIN from cache (equivalent to `default-cache-ttl` and `max-cache-ttl`), unplug YubiKey, or set `forcesig` 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 this error appears: `sign_and_send_pubkey: signing failed: agent refused operation`, 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 $DBUS_SESSION_BUS_ADDRESS found`, falls back to `curses` but doesn't find the expected `tty`. On macOS, use `brew install pinentry-mac` and set the program path to `pinentry-program /usr/local/bin/pinentry-mac` for Intel Macs, `/opt/homebrew/bin/pinentry-mac` for ARM/Apple Silicon Macs or `pinentry-program /usr/local/MacGPG2/libexec/pinentry-mac.app/Contents/MacOS/pinentry-mac` if using MacGPG Suite. For the configuration to take effect, run `gpgconf --kill gpg-agent` ## Replace agents To launch `gpg-agent` for use by SSH, use the `gpg-connect-agent /bye` or `gpgconf --launch gpg-agent` commands. Add the following to the shell rc file: ```console export GPG_TTY="$(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 ``` When using `ForwardAgent` for ssh-agent forwarding, `SSH_AUTH_SOCK` only needs to be set on the *local* host, where YubiKey is connected. On the *remote* host, `ssh` will set `SSH_AUTH_SOCK` to something like `/tmp/ssh-mXzCzYT2Np/agent.7541` upon connection. Do **not** set `SSH_AUTH_SOCK` on the remote host - doing so will break [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding). For `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 GnuPG 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://words.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 YubiKey, `IdentityFile` must point to the _public_ key file, and `ssh` will select the appropriate private key from those available via ssh-agent. To prevent `ssh` from trying all keys in the agent, use `IdentitiesOnly yes` along with one or more `-i` or `IdentityFile` options for the target host. To reiterate, with `IdentitiesOnly yes`, `ssh` will not enumerate public keys loaded into `ssh-agent` or `gpg-agent`. This means public-key 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 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, use the [ControlMaster](https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing) ssh option. ## 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 GnuPG 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 environment variables such as `$SSH_AUTH_SOCK` when connecting to remote hosts and attaching an existing session. For each shell, find the socket and `export SSH_AUTH_SOCK=/tmp/ssh-agent-xxx/xxxx.socket`. However, with `S.gpg-agent.ssh` in a fixed place, it can be used as the ssh-agent in shell rc files. ### Use ssh-agent You should now be able to use `ssh -A remote` on the _local_ host to log into _remote_ host, and should then be able to use YubiKey as if it were connected to the remote host. For example, using e.g. `ssh-add -l` on that remote host will show the public key from the YubiKey (`cardno:`). Always use `ForwardAgent yes` only for a single host, 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: ```console 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 $KEYID ``` The `user.email` option must match the email address associated with the PGP identity. To sign commits or tags, use the `-S` option. **Windows** To configure authentication: ```console git config --global core.sshcommand "plink -agent" git config --global gpg.program 'C:\Program Files (x86)\GnuPG\bin\gpg.exe' ``` Update the repository URL to `git@github.com:USERNAME/repository` and any authenticated commands will be authorized by YubiKey. **Note** For the error `gpg: signing failed: No secret key` - run `gpg --card-status` with YubiKey plugged in and try the git command again. ## OpenBSD Install and enable tools for use with PC/SC drivers, cards, readers, then reboot to recognize YubiKey: ```console doas pkg_add pcsc-tools doas rcctl enable pcscd doas reboot ``` ## Windows Windows can already have some virtual smart card readers installed, like the one provided for Windows Hello. To verify YubiKey is the correct one used by scdaemon, add it to its configuration. Find the YubiKey label using PowerShell: ```powershell PS C:\WINDOWS\system32> Get-PnpDevice -Class SoftwareDevice | Where-Object {$_.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 ``` 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 ``` Retrieve the public key id: ```console gpg --list-public-keys ``` Export the SSH public key: ```console gpg --export-ssh-key ``` 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 GnuPG, which will prompt for a PIN to authorize the operation. ### WSL The goal is to configure SSH client inside WSL work together with the Windows agent, such as gpg-agent.exe. ![WSL agent architecture](media/schema_gpg.png) **Note** this works only for SSH agent forwarding. GnuPG forwarding for cryptographic operations is not supported. See [vuori/weasel-pageant](https://github.com/vuori/weasel-pageant) for more information. 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 ssh socket forwarding, do not enable `ForwardAgent` in ssh config. See [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding) for more information. This requires Ubuntu 16.04 or newer for WSL and Kleopatra. Download [vuori/weasel-pageant](https://github.com/vuori/weasel-pageant). Add `eval $(/mnt/c//weasel-pageant -r -a /tmp/S.weasel-pageant)` to the shell rc file. Use a named socket here so it can be used in the `RemoteForward` directive of `~/.ssh/config`. Source it with `source ~/.bashrc`. Display the SSH key with `$ ssh-add -l` Edit `~/.ssh/config` to add the following for each agent forwarding host: ```console RemoteForward /tmp/S.weasel-pageant ``` **Note** The remote SSH socket path can be found with `gpgconf --list-dirs agent-ssh-socket` Add the following to the shell rc file: ```console export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket) ``` Add the following to `/etc/ssh/sshd_config`: ```console StreamLocalBindUnlink yes ``` Reload SSH daemon: ```console sudo service sshd reload ``` Unplug YubiKey, disconnect or reboot. Log back into Windows, open a WSL console and enter `ssh-add -l` - no output should appear. Plug in YubiKey, enter the same command to display the ssh key. Connect to the remote host and use `ssh-add -l` to confirm forwarding works. Agent forwarding may be chained through multiple hosts. Follow the same [protocol](#remote-host-configuration) to configure each host. ## macOS To use gui applications on macOS, [a little bit more setup is needed](https://jms1.net/yubikey/make-ssh-use-gpg-agent.md). Create `$HOME/Library/LaunchAgents/gnupg.gpg-agent.plist` with the following contents: ``` Label gnupg.gpg-agent RunAtLoad KeepAlive ProgramArguments /usr/local/MacGPG2/bin/gpg-connect-agent /bye ``` ```console launchctl load $HOME/Library/LaunchAgents/gnupg.gpg-agent.plist ``` Create `$HOME/Library/LaunchAgents/gnupg.gpg-agent-symlink.plist` with the following contens: ``` Label gnupg.gpg-agent-symlink ProgramArguments /bin/sh -c /bin/ln -sf $HOME/.gnupg/S.gpg-agent.ssh $SSH_AUTH_SOCK RunAtLoad ``` ```console launchctl load $HOME/Library/LaunchAgents/gnupg.gpg-agent-symlink.plist ``` Reboot or log out and log back in to activate these changes. # Remote Machines (GPG Agent Forwarding) YubiKey can be used sign git commits and decrypt files on remote hosts with 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). `gpg-agent.conf` is not needed on the remote host; after forwarding, remote GnuPG directly communicates with `S.gpg-agent` without starting `gpg-agent` on the remote host. On the remote host, edit `/etc/ssh/sshd_config` to set `StreamLocalBindUnlink yes` **Optional** Without root access on the remote host to edit `/etc/ssh/sshd_config`, socket located at `gpgconf --list-dir agent-socket` on the remote host will need to be removed before forwarding works. See [AgentForwarding GNUPG wiki page](https://wiki.gnupg.org/AgentForwarding) for more information. Import public keys on the remote host. On the local host, copy the public keyring to the remote host: ```console scp ~/.gnupg/pubring.kbx remote:~/.gnupg/ ``` On modern distributions such as Fedora 30, there is no need to set `RemoteForward` in `~/.ssh/config` ## Steps for older distributions On the local host, 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//.gnupg/S/gpg-agent.extra` Find the agent socket on the **remote** host: ```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 host by adding the following to the local host's `~/.ssh/config` (agent sockets may differ): ``` 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] ``` It may be necessary to edit `gpg-agent.conf` on the *local* host 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 the *local* host, not remote. **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 GnuPG 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 <&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: ```console gpg --expert --edit-key $KEYID ``` Add the identity and set ultimate trust: ```console gpg> adduid gpg> trust Your decision? 5 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 ``` **Note** On Windows, using an extension other than `.gpg` or attempting IO redirection to a file will result in a nonfunctional private key. ```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** 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 YubiKey](#configure-yubikey) again. Next, [Transfer Keys](#transfer-keys) and reboot or securely erase the 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 cryptographic operations without requiring any action from the user after the key is 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: Encryption: ```console ykman openpgp keys set-touch dec on ``` Signature: ```console ykman openpgp keys set-touch sig on ``` Authentication: ```console ykman openpgp keys set-touch aut 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` To view and adjust policy options: ``` ykman openpgp keys set-touch -h ``` If the YubiKey is going to be used within an email client which opens and verifies mail, `Cached` or `Cached-Fixed` may be desirable. YubiKey will blink when it is waiting for a touch. On Linux, [maximbaz/yubikey-touch-detector](https://github.com/maximbaz/yubikey-touch-detector) can be used to indicate YubiKey is waiting for a touch. # Email YubiKey can be used to decrypt and 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 EFF guide](https://ssd.eff.org/en/module/how-use-pgp-linux) for more information. Mutt has OAuth 2 support since version 2.0. ## Mailvelope [Mailvelope](https://www.mailvelope.com/en) allows 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` to 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 GnuPG: ```console sudo launchctl config user path /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin ``` Finally, install the [Mailvelope extension](https://chromewebstore.google.com/detail/mailvelope/kajibbejlbohfaggdiogboambcijhkke) from the Chrome web 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 to a file and run `gpg-connect-agent -r $file` to lock and terminate the card. Then re-insert YubiKey to complete 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 reset 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 YubiKey](#configure-yubikey). # Notes 1. 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`. OTP mode can be swapped to the second configuration via the YubiKey Personalization tool or disabled entirely using [YubiKey Manager](https://developers.yubico.com/yubikey-manager): `ykman config usb -d OTP` 1. Using YubiKey for GnuPG keys does not prevent use of other features, such as [WebAuthn](https://en.wikipedia.org/wiki/WebAuthn), [OTP](https://www.yubico.com/resources/glossary/otp/) and [static password](https://support.yubico.com/hc/en-us/articles/360016614980-Understanding-Core-Static-Password-Features). 1. Setting a key expiry forces identity and credential lifecycle management. However, setting an expiry on the primary Certify key is useless, because it can be used to simply extend itself. [Revocation certificates](https://security.stackexchange.com/questions/14718/does-openpgp-key-expiration-add-to-security/79386#79386) should be used instead. 1. To switch between multiple 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 insert the other YubiKey and run `gpg-connect-agent updatestartuptty /bye` 1. To use YubiKey on multiple computers, import the corresponding public keys on them. Confirm see YubiKey is visible with `gpg --card-status`, then trust the imported public keys ultimately. `gpg --list-secret-keys` will show the correct and trusted key. # Troubleshooting - Use `man gpg` to understand GnuPG 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 GnuPG - 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 verify the public key from the card is being offered: `Offering public key: RSA SHA256:abcdefg... cardno:00060123456`. If it is, verify the correct user the target system - not 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 the GnuPG public key is lost, 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) - SSH and GnuPG agent which can be used with PIV devices * [`keytotpm`](https://www.gnupg.org/documentation/manuals/gnupg/OpenPGP-Key-Management.html) - 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 ``` Verify the Certify key: ```console gpg --list-key ``` Export the Certify key ID and create the Subkeys: ```console export KEYID=0xF0F2CFEB04341FB5 gpg --quick-add-key "$KEYID" rsa4096 sign 2y gpg --quick-add-key "$KEYID" rsa4096 encrypt 2y gpg --quick-add-key "$KEYID" rsa4096 auth 2y ``` # 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)