This is a guide to using [YubiKey](https://www.yubico.com/products/) as a [SmartCard](https://security.stackexchange.com/questions/38924/how-does-storing-gpg-ssh-private-keys-on-smart-cards-compare-to-plain-usb-drives) for storing GPG encryption, signing and authentication keys, which can also be used for SSH. Many of the principles in this document are applicable to other smart card devices. 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-) (as opposed to filesystem-based keys that are stored on disk), while remaining convenient for daily use. Instead of having to remember and enter complicated passphrases to unlock SSH/GPG keys, YubiKey needs only a physical touch after being unlocked with a PIN. **Security Note** If you followed this guide before Jan 2021, your GPG *PIN* and *Admin PIN* may be set to their default values (`123456` and `12345678` respectively). This would allow an attacker to use your YubiKey or reset your PIN. Please see the [Change PIN](#change-pin) section for details on how to change your 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 GPG and YubiKey to securely store and use credentials. - [Purchase](#purchase) - [Prepare environment](#prepare-environment) - [Required software](#required-software) * [Debian and Ubuntu](#debian-and-ubuntu) * [Fedora](#fedora) * [Arch](#arch) * [RHEL7](#rhel7) * [NixOS](#nixos) * [OpenBSD](#openbsd) * [macOS](#macos) * [Windows](#windows) - [Entropy](#entropy) * [YubiKey](#yubikey) * [OneRNG](#onerng) - [Creating keys](#creating-keys) * [Temporary working directory](#temporary-working-directory) * [Harden configuration](#harden-configuration) - [Certify key](#certify-key) - [Sign with existing key](#sign-with-existing-key) - [Subkeys](#subkeys) * [Signing](#signing) * [Encryption](#encryption) * [Authentication](#authentication) * [Add extra identities](#add-extra-identities) - [Verify](#verify) - [Export secret keys](#export-secret-keys) - [Revocation certificate](#revocation-certificate) - [Backup](#backup) - [Export public keys](#export-public-keys) - [Configure Smartcard](#configure-smartcard) * [Enable KDF](#enable-kdf) * [Change PIN](#change-pin) * [Set information](#set-information) - [Transfer keys](#transfer-keys) * [Signing](#signing-1) * [Encryption](#encryption-1) * [Authentication](#authentication-1) - [Verify card](#verify-card) - [Multiple YubiKeys](#multiple-yubikeys) * [Switching between YubiKeys](#switching-between-yubikeys) - [Multiple Hosts](#multiple-hosts) - [Finish](#finish) - [Using keys](#using-keys) - [Rotating keys](#rotating-keys) * [Setup environment](#setup-environment) * [Renewing Subkeys](#renewing-subkeys) * [Rotating keys](#rotating-keys-1) - [Adding notations](#adding-notations) - [SSH](#ssh) * [Create configuration](#create-configuration) * [Replace agents](#replace-agents) * [Copy public key](#copy-public-key) * [(Optional) Save public key for identity file configuration](#optional-save-public-key-for-identity-file-configuration) * [Connect with public key authentication](#connect-with-public-key-authentication) * [Import SSH keys](#import-ssh-keys) * [Remote Machines (SSH Agent Forwarding)](#remote-machines-ssh-agent-forwarding) + [Use ssh-agent ](#use-ssh-agent) + [Use S.gpg-agent.ssh](#use-sgpg-agentssh) + [Chained SSH Agent Forwarding](#chained-ssh-agent-forwarding) * [GitHub](#github) * [OpenBSD](#openbsd-1) * [Windows](#windows-1) + [WSL](#wsl) - [Use ssh-agent or use S.weasel-pageant](#use-ssh-agent-or-use-sweasel-pageant) - [Prerequisites](#prerequisites) - [WSL configuration](#wsl-configuration) - [Remote host configuration](#remote-host-configuration) * [macOS](#macos-1) - [Remote Machines (GPG Agent Forwarding)](#remote-machines-gpg-agent-forwarding) * [Steps for older distributions](#steps-for-older-distributions) * [Chained GPG Agent Forwarding](#chained-gpg-agent-forwarding) - [Using Multiple Keys](#using-multiple-keys) - [Adding an identity](#adding-an-identity) * [Updating YubiKey](#updating-yubikey) - [Require touch](#require-touch) - [Email](#email) * [Mailvelope](#mailvelope) * [Mutt](#mutt) - [Reset](#reset) * [Recovery after reset](#recovery-after-reset) - [Notes](#notes) - [Troubleshooting](#troubleshooting) - [Alternatives](#alternatives) * [Create keys with batch](#create-keys-with-batch) - [Additional resources](#additional-resources) # Purchase All YubiKeys except the blue "security key" model and the "Bio Series - FIDO Edition" are compatible with this guide. NEO models are limited to 2048-bit RSA keys. Compare YubiKeys [here](https://www.yubico.com/products/yubikey-hardware/compare-products-series/). A list of the YubiKeys compatible with OpenPGP is available [here](https://support.yubico.com/hc/en-us/articles/360013790259-Using-Your-YubiKey-with-OpenPGP). In May 2021, Yubico also released a press release and blog post about supporting resident ssh keys on their YubiKeys including blue "security key 5 NFC" with OpenSSH 8.2 or later, see [here](https://www.yubico.com/blog/github-now-supports-ssh-security-keys/) for details. To verify a YubiKey is genuine, open a [browser with U2F support](https://support.yubico.com/hc/en-us/articles/360013723419-How-to-Confirm-Your-Yubico-Device-is-Genuine) to [https://www.yubico.com/genuine/](https://www.yubico.com/genuine/). Insert a Yubico device, and select *Verify Device* to begin the process. Touch the YubiKey when prompted, and if asked, allow it to see the make and model of the device. If you see *Verification complete*, the device is authentic. This website verifies YubiKey device attestation certificates signed by a set of Yubico certificate authorities, and helps mitigate [supply chain attacks](https://media.defcon.org/DEF%20CON%2025/DEF%20CON%2025%20presentations/DEF%20CON%2025%20-%20r00killah-and-securelyfitz-Secure-Tokin-and-Doobiekeys.pdf). You will also need several portable storage devices (microSD cards work well) for storing encrypted backups of your keys. # Prepare environment To create cryptographic keys, a secure environment that can be reasonably assured to be free of adversarial control is recommended. The following is a general ranking of environments most to least likely to be compromised: 1. Daily-use system with full Internet access 1. Virtual machine on daily-use host OS (using [virt-manager](https://virt-manager.org/), VirtualBox or VMware) 1. Separate hardened [Debian](https://www.debian.org/) or [OpenBSD](https://www.openbsd.org/) installation which can be dual booted 1. Live image, such as [Debian Live](https://www.debian.org/CD/live/) or [Tails](https://tails.boum.org/index.en.html) 1. Secure hardware/firmware ([Coreboot](https://www.coreboot.org/), [Intel ME removed](https://github.com/corna/me_cleaner)) 1. Dedicated air-gapped system without networking capabilities (Raspberry Pi or equivalent) This guide recommends using a bootable "live" Debian Linux image to provide such an environment, however, depending on your threat model, you may want to take fewer or more steps to secure it. To use Debian Live, download the latest image and corresponding 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)" ``` Verify the signature of the hashes file with GPG: ```console gpg --keyserver hkps://keyring.debian.org --recv DF9B9C49EAA9298432589D76DA87E80D6294BE9B gpg --verify SHA512SUMS.sign SHA512SUMS ``` Verify `gpg: Good signature from "Debian CD signing key "` appears in the output. If the public key cannot be received, try changing the DNS resolver and/or use a different keyserver: ```console gpg --keyserver hkps://keyserver.ubuntu.com:443 --recv DF9B9C49EAA9298432589D76DA87E80D6294BE9B ``` Ensure the SHA512 hash of the live image matches the one in the signed file - if there following command produces output, it is correct: ```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 your architecture (likely `amd64`) and then an ftp server. Edit `/etc/apt/sources.list` and add the ftp server: ``` deb http://ftp.debian.org/debian sid main ``` Then add this to `/etc/apt/preferences` (or a fragment, e.g. `/etc/apt/preferences.d/00-sid`) so that APT still prioritizes packages from the stable repository over sid. ``` Package: * Pin: release n=sid Pin-Priority: 10 ``` **Note** Live Ubuntu images [may require modification](https://github.com/drduh/YubiKey-Guide/issues/116) to `/etc/apt/sources.list` and may need additional packages: ```console sudo apt -y install libssl-dev swig libpcsclite-dev ``` **Optional** Install the `ykman` utility, which will allow you to enable touch policies (requires admin PIN): ```console sudo apt -y install python3-pip python3-pyscard pip3 install PyOpenSSL pip3 install yubikey-manager sudo service pcscd start ~/.local/bin/ykman openpgp info ``` **Note** Debian 12 does not recommend installing non-Debian packaged Python applications globally. But fortunately, it is not necessary as `yubikey-manager` is available in the stable main repository: ```console sudo apt install yubikey-manager ``` ## Fedora ```console sudo dnf install wget wget https://github.com/rpmsphere/noarch/raw/master/r/rpmsphere-release-38-1.noarch.rpm sudo rpm -Uvh rpmsphere-release*rpm sudo dnf install \ gnupg2 dirmngr cryptsetup gnupg2-smime \ pcsc-tools opensc pcsc-lite secure-delete \ pgp-tools yubikey-personalization-gui ``` ## Arch ```console sudo pacman -Syu gnupg pcsclite ccid hopenpgp-tools yubikey-personalization ``` ## RHEL7 ```console sudo yum install -y gnupg2 pinentry-curses pcsc-lite pcsc-lite-libs gnupg2-smime ``` ## NixOS Generate 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 ``` Generate the ISO: ```console nix build --experimental-features "nix-command flakes" .#nixosConfigurations.yubikeyLive.x86_64-linux.config.system.build.isoImage ``` Copy it to a USB drive: ```console sudo cp -v result/iso/yubikeyLive.iso /dev/sdb; sync ``` With this image, you won't need to create a [temporary working directory](#temporary-working-directory) or [harden the configuration](#harden-configuration), as it was done when creating the image. ## OpenBSD ```console doas pkg_add gnupg pcsc-tools ``` ## macOS Download and install [Homebrew](https://brew.sh/) and the following packages: ```console brew install gnupg yubikey-personalization hopenpgp-tools ykman pinentry-mac wget ``` **Note** An additional Python package dependency may need to be installed to use [`ykman`](https://support.yubico.com/support/solutions/articles/15000012643-yubikey-manager-cli-ykman-user-guide) - `pip install yubikey-manager` ## Windows Download and install [Gpg4Win](https://www.gpg4win.org/) and [PuTTY](https://putty.org). You may also need more recent versions of [yubikey-personalization](https://developers.yubico.com/yubikey-personalization/Releases/) and [yubico-c](https://developers.yubico.com/yubico-c/Releases/). # Entropy Generating cryptographic keys requires high-quality [randomness](https://www.random.org/randomness/), measured as entropy. Most operating systems use software-based pseudorandom number generators or CPU-based hardware random number generators (HRNG). **Optional** A hardware device such as [OneRNG](https://onerng.info/onerng/) may be used to [increase the speed](https://lwn.net/Articles/648550/) and possibly the quality of available entropy. ## YubiKey YubiKey firmware version 5.2.3 introduced "Enhancements to OpenPGP 3.4 Support" - which can gather additional entropy from YubiKey via the SmartCard interface. To seed the kernel's PRNG with additional 512 bytes retrieved from the YubiKey: ```console echo "SCD RANDOM 512" | gpg-connect-agent | sudo tee /dev/random | hexdump -C ``` ## OneRNG Configure [rng-tools](https://wiki.archlinux.org/index.php/Rng-tools) software: ```console sudo apt -y install at rng-tools python3-gnupg openssl wget https://github.com/OneRNG/onerng.github.io/raw/master/sw/onerng_3.7-1_all.deb ``` Verify the package: ```console $ sha256sum onerng_3.7-1_all.deb b7cda2fe07dce219a95dfeabeb5ee0f662f64ba1474f6b9dddacc3e8734d8f57 onerng_3.7-1_all.deb ``` Install the package: ```console sudo dpkg -i onerng_3.7-1_all.deb echo "HRNGDEVICE=/dev/ttyACM0" | sudo tee /etc/default/rng-tools ``` Plug in the device and restart rng-tools: ```console sudo atd sudo service rng-tools restart ``` # Creating 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) ``` Otherwise, to preserve the working environment, set the GnuPG directory to your home folder: ```console export GNUPGHOME=~/gnupg-workspace ``` ## Harden configuration Create a hardened configuration in the temporary working directory: ```console wget -O $GNUPGHOME/gpg.conf https://raw.githubusercontent.com/drduh/config/master/gpg.conf ``` The options will look like: ```console $ grep -ve "^#" $GNUPGHOME/gpg.conf personal-cipher-preferences AES256 AES192 AES personal-digest-preferences SHA512 SHA384 SHA256 personal-compress-preferences ZLIB BZIP2 ZIP Uncompressed default-preference-list SHA512 SHA384 SHA256 AES256 AES192 AES ZLIB BZIP2 ZIP Uncompressed cert-digest-algo SHA512 s2k-digest-algo SHA512 s2k-cipher-algo AES256 charset utf-8 fixed-list-mode no-comments no-emit-version no-greeting keyid-format 0xlong list-options show-uid-validity verify-options show-uid-validity with-fingerprint require-cross-certification no-symkey-cache use-agent throw-keyids ``` **Tip** Networking can be disabled for the remainder of the setup. # Certify key The primary key to generate is the Certify key, which will be used to issue Subkeys for Encrypt, Sign and Authenticate operations. **Important** The Certify key should be kept offline at all times and only accessed from a secure environment to revoke or issue new Subkeys. Keys can also be generated on the YubiKey itself to ensure no other copies exist, however for usability and durability reasons this guide recommends against doing so. Generate a passphrase which will be needed throughout the guide to create and export Subkeys. **Important** The passphrase should be memorized or written down in a secure place separate from the backup storage disk. The passphrase should consist of only upper case letters and numbers for improved readability. The following command should generate strong passphrases while avoiding ambiguous characters: ```console LC_ALL=C tr -dc 'A-Z1-9' < /dev/urandom | \ tr -d "1IOS5U" | fold -w 30 | head -n10 | \ sed "-es/./ /"{1..26..5} | cut -c2- | tr " " "-" ``` Such as: ```console A4ZK-YRRJ-8WPM-82NY-CX9T-AGKT PH9Z-HFDX-QDB9-YMMC-GQZB-Z3EV EC3H-C42G-8E9K-VF7F-ZWT7-BTL6 B3CA-QCCE-JMNE-VAZG-ZEYD-J3XP YKP4-M42X-4WWE-WEKR-C3J7-GZYF ZQWC-E7MN-M7CT-4Y4Z-9QFV-44VY KY4F-C83Q-BTYQ-V8EM-WGCR-DPZN GYWQ-WNAC-ERWM-XGAD-6XVD-ZCLD L8JL-EK8H-Z4ZF-MA93-NND8-FPKA WM2J-XF7L-QV6D-AWLY-Y2D8-4TQQ ``` **Tip** On Linux or OpenBSD, select the password using the mouse or by double-clicking on it to copy to clipboard. Paste using the middle mouse button or `Shift`-`Insert` Generate a new key with GPG, selecting `(8) RSA (set your own capabilities)`, `Certify` capability only and `4096` bit key size. Do **not** set the Certify key to expire - see [Note #3](#notes). ```console $ gpg --expert --full-generate-key Please select what kind of key you want: (1) RSA and RSA (default) (2) DSA and Elgamal (3) DSA (sign only) (4) RSA (sign only) (7) DSA (set your own capabilities) (8) RSA (set your own capabilities) (9) ECC and ECC (10) ECC (sign only) (11) ECC (set your own capabilities) (13) Existing key (14) Existing key from card Your selection? 8 Possible actions for a RSA key: Sign Certify Encrypt Authenticate Current allowed actions: Sign Certify Encrypt (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? E Possible actions for a RSA key: Sign Certify Encrypt Authenticate Current allowed actions: Sign Certify (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? S Possible actions for a RSA key: Sign Certify Encrypt Authenticate Current allowed actions: Certify (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? Q RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048) 4096 Requested keysize is 4096 bits Please specify how long the key should be valid. 0 = key does not expire = 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 name and email address (it doesn't have to be valid): ```console GnuPG needs to construct a user ID to identify your key. Real name: Dr Duh Email address: doc@duh.to Comment: [Optional - leave blank] You selected this USER-ID: "Dr Duh " Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o We need to generate a lot of random bytes. It is a good idea to perform some other action (type on the keyboard, move the mouse, utilize the disks) during the prime generation; this gives the random number generator a better chance to gain enough entropy. gpg: /tmp.FLZC0xcM/trustdb.gpg: trustdb created gpg: key 0xFF3E7D88647EBCDB marked as ultimately trusted gpg: directory '/tmp.FLZC0xcM/openpgp-revocs.d' created gpg: revocation certificate stored as '/tmp.FLZC0xcM/openpgp-revocs.d/011CE16BD45B27A55BA8776DFF3E7D88647EBCDB.rev' public and secret key created and signed. pub rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [C] Key fingerprint = 011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB uid Dr Duh ``` Export the key ID as a [variable](https://stackoverflow.com/questions/1158091/defining-a-variable-with-or-without-export/1158231#1158231) (`KEYID`) for use later: ```console export KEYID=0xFF3E7D88647EBCDB ``` # Sign with existing key (Optional) If you already have a PGP key, you may want to sign the new key with the old one to prove that the new key is controlled by you. Export your existing key to move it to the working keyring: ```console gpg --export-secret-keys --armor --output /tmp/new.sec ``` Then sign the new key: ```console gpg --default-key $OLDKEY --sign-key $KEYID ``` # Subkeys Edit the identity to add Subkeys: ```console $ gpg --expert --edit-key $KEYID Secret key is available. sec rsa4096/0xEA5DE91459B80592 created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate [ultimate] (1). Dr Duh ``` Use of 4096-bit RSA keys is recommended. Subkeys are recommended to have one or several year expirations. They must be renewed using the Certify key. See [rotating keys](#rotating-keys). ## Signing Create a [signing key](https://stackoverflow.com/questions/5421107/can-rsa-be-both-used-as-encryption-and-signature/5432623#5432623) by selecting `addkey` then `(4) RSA (sign only)`: ```console gpg> addkey Key is protected. You need a passphrase to unlock the secret key for user: "Dr Duh " 4096-bit RSA key, ID 0xFF3E7D88647EBCDB, created 2016-05-24 Please select what kind of key you want: (3) DSA (sign only) (4) RSA (sign only) (5) Elgamal (encrypt only) (6) RSA (encrypt only) (7) DSA (set your own capabilities) (8) RSA (set your own capabilities) Your selection? 4 RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048) 4096 Requested keysize is 4096 bits Please specify how long the key should be valid. 0 = key does not expire = 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) 1y Key expires at Mon 10 Sep 2018 00:00:00 PM UTC Is this correct? (y/N) y Really create? (y/N) y We need to generate a lot of random bytes. It is a good idea to perform some other action (type on the keyboard, move the mouse, utilize the disks) during the prime generation; this gives the random number generator a better chance to gain enough entropy. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S [ultimate] (1). Dr Duh ``` ## Encryption Next, create an [encryption key](https://www.cs.cornell.edu/courses/cs5430/2015sp/notes/rsa_sign_vs_dec.php) by selecting `(6) RSA (encrypt only)`: ```console gpg> addkey Please select what kind of key you want: (3) DSA (sign only) (4) RSA (sign only) (5) Elgamal (encrypt only) (6) RSA (encrypt only) (7) DSA (set your own capabilities) (8) RSA (set your own capabilities) (10) ECC (sign only) (11) ECC (set your own capabilities) (12) ECC (encrypt only) (13) Existing key Your selection? 6 RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048) 4096 Requested keysize is 4096 bits Please specify how long the key should be valid. 0 = key does not expire = 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) 1y Key expires at Mon 10 Sep 2018 00:00:00 PM UTC Is this correct? (y/N) y Really create? (y/N) y We need to generate a lot of random bytes. It is a good idea to perform some other action (type on the keyboard, move the mouse, utilize the disks) during the prime generation; this gives the random number generator a better chance to gain enough entropy. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E [ultimate] (1). Dr Duh ``` ## Authentication Finally, create an [authentication key](https://superuser.com/questions/390265/what-is-a-gpg-with-authenticate-capability-used-for). GPG doesn't provide an authenticate-only key type, so select `(8) RSA (set your own capabilities)` and toggle the required capabilities until the only allowed action is `Authenticate`: ```console gpg> addkey Please select what kind of key you want: (3) DSA (sign only) (4) RSA (sign only) (5) Elgamal (encrypt only) (6) RSA (encrypt only) (7) DSA (set your own capabilities) (8) RSA (set your own capabilities) (10) ECC (sign only) (11) ECC (set your own capabilities) (12) ECC (encrypt only) (13) Existing key Your selection? 8 Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: Sign Encrypt (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? S Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: Encrypt (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? E Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? A Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: Authenticate (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? Q RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048) 4096 Requested keysize is 4096 bits Please specify how long the key should be valid. 0 = key does not expire = 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) 1y Key expires at Mon 10 Sep 2018 00:00:00 PM UTC Is this correct? (y/N) y Really create? (y/N) y We need to generate a lot of random bytes. It is a good idea to perform some other action (type on the keyboard, move the mouse, utilize the disks) during the prime generation; this gives the random number generator a better chance to gain enough entropy. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh ``` Finish by saving the keys. ```console gpg> save ``` ## Add extra identities **Optional** To add additional email addresses or identities, use `adduid`. Edit the identity: ```console gpg --expert --edit-key $KEYID ``` Then add the new identity: ```console gpg> adduid Real name: Dr Duh Email address: DrDuh@other.org Comment: You selected this USER-ID: "Dr Duh " sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: never usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: never usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: never usage: A [ultimate] (1). Dr Duh [ unknown] (2). Dr Duh gpg> trust sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: never usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: never usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: never usage: A [ultimate] (1). Dr Duh [ unknown] (2). Dr Duh Please decide how far you trust this user to correctly verify other users' keys (by looking at passports, checking fingerprints from different sources, etc.) 1 = I don't know or won't say 2 = I do NOT trust 3 = I trust marginally 4 = I trust fully 5 = I trust ultimately m = back to the main menu Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: never usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: never usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: never usage: A [ultimate] (1). Dr Duh [ unknown] (2). Dr Duh gpg> uid 1 sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: never usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: never usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: never usage: A [ultimate] (1)* Dr Duh [ unknown] (2). Dr Duh gpg> primary sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: never usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: never usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: never usage: A [ultimate] (1)* Dr Duh [ unknown] (2) Dr Duh gpg> save ``` By default, the last identity added will be the primary user ID - use `primary` to change that. # Verify List available secret keys: ```console gpg -K ``` Verify output: ```console /tmp.FLZC0xcM/pubring.kbx ------------------------------------------------------------------------- sec rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [C] Key fingerprint = 011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB uid Dr Duh ssb rsa4096/0xBECFA3C1AE191D15 2017-10-09 [S] [expires: 2018-10-09] ssb rsa4096/0x5912A795E90DD2CF 2017-10-09 [E] [expires: 2018-10-09] ssb rsa4096/0x3F29127E79649A3D 2017-10-09 [A] [expires: 2018-10-09] ``` Add any additional identities or email addresses as needed using the `adduid` command. **Tip** Verify with a OpenPGP [key best practice checker](https://riseup.net/en/security/message-security/openpgp/best-practices#openpgp-key-checks): ```console gpg --export $KEYID | hokey lint ``` The output will display any problems with your key in red text. If everything is green, your key passes each of the tests. If it is red, your key has failed one of the tests. > hokey may warn (orange text) about cross certification for the authentication key. GPG's [Signing Subkey Cross-Certification](https://gnupg.org/faq/subkey-cross-certify.html) documentation has more detail on cross certification, and gpg v2.2.1 notes "subkey does not sign and so does not need to be cross-certified". hokey may also indicate a problem (red text) with `Key expiration times: []` on the primary key (see [Note #3](#notes) about not setting an expiry for the primary key). # Export secret keys The Certify key and Subkeys will be encrypted with your passphrase when exported. Save a copy of all keys: ```console gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/certify.key gpg --armor --export-secret-subkeys $KEYID > $GNUPGHOME/subkeys.key ``` On Windows, note that using any extension other than `.gpg` or attempting IO redirection to a file will garble the secret key, making it impossible to import it again at a later date: ```console gpg -o \path\to\dir\certify.gpg --armor --export-secret-keys $KEYID gpg -o \path\to\dir\subkeys.gpg --armor --export-secret-subkeys $KEYID ``` # Revocation certificate Although we will backup and store the Certify key in a safe place, it is best practice to never rule out the possibility of losing it or having the backup fail. Without the Certify key, it will be impossible to renew or rotate Subkeys or generate a revocation certificate; the PGP identity will be useless. Even worse, we cannot advertise this fact in any way to those that are using our keys. It is reasonable to assume this *will* occur at some point and the only remaining way to deprecate orphaned keys is a revocation certificate. To create the revocation certificate: ``` console gpg --output $GNUPGHOME/revoke.asc --gen-revoke $KEYID ``` The `revoke.asc` certificate file should be stored (or printed) in a (secondary) place that allows retrieval in case the main backup fails. # Backup Create an **encrypted** backup on portable storage to be kept offline in a secure and durable location. **Tip** The ext2 filesystem (without encryption) can be mounted on both Linux and OpenBSD. Consider using a FAT32/NTFS filesystem for MacOS/Windows compatibility instead. As an additional backup measure, consider using a [paper copy](https://www.jabberwocky.com/software/paperkey/) of the keys. The [Linux Kernel Maintainer PGP Guide](https://www.kernel.org/doc/html/latest/process/maintainer-pgp-guide.html#back-up-your-master-key-for-disaster-recovery) points out that such printouts *are still password-protected*. It recommends to *write the password on the paper*, since it will be unlikely that you remember the original key password that was used when the paper backup was created. Obviously, you need a really good place to keep such a printout. It is strongly recommended to keep even encrypted OpenPGP private key material offline to deter [key overwriting attacks](https://www.kopenpgp.com/), for example. **Linux** Attach another portable storage device and check its label: ```console $ sudo dmesg | tail mmc0: new high speed SDHC card at address a001 mmcblk0: mmc0:a001 SS16G 14.8 GiB $ sudo fdisk -l /dev/mmcblk0 Disk /dev/mmcblk0: 14.9 GiB, 15931539456 bytes, 31116288 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes ``` Write it with random data to prepare for encryption: ```console sudo dd if=/dev/urandom of=/dev/mmcblk0 bs=4M status=progress ``` Erase and create a new partition table: ```console $ sudo fdisk /dev/mmcblk0 Welcome to fdisk (util-linux 2.33.1). Changes will remain in memory only, until you decide to write them. Be careful before using the write command. Device does not contain a recognized partition table. Created a new DOS disklabel with disk identifier 0x3c1ad14a. Command (m for help): g Created a new GPT disklabel (GUID: 4E7495FD-85A3-3E48-97FC-2DD8D41516C3). Command (m for help): w The partition table has been altered. Calling ioctl() to re-read partition table. Syncing disks. ``` Create a new partition with a 25 Megabyte size: ```console $ sudo fdisk /dev/mmcblk0 Welcome to fdisk (util-linux 2.36.1). Changes will remain in memory only, until you decide to write them. Be careful before using the write command. Command (m for help): n Partition number (1-128, default 1): First sector (2048-30261214, default 2048): Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-30261214, default 30261214): +25M Created a new partition 1 of type 'Linux filesystem' and of size 25 MiB. Command (m for help): w The partition table has been altered. Calling ioctl() to re-read partition table. Syncing disks. ``` Use [LUKS](https://askubuntu.com/questions/97196/how-secure-is-an-encrypted-luks-filesystem) to encrypt the new partition. Generate a different password which will be used to protect the filesystem: ```console sudo cryptsetup luksFormat /dev/mmcblk0p1 ``` Mount the partition: ```console sudo cryptsetup luksOpen /dev/mmcblk0p1 secret ``` Create an ext2 filesystem: ```console sudo mkfs.ext2 /dev/mapper/secret -L gpg-$(date +%F) ``` Mount the filesystem and copy the temporary GnuPG directory with keyring: ```console sudo mkdir /mnt/encrypted-storage sudo mount /dev/mapper/secret /mnt/encrypted-storage sudo cp -avi $GNUPGHOME /mnt/encrypted-storage/ ``` **Optional** Backup the OneRNG package: ```console sudo cp onerng_3.7-1_all.deb /mnt/encrypted-storage/ ``` **Note** If you plan on setting up multiple keys, keep the backup mounted or remember to terminate the gpg process before [saving](https://lists.gnupg.org/pipermail/gnupg-users/2016-July/056353.html). Unmount, close and disconnect the encrypted volume: ```console sudo umount /mnt/encrypted-storage/ sudo cryptsetup luksClose secret ``` **OpenBSD** Attach a USB disk and determine its label: ```console $ dmesg | grep sd.\ at sd2 at scsibus5 targ 1 lun 0: SCSI4 0/direct removable serial.00000000000000000000 ``` Print the existing partitions to make sure it's the right device: ```console doas disklabel -h sd2 ``` Initialize the disk by creating an `a` partition with FS type `RAID` and size of 25 Megabytes: ```console $ doas fdisk -giy sd2 Writing MBR at offset 0. Writing GPT. $ doas disklabel -E sd2 Label editor (enter '?' for help at any prompt) sd2> a a offset: [64] size: [31101776] 25M FS type: [4.2BSD] RAID sd2*> w sd2> q No label changes ``` Encrypt with bioctl: ```console $ doas bioctl -c C -l sd2a softraid0 New passphrase: Re-type passphrase: softraid0: CRYPTO volume attached as sd3 ``` Create an `i` partition on the new crypto volume and the filesystem: ```console $ doas fdisk -giy sd3 Writing MBR at offset 0. Writing GPT. $ doas disklabel -E sd3 Label editor (enter '?' for help at any prompt) sd3> a i offset: [64] size: [16001] FS type: [4.2BSD] sd3*> w sd3> q No label changes. $ doas newfs sd3i ``` Mount the filesystem and copy the temporary directory with the keyring: ```console doas mkdir /mnt/encrypted-storage doas mount /dev/sd3i /mnt/encrypted-storage doas cp -avi $GNUPGHOME /mnt/encrypted-storage ``` **Note** If you plan on setting up multiple keys, keep the backup mounted or remember to terminate the gpg process before [saving](https://lists.gnupg.org/pipermail/gnupg-users/2016-July/056353.html). Otherwise, unmount and disconnect the encrypted volume: ```console doas umount /mnt/encrypted-storage doas bioctl -d sd3 ``` See [OpenBSD FAQ#14](https://www.openbsd.org/faq/faq14.html#softraidCrypto) for more information. # Export public keys **Important** Without the *public* key, you will not be able to use GPG to encrypt, decrypt, nor sign messages. However, you will still be able to use YubiKey for SSH authentication. Create another partition on the portable storage device to store the public key, or reconnect networking and upload to a key server. **Linux** ```console $ sudo fdisk /dev/mmcblk0 Welcome to fdisk (util-linux 2.36.1). Changes will remain in memory only, until you decide to write them. Be careful before using the write command. Command (m for help): n Partition number (2-128, default 2): First sector (53248-30261214, default 53248): Last sector, +/-sectors or +/-size{K,M,G,T,P} (53248-30261214, default 30261214): +25M Created a new partition 2 of type 'Linux filesystem' and of size 25 MiB. Command (m for help): w The partition table has been altered. Calling ioctl() to re-read partition table. Syncing disks. $ sudo mkfs.ext2 /dev/mmcblk0p2 $ sudo mkdir /mnt/public $ sudo mount /dev/mmcblk0p2 /mnt/public/ $ gpg --armor --export $KEYID | sudo tee /mnt/public/gpg-$KEYID-$(date +%F).asc ``` **OpenBSD** ```console $ doas disklabel -E sd2 Label editor (enter '?' for help at any prompt) sd2> a b offset: [32130] size: [31069710] 25M FS type: [swap] 4.2BSD sd2*> w sd2> q No label changes. $ doas newfs sd2b $ doas mkdir /mnt/public $ doas mount /dev/sd2b /mnt/public $ gpg --armor --export $KEYID | doas tee /mnt/public/gpg-$KEYID-$(date +%F).asc ``` **Windows** ```console gpg -o \path\to\dir\pubkey.gpg --armor --export $KEYID ``` **Keyserver** (Optional) Upload the public key to a [public keyserver](https://debian-administration.org/article/451/Submitting_your_GPG_key_to_a_keyserver): ```console gpg --send-key $KEYID gpg --keyserver keys.gnupg.net --send-key $KEYID gpg --keyserver hkps://keyserver.ubuntu.com:443 --send-key $KEYID ``` Or if [uploading to keys.openpgp.org](https://keys.openpgp.org/about/usage): ```console gpg --send-key $KEYID | curl -T - https://keys.openpgp.org ``` # Configure Smartcard Plug in a YubiKey and use GPG to configure it as a smartcard: ```console $ gpg --card-edit Reader ...........: Yubico Yubikey 4 OTP U2F CCID Application ID ...: D2760001240102010006055532110000 Application type .: OpenPGP Version ..........: 3.4 Manufacturer .....: Yubico Serial number ....: 05553211 Name of cardholder: [not set] Language prefs ...: [not set] Salutation .......: URL of public key : [not set] Login data .......: [not set] Signature PIN ....: not forced Key attributes ...: rsa2048 rsa2048 rsa2048 Max. PIN lengths .: 127 127 127 PIN retry counter : 3 0 3 Signature counter : 0 KDF setting ......: off Signature key ....: [none] Encryption key....: [none] Authentication key: [none] General key info..: [none] ``` Enter administrative mode: ```console gpg/card> admin Admin commands are allowed ``` **Note** If the card is locked, see [Reset](#reset). **Windows** Use the [YubiKey Manager](https://developers.yubico.com/yubikey-manager) application (note, this is not the similarly named older YubiKey NEO Manager) to enable CCID functionality. ## Enable KDF Key Derived Function (KDF) enables YubiKey to store the hash of PIN, preventing the PIN from being passed as plain text. Note that this requires a relatively new version of GnuPG to work, and may not be compatible with other GPG clients (notably mobile clients). These incompatible clients will be unable to use the YubiKey GPG functions as the PIN will always be rejected. If you are not sure you will only be using your YubiKey on supported platforms, it may be better to skip this step. ```console gpg/card> kdf-setup ``` ## Change PIN The [GPG interface](https://developers.yubico.com/PGP/) is separate from other modules on a YubiKey such as the [PIV interface](https://developers.yubico.com/PIV/Introduction/YubiKey_and_PIV.html). The GPG interface has its own *PIN*, *Admin PIN*, and *Reset Code* - these should be changed from default values! Entering the user *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 GPG data on the card and it will need to be reconfigured. Name | Default Value | Use -----------|---------------|------------------------------------------------------------- PIN | `123456` | decrypt and authenticate (SSH) Admin PIN | `12345678` | reset *PIN*, change *Reset Code*, add keys and owner information Reset code | _**None**_ | reset *PIN* ([more information](https://forum.yubico.com/viewtopicd01c.html?p=9055#p9055)) Values are valid up to 127 ASCII characters and must be at least 6 (*PIN*) or 8 (*Admin PIN*, *Reset Code*) characters. See the GnuPG documentation on [Managing PINs](https://www.gnupg.org/howtos/card-howto/en/ch03s02.html) for details. To update GPG PINs on the YubiKey: ```console gpg/card> passwd gpg: OpenPGP card no. D2760001240102010006055532110000 detected 1 - change PIN 2 - unblock PIN 3 - change Admin PIN 4 - set the Reset Code Q - quit Your selection? 3 PIN changed. 1 - change PIN 2 - unblock PIN 3 - change Admin PIN 4 - set the Reset Code Q - quit Your selection? 1 PIN changed. 1 - change PIN 2 - unblock PIN 3 - change Admin PIN 4 - set the Reset Code Q - quit Your selection? q ``` **Note** The number of retry attempts can be changed later with the following command, documented [here](https://docs.yubico.com/software/yubikey/tools/ykman/OpenPGP_Commands.html#ykman-openpgp-access-set-retries-options-pin-retries-reset-code-retries-admin-pin-retries): ```bash ykman openpgp access set-retries 5 5 5 -f -a YOUR_ADMIN_PIN ``` ## Set information Some fields are optional. ```console gpg/card> name Cardholder's surname: Duh Cardholder's given name: Dr gpg/card> lang Language preferences: en gpg/card> login Login data (account name): doc@duh.to gpg/card> list Application ID ...: D2760001240102010006055532110000 Version ..........: 3.4 Manufacturer .....: unknown Serial number ....: 05553211 Name of cardholder: Dr Duh Language prefs ...: en Sex ..............: unspecified URL of public key : [not set] Login data .......: doc@duh.to Private DO 4 .....: [not set] Signature PIN ....: not forced Key attributes ...: rsa2048 rsa2048 rsa2048 Max. PIN lengths .: 127 127 127 PIN retry counter : 3 0 3 Signature counter : 0 KDF setting ......: on Signature key ....: [none] Encryption key....: [none] Authentication key: [none] General key info..: [none] gpg/card> quit ``` # Transfer keys **Important** Transferring keys to YubiKey using `keytocard` is a one-way/destructive operation. Make sure a backup was made before proceeding. `keytocard` converts the local, on-disk key into a stub, which means the on-disk copy is no longer usable to transfer to subsequent YubiKeys. Previous GPG versions required the `toggle` command before selecting keys. The currently selected key(s) are indicated with an `*`. When moving keys only one key should be selected at a time. ```console $ gpg --edit-key $KEYID Secret key is available. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh ``` ## Signing **Important** You will be prompted for the Certify key passphrase and Admin PIN. Select and transfer the signature key. ```console gpg> key 1 sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb* rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh gpg> keytocard Please select where to store the key: (1) Signature key (3) Authentication key Your selection? 1 You need a passphrase to unlock the secret key for user: "Dr Duh " 4096-bit RSA key, ID 0xBECFA3C1AE191D15, created 2016-05-24 ``` ## Encryption Type `key 1` again to de-select and `key 2` to select the next key: ```console gpg> key 1 gpg> key 2 sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb* rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh gpg> keytocard Please select where to store the key: (2) Encryption key Your selection? 2 [...] ``` ## Authentication Type `key 2` again to deselect and `key 3` to select the last key: ```console gpg> key 2 gpg> key 3 sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb* rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh gpg> keytocard Please select where to store the key: (3) Authentication key Your selection? 3 ``` Save and quit: ```console gpg> save ``` # Verify card Verify the Subkeys have been moved to YubiKey as indicated by `ssb>` ```console $ gpg -K /tmp.FLZC0xcM/pubring.kbx ------------------------------------------------------------------------- sec rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [C] Key fingerprint = 011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB uid Dr Duh ssb> rsa4096/0xBECFA3C1AE191D15 2017-10-09 [S] [expires: 2018-10-09] ssb> rsa4096/0x5912A795E90DD2CF 2017-10-09 [E] [expires: 2018-10-09] ssb> rsa4096/0x3F29127E79649A3D 2017-10-09 [A] [expires: 2018-10-09] ``` # Multiple YubiKeys To provision additional YubiKeys, restore the Certify key backup and repeat [Configure Smartcard](#configure-smartcard). ```console $ mv -vi $GNUPGHOME $GNUPGHOME.1 renamed '/tmp.FLZC0xcM' -> '/tmp.FLZC0xcM.1' $ cp -avi /mnt/encrypted-storage/tmp.XXX $GNUPGHOME '/mnt/encrypted-storage/tmp.FLZC0xcM' -> '/tmp.FLZC0xcM' $ cd $GNUPGHOME ``` ## Switching between YubiKeys When you add a GPG key to a YubiKey using the *keytocard* command, GPG deletes the key from your keyring and adds a *stub* pointing to that exact YubiKey (the stub identifies the GPG KeyID and the YubiKey serial number). However, when you do this same operation for a second YubiKey, the stub in your keyring is overwritten by the *keytocard* operation and now the stub points to your second YubiKey. Adding more repeats this overwriting operation. In other words, the stub will point only to the last YubiKey written to. When using GPG key operations with the GPG key you placed onto the YubiKeys, GPG will request a specific YubiKey asking that you insert a YubiKey with a given serial number (referenced by the stub). GPG will not recognise another YubiKey with a different serial number without manual intervention. You can force GPG to scan the card and re-create stubs to point to another YubiKey. Having created two (or more YubiKeys) with the same GPG key (as described above) where the stubs are pointing to the second YubiKey: Insert the first YubiKey (which has a different serial number) and run the following command: ```console gpg-connect-agent "scd serialno" "learn --force" /bye ``` GPG will then scan your first YubiKey for GPG keys and recreate the stubs to point to the GPG keyID and YubiKey serial number of this first YubiKey. To return to using the second YubiKey just repeat (insert other YubiKey and re-run command). Obviously this command is not easy to remember so it is recommended to either create a script or a shell alias to make this more user friendly. # Multiple Hosts It can be convenient to use your YubiKey on multiple hosts: * a desktop plus a laptop * home and work computers * an environment like [Tails](https://tails.boum.org) The simplest way to set up a second host is to begin by exporting your public key and trust settings on the host where your YubiKey is already working: ``` console gpg --armor --export $KEYID > gpg-public-key-$KEYID.asc gpg --export-ownertrust > gpg-owner-trust.txt ``` Move both files to the second host. Then, on the second host: 1. Define the KEYID: ``` console export KEYID=0xFF3E7D88647EBCDB ``` 2. Import the public key: ``` console gpg --import gpg-public-key-$KEYID.asc ``` 3. Import the trust settings: ``` console gpg --import-ownertrust < gpg-owner-trust.txt ``` 4. Insert your YubiKey into a USB port. 5. Import the private key stubs from the YubiKey: ``` console gpg --card-status ``` If you need to set up a second host when you are travelling and don't have ready access to your primary host, you can import your public key from a key-server and set trust: 1. Define the identity: ```console export KEYID=0xFF3E7D88647EBCDB ``` 2. Fetch the public key from a key-server. For example: ```console gpg --keyserver hkps://keyserver.ubuntu.com:443 --recv $KEYID ``` 3. Set ultimate trust: ``` console $ gpg --edit-key $KEYID gpg> trust Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y gpg> quit ``` 4. Insert YubiKey into a USB port. 5. Import the private key stubs from the YubiKey: ```console gpg --card-status ``` Another approach is to add the URL of your public key to your YubiKey: 1. Define the identity: ```console KEYID=0xFF3E7D88647EBCDB ``` 2. Construct the URL (based on [Shaw 2003](https://datatracker.ietf.org/doc/html/draft-shaw-openpgp-hkp-00)): ```console [[ ! "$KEYID" =~ ^"0x" ]] && KEYID="0x${KEYID}" URL="hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=${KEYID}" ``` 3. Insert YubiKey into a USB port. 4. Add the URL to your YubiKey (will prompt for your YubiKey's admin PIN): ```console $ gpg --edit-card gpg/card> admin gpg/card> url URL to retrieve public key: hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF3E7D88647EBCDB gpg/card> quit ``` **Note** You do not have to use a *keyserver* URL. You can export your public key as an armored ASCII file and upload it to any place on the web where it can be downloaded using HTTP/HTTPS. Once the URL of your public key is present on your YubiKey, setting up a new host becomes: 1. Insert YubiKey into a USB port. 2. Use the `fetch` sub-command to retrieve your public key using the URL stored on the card: ```console $ gpg --edit-card gpg/card> fetch gpg: requesting key from 'hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF3E7D88647EBCDB' gpg: /home/pi/.gnupg/trustdb.gpg: trustdb created gpg: key FF3E7D88647EBCDB: public key "Dr Duh " imported gpg: Total number processed: 1 gpg: imported: 1 gpg/card> quit ``` This step also imports the private key stubs from the YubiKey. 3. Define your KEYID (which appears in the output in the previous step): ```console export KEYID=0xFF3E7D88647EBCDB ``` 4. Set ultimate trust: ```console $ gpg --edit-key $KEYID gpg> trust Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y gpg> quit ``` # Finish Before completing setup, verify the following: - [ ] Saved encryption, signing and authentication Subkeys to YubiKey (`gpg -K` should show `ssb>` for Subkeys) - [ ] Saved YubiKey user and admin PINs, which are unique and were changed from default values - [ ] Saved Certify key passphrase to a secure and durable location - [ ] Saved Certify key, Subkeys and revocation certificate on encrypted portable storage, to be kept offline - [ ] Saved password to encrypted volume on portable storage - [ ] Saved copy of public key where is can be easily accessed later Reboot to finish. If an ephemeral environment was not used for setup, delete secret keys from the keyring and [securely delete](https://srm.sourceforge.net/) `$GNUPGHOME`. ```console gpg --delete-secret-key $KEYID sudo srm -r $GNUPGHOME || sudo rm -rf $GNUPGHOME unset GNUPGHOME ``` # Using keys The following command creates and initialises the `~/.gnupg` if it does not exist already: ```console gpg -k ``` Change the working directory: ```console cd ~/.gnupg ``` Download [drduh/config/gpg.conf](https://github.com/drduh/config/blob/master/gpg.conf): ```console wget https://raw.githubusercontent.com/drduh/config/master/gpg.conf chmod 600 gpg.conf ``` Set the following option. This avoids the problem where GnuPG will prompt, repeatedly, for the insertion of an already-inserted YubiKey: ```console touch scdaemon.conf echo "disable-ccid" >>scdaemon.conf ``` > The `disable-ccid` option is only required for GnuPG versions 2.3 or later. However, setting this option does not appear to interfere with the operation of earlier versions of GnuPG so it is recommended for all installations. Install the required packages and mount the non-encrypted volume created earlier: **Linux** ```console sudo apt update sudo apt install -y gnupg2 gnupg-agent gnupg-curl scdaemon pcscd sudo mount /dev/mmcblk0p2 /mnt ``` **OpenBSD** ```console doas pkg_add gnupg pcsc-tools doas mount /dev/sd2b /mnt ``` Import the public key file: ```console gpg --import /mnt/gpg-0x*.asc ``` Or download the public key from a keyserver: ```console gpg --recv $KEYID ``` Edit the Certify key: ```console export KEYID=0xFF3E7D88647EBCDB gpg --edit-key $KEYID ``` Assign ultimate trust by selecting `trust` and `5`: ```console gpg> trust pub 4096R/0xFF3E7D88647EBCDB created: 2016-05-24 expires: never usage: C trust: unknown validity: unknown sub 4096R/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S sub 4096R/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E sub 4096R/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ unknown] (1). Dr Duh Please decide how far you trust this user to correctly verify other users' keys (by looking at passports, checking fingerprints from different sources, etc.) 1 = I don't know or won't say 2 = I do NOT trust 3 = I trust marginally 4 = I trust fully 5 = I trust ultimately m = back to the main menu Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y pub 4096R/0xFF3E7D88647EBCDB created: 2016-05-24 expires: never usage: C trust: ultimate validity: unknown sub 4096R/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S sub 4096R/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E sub 4096R/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ unknown] (1). Dr Duh gpg> quit ``` Remove and re-insert YubiKey and verify the status: ```console $ gpg --card-status Reader ...........: Yubico YubiKey OTP FIDO CCID 00 00 Application ID ...: D2760001240102010006055532110000 Version ..........: 3.4 Manufacturer .....: Yubico Serial number ....: 05553211 Name of cardholder: Dr Duh Language prefs ...: en Sex ..............: unspecified URL of public key : [not set] Login data .......: doc@duh.to Signature PIN ....: not forced Key attributes ...: rsa4096 rsa4096 rsa4096 Max. PIN lengths .: 127 127 127 PIN retry counter : 3 3 3 Signature counter : 0 KDF setting ......: on Signature key ....: 07AA 7735 E502 C5EB E09E B8B0 BECF A3C1 AE19 1D15 created ....: 2016-05-24 23:22:01 Encryption key....: 6F26 6F46 845B BEB8 BDF3 7E9B 5912 A795 E90D D2CF created ....: 2016-05-24 23:29:03 Authentication key: 82BE 7837 6A3F 2E7B E556 5E35 3F29 127E 7964 9A3D created ....: 2016-05-24 23:36:40 General key info..: pub 4096R/0xBECFA3C1AE191D15 2016-05-24 Dr Duh sec# 4096R/0xFF3E7D88647EBCDB created: 2016-05-24 expires: never ssb> 4096R/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 card-no: 0006 05553211 ssb> 4096R/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 card-no: 0006 05553211 ssb> 4096R/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 card-no: 0006 05553211 ``` `sec#` indicates the corresponding key is not available. **Note** If you see `General key info..: [none]` in the output instead - go back and import the public key using the previous step. Encrypt a message to your own key (useful for storing password credentials and other data): ```console echo "test message string" | gpg --encrypt --armor --recipient $KEYID -o encrypted.txt ``` To encrypt to multiple recipients (or to multiple keys): ```console echo "test message string" | \ gpg --encrypt --armor --recipient $KEYID_0 --recipient $KEYID_1 --recipient $KEYID_2 -o encrypted.txt ``` Decrypt the message: ```console $ gpg --decrypt --armor encrypted.txt gpg: anonymous recipient; trying secret key 0x0000000000000000 ... gpg: okay, we are the anonymous recipient. gpg: encrypted with RSA key, ID 0x0000000000000000 test message string ``` Sign a message: ```console echo "test message string" | gpg --armor --clearsign > signed.txt ``` Verify the signature: ```console $ gpg --verify signed.txt gpg: Signature made Wed 25 May 2016 00:00:00 AM UTC gpg: using RSA key 0xBECFA3C1AE191D15 gpg: Good signature from "Dr Duh " [ultimate] Primary key fingerprint: 011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB Subkey fingerprint: 07AA 7735 E502 C5EB E09E B8B0 BECF A3C1 AE19 1D15 ``` Use a [shell function](https://github.com/drduh/config/blob/master/zshrc) to make encrypting files easier: ``` secret () { output=~/"${1}".$(date +%s).enc gpg --encrypt --armor --output ${output} -r 0x0000 -r 0x0001 -r 0x0002 "${1}" && echo "${1} -> ${output}" } reveal () { output=$(echo "${1}" | rev | cut -c16- | rev) gpg --decrypt --output ${output} "${1}" && echo "${1} -> ${output}" } ``` ```console $ secret document.pdf document.pdf -> document.pdf.1580000000.enc $ reveal document.pdf.1580000000.enc gpg: anonymous recipient; trying secret key 0xFF3E7D88647EBCDB ... gpg: okay, we are the anonymous recipient. gpg: encrypted with RSA key, ID 0x0000000000000000 document.pdf.1580000000.enc -> document.pdf ``` # Rotating keys PGP does not provide [forward secrecy](https://en.wikipedia.org/wiki/Forward_secrecy) - a compromised key may be used to decrypt all past messages. Although keys stored on YubiKey are difficult to steal, it is not impossible; the key and PIN could be taken, or a vulnerability may be discovered in key hardware or the random number generator used to create them, for example. Therefore, it is good practice to occassionally rotate Subkeys. When a Subkey expires, it can either be renewed or replaced. Both actions require access to the Certify key. - Renewing Subkeys by updating expiration dates indicates you are still in possession of the Certify key and is more convenient. - Replacing keys, on the other hand, is less convenient but 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. However, you will always be able to decrypt previous messages using the encrypted backup of the original keys. Neither rotation method is superior and it's up to personal philosophy on identity management and individual threat model to decide which one to use, or whether to expire Subkeys at all. Ideally, Subkeys would be ephemeral: used only once for each unique encryption, signing and authentication event, however in practice that is not really feasible nor worthwhile with YubiKey. Advanced users may want to dedicate an air-gapped machine for more frequent rotations and ease of provisioning key materials. ## Setup environment To renew or rotate Subkeys, follow the same process as generating keys: boot to a secure environment, install required software and disconnect networking. Connect the portable storage device with the Certify key and identify the disk label: ```console $ sudo dmesg | tail mmc0: new high speed SDHC card at address a001 mmcblk0: mmc0:a001 SS16G 14.8 GiB (ro) mmcblk0: p1 p2 ``` Decrypt and mount the encrypted volume: ```console sudo cryptsetup luksOpen /dev/mmcblk0p1 secret sudo mount /dev/mapper/secret /mnt/encrypted-storage ``` Import the Certify key and configuration to a temporary working directory. Note that Windows users should import certify.gpg: ```console export GNUPGHOME=$(mktemp -d -t gnupg_$(date +%Y%m%d%H%M)_XXX) gpg --import /mnt/encrypted-storage/tmp.XXX/certify.key cp -v /mnt/encrypted-storage/tmp.XXX/gpg.conf $GNUPGHOME ``` Edit the Certify key: ```console export KEYID=0xFF3E7D88647EBCDB gpg --expert --edit-key $KEYID ``` ## Renewing Subkeys Renewing Subkeys is simpler: you do not need to generate new keys, move keys to the YubiKey, or update any SSH public keys linked to the GPG key. All you need to do is to change the expiry time associated with the public key (which requires access to the Certify key you just loaded) and then to export that public key and import it on any computer where you wish to use the **GPG** (as distinct from the SSH) key. To change the expiration date of all Subkeys, start by selecting all keys: ```console $ gpg --edit-key $KEYID Secret key is available. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh gpg> key 1 Secret key is available. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb* rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh gpg> key 2 Secret key is available. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb* rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb* rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh gpg> key 3 Secret key is available. sec rsa4096/0xFF3E7D88647EBCDB created: 2017-10-09 expires: never usage: C trust: ultimate validity: ultimate ssb* rsa4096/0xBECFA3C1AE191D15 created: 2017-10-09 expires: 2018-10-09 usage: S ssb* rsa4096/0x5912A795E90DD2CF created: 2017-10-09 expires: 2018-10-09 usage: E ssb* rsa4096/0x3F29127E79649A3D created: 2017-10-09 expires: 2018-10-09 usage: A [ultimate] (1). Dr Duh ``` Then, use the `expire` command to set a new expiration date. This will **not** cause currently valid keys to become expired. ```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) ``` Follow these prompts to set a new expiration date, then `save` Next, [export the public key](#export-public-keys): ```console gpg --armor --export $KEYID > gpg-$KEYID-$(date +%F).asc ``` Transfer the public key to the computer from which you use your GPG key, and then import it with: ```console gpg --import gpg-0x*.asc ``` Alternatively, use a public key server (it will update the key if already on the server): ```console gpg --send-key $KEYID ``` and import the newly updated key on any computer where you wish to use it (it will update the key if previously imported): ```console gpg --recv $KEYID ``` This will extend the validity of the GPG key and will allow you to use it for SSH authorization. Note that you do **not** need to update the SSH public key located on remote servers. ## Rotating keys Follow the original steps to generate and add each Subkey. Previous Subkeys may be kept or deleted from the identity. Finish by exporting new keys: ```console gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/certify.key gpg --armor --export-secret-subkeys $KEYID > $GNUPGHOME/subkeys.key ``` Copy the **new** temporary working directory to encrypted storage, which should still be mounted: ```console sudo cp -avi $GNUPGHOME /mnt/encrypted-storage ``` There should now be at least two versions of the Certify and Subkeys: ```console $ ls /mnt/encrypted-storage lost+found tmp.ykhTOGjR36 tmp.2gyGnyCiHs ``` Unmount and close the encrypted volume: ```console sudo umount /mnt/encrypted-storage sudo cryptsetup luksClose /dev/mapper/secret ``` Export the updated public key: ```console sudo mkdir /mnt/public sudo mount /dev/mmcblk0p2 /mnt/public gpg --armor --export $KEYID | sudo tee /mnt/public/$KEYID-$(date +%F).asc sudo umount /mnt/public ``` Disconnect the storage device and follow the original steps to transfer new keys (4, 5 and 6) to the YubiKey, replacing existing ones. Reboot or securely erase the GPG temporary working directory. # Adding notations Notations can be added to user ID(s) and can be used in conjunction with [Keyoxide](https://keyoxide.org) to create [OpenPGP identity proofs](https://docs.keyoxide.org/wiki/identity-proof-formats/). Adding notations requires access to the Certify key so we can follow the setup instructions taken from this [section](#setup-environment) of this guide. Please note that there is no need to connect the YubiKey to the setup environment and that we do not need to generate new keys, move keys to the YubiKey, or update any SSH public keys linked to the GPG key. After having completed the environment setup, it is possible to follow any of the guides listed under "Adding proofs" in the Keyoxide ["Guides"](https://keyoxide.org/guides/) page up until the notation is saved using the `save` command. At this point the public key can be exported: ```console gpg --export $KEYID > pubkey.asc ``` The public key can now be transferred to the computer where the GPG key is used and it is imported with: ```console gpg --import pubkey.asc ``` N.B.: The `showpref` command can be issued to ensure that the notions were correctly added. # SSH **Tip** If you want to use a YubiKey for SSH only (and don't really care about PGP/GPG), then [since OpenSSH v8.2](https://www.openssh.com/txt/release-8.2) you alternatively can simply `ssh-keygen -t ed25519-sk` (without requiring anything else from this guide!), as explained [in this guide](https://github.com/vorburger/vorburger.ch-Notes/blob/develop/security/ed25519-sk.md). Yubico also recently announced support for resident ssh keys under OpenSSH 8.2+ on their blue "security key 5 nfc" as mentioned in their [blog post](https://www.yubico.com/blog/github-now-supports-ssh-security-keys/)._ [gpg-agent](https://wiki.archlinux.org/index.php/GnuPG#SSH_agent) supports the OpenSSH ssh-agent protocol (`enable-ssh-support`), as well as Putty's Pageant on Windows (`enable-putty-support`). This means it can be used instead of the traditional ssh-agent / pageant. There are some differences from ssh-agent, notably that gpg-agent does not _cache_ keys rather it converts, encrypts and stores them - persistently - as GPG keys and then makes them available to ssh clients. Any existing ssh private keys that you'd like to keep in `gpg-agent` should be deleted after they've been imported to the GPG agent. When importing the key to `gpg-agent`, you'll be prompted for a passphrase to protect that key within GPG's key store - you may want to use the same passphrase as the original's ssh version. GPG can both cache passphrases for a determined period (ref. `gpg-agent`'s various `cache-ttl` options), and since version 2.1 can store and fetch passphrases via the macOS keychain. Note than when removing the old private key after importing to `gpg-agent`, keep the `.pub` key file around for use in specifying ssh identities (e.g. `ssh -i /path/to/identity.pub`). Probably the biggest thing missing from `gpg-agent`'s ssh agent support is being able to remove keys. `ssh-add -d/-D` have no effect. Instead, you need to use the `gpg-connect-agent` utility to lookup a key's keygrip, match that with the desired ssh key fingerprint (as an MD5) and then delete that keygrip. The [gnupg-users mailing list](https://lists.gnupg.org/pipermail/gnupg-users/2016-August/056499.html) has more information. ## Create configuration Create a hardened configuration for gpg-agent by downloading [drduh/config/gpg-agent.conf](https://github.com/drduh/config/blob/master/gpg-agent.conf): ```console cd ~/.gnupg wget https://raw.githubusercontent.com/drduh/config/master/gpg-agent.conf ``` **Important** The `cache-ttl` options do **NOT** apply when using a YubiKey as a smartcard as the PIN is [cached by the smartcard itself](https://dev.gnupg.org/T3362). Therefore, in order to clear the PIN from cache (smartcard equivalent to `default-cache-ttl` and `max-cache-ttl`), you need to unplug the YubiKey, or set the `forcesig` flag when editing the card to be prompted for the PIN each time. **Tip** Set `pinentry-program /usr/bin/pinentry-gnome3` for a GUI-based prompt. If the _pinentry_ graphical dialog doesn't show and you get this error: `sign_and_send_pubkey: signing failed: agent refused operation`, you may need to install the `dbus-user-session` package and restart the computer for the `dbus` user session to be fully inherited; this is because behind the scenes, `pinentry` complains about `No $DBUS_SESSION_BUS_ADDRESS found`, falls back to `curses` but doesn't find the expected `tty`. On macOS, use `brew install pinentry-mac` and set the program path to `pinentry-program /usr/local/bin/pinentry-mac` for Intel Macs, `/opt/homebrew/bin/pinentry-mac` for ARM/Apple Silicon Macs or `pinentry-program /usr/local/MacGPG2/libexec/pinentry-mac.app/Contents/MacOS/pinentry-mac` if using MacGPG Suite. For the configuration to take effect you have to run `gpgconf --kill gpg-agent`. ## Replace agents To launch `gpg-agent` for use by SSH, use the `gpg-connect-agent /bye` or `gpgconf --launch gpg-agent` commands. Add these to the shell `rc` file: ```console export GPG_TTY="$(tty)" export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh" gpg-connect-agent updatestartuptty /bye > /dev/null ``` On modern systems, `gpgconf --list-dirs agent-ssh-socket` will automatically set `SSH_AUTH_SOCK` to the correct value and is better than hard-coding to `run/user/$UID/gnupg/S.gpg-agent.ssh`, if available: ```console export GPG_TTY="$(tty)" export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket) gpgconf --launch gpg-agent ``` For fish, `config.fish` would look like this (consider putting them into the `is-interactive` block): ```fish set -x GPG_TTY (tty) set -x SSH_AUTH_SOCK (gpgconf --list-dirs agent-ssh-socket) gpgconf --launch gpg-agent ``` Note that if you use `ForwardAgent` for ssh-agent forwarding, `SSH_AUTH_SOCK` only needs to be set on the *local* laptop (workstation), where the YubiKey is plugged in. On the *remote* server, `ssh` will automatically set `SSH_AUTH_SOCK` to something like `/tmp/ssh-mXzCzYT2Np/agent.7541` upon connection. Do **not** manually set `SSH_AUTH_SOCK` on the server - doing so would break [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding). If you use `S.gpg-agent.ssh` (see [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding) for more info), `SSH_AUTH_SOCK` should also be set on the *remote*. However, `GPG_TTY` should not be set on the *remote*, explanation specified in that section. ## Copy public key **Note** It is **not** necessary to import the corresponding GPG public key in order to use SSH. Copy and paste the output from `ssh-add` to the server's `authorized_keys` file: ```console $ ssh-add -L ssh-rsa AAAAB4NzaC1yc2EAAAADAQABAAACAz[...]zreOKM+HwpkHzcy9DQcVG2Nw== cardno:000605553211 ``` ## (Optional) Save public key for identity file configuration By default, SSH attempts to use all the identities available via the agent. It's often a good idea to manage exactly which keys SSH will use to connect to a server, for example to separate different roles or [to avoid being fingerprinted by untrusted ssh servers](https://blog.filippo.io/ssh-whoami-filippo-io/). To do this you'll need to use the command line argument `-i [identity_file]` or the `IdentityFile` and `IdentitiesOnly` options in `.ssh/config`. The argument provided to `IdentityFile` is traditionally the path to the _private_ key file (for example `IdentityFile ~/.ssh/id_rsa`). For the YubiKey - indeed, in general for keys stored in an ssh agent - `IdentityFile` should point to the _public_ key file, `ssh` will select the appropriate private key from those available via the ssh agent. To prevent `ssh` from trying all keys in the agent use the `IdentitiesOnly yes` option along with one or more `-i` or `IdentityFile` options for the target host. To reiterate, with `IdentitiesOnly yes`, `ssh` will not automatically enumerate public keys loaded into `ssh-agent` or `gpg-agent`. This means `publickey` authentication will not proceed unless explicitly named by `ssh -i [identity_file]` or in `.ssh/config` on a per-host basis. In the case of YubiKey usage, to extract the public key from the ssh agent: ```console ssh-add -L | grep "cardno:000605553211" > ~/.ssh/id_rsa_yubikey.pub ``` Then you can explicitly associate this YubiKey-stored key for used with a host, `github.com` for example, as follows: ```console $ cat << EOF >> ~/.ssh/config Host github.com IdentitiesOnly yes IdentityFile ~/.ssh/id_rsa_yubikey.pub EOF ``` ## Connect with public key authentication ```console $ ssh git@github.com -vvv [...] debug2: key: cardno:000605553211 (0x1234567890), debug1: Authentications that can continue: publickey debug3: start over, passed a different list publickey debug3: preferred gssapi-keyex,gssapi-with-mic,publickey,keyboard-interactive,password debug3: authmethod_lookup publickey debug3: remaining preferred: keyboard-interactive,password debug3: authmethod_is_enabled publickey debug1: Next authentication method: publickey debug1: Offering RSA public key: cardno:000605553211 debug3: send_pubkey_test debug2: we sent a publickey packet, wait for reply debug1: Server accepts key: pkalg ssh-rsa blen 535 debug2: input_userauth_pk_ok: fp e5:de:a5:74:b1:3e:96:9b:85:46:e7:28:53:b4:82:c3 debug3: sign_and_send_pubkey: RSA e5:de:a5:74:b1:3e:96:9b:85:46:e7:28:53:b4:82:c3 debug1: Authentication succeeded (publickey). [...] ``` **Tip** To make multiple connections or securely transfer many files, consider using the [ControlMaster](https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing) ssh option. Also see [drduh/config/ssh_config](https://github.com/drduh/config/blob/master/ssh_config). ## Import SSH keys If there are existing SSH keys to make available via `gpg-agent`, they will need to be imported. Then, remove the original private keys. When importing the key, `gpg-agent` uses the key filename as the label - this makes it easier to follow where the key originated from. In this example, we're starting with just the YubiKey in place and importing `~/.ssh/id_rsa`: ```console $ ssh-add -l 4096 SHA256:... cardno:00060123456 (RSA) $ ssh-add ~/.ssh/id_rsa && rm ~/.ssh/id_rsa ``` When invoking `ssh-add`, a prompt for the SSH key passphrase will appear, then the `pinentry` program will prompt and confirm a new passphrase to encrypt the converted key within the GPG key store. The migrated key will be listed in `ssh-add -l`: ```console $ ssh-add -l 4096 SHA256:... cardno:00060123456 (RSA) 2048 SHA256:... /Users/username/.ssh/id_rsa (RSA) ``` To show the keys with MD5 fingerprints, as used by `gpg-connect-agent`'s `KEYINFO` and `DELETE_KEY` commands: ```console $ ssh-add -E md5 -l 4096 MD5:... cardno:00060123456 (RSA) 2048 MD5:... /Users/username/.ssh/id_rsa (RSA) ``` When using the key `pinentry` will be invoked to request the key passphrase. The passphrase will be cached for up to 10 idle minutes between uses, up to a maximum of 2 hours. ## Remote Machines (SSH Agent Forwarding) **Note** SSH Agent Forwarding can [add additional risk](https://matrix.org/blog/2019/05/08/post-mortem-and-remediations-for-apr-11-security-incident/#ssh-agent-forwarding-should-be-disabled) - proceed with caution! There are two methods for ssh-agent forwarding, one is provided by OpenSSH and the other is provided by GnuPG. The latter one may be more insecure as raw socket is just forwarded (not like `S.gpg-agent.extra` with only limited functionality; if `ForwardAgent` implemented by OpenSSH is just forwarding the raw socket, then they are insecure to the same degree). But for the latter one, one convenience is that one may forward once and use this agent everywhere in the remote. So again, proceed with caution! For example, `tmux` does not have some environment variables like `$SSH_AUTH_SOCK` when you ssh into remote and attach an old `tmux` session. In this case if you use `ForwardAgent`, you need to find the socket and `export SSH_AUTH_SOCK=/tmp/ssh-agent-xxx/xxxx.socket` for each shell. But with `S.gpg-agent.ssh` in fixed place, one can just use it as ssh-agent in their shell rc file. ### Use ssh-agent In the above steps, you have successfully configured a local ssh-agent. You should now be able to use `ssh -A remote` on the _local_ machine to log into _remote_, and should then be able to use YubiKey as if it were connected to the remote machine. For example, using e.g. `ssh-add -l` on that remote machine should show the public key from the YubiKey (note `cardno:`). (If you don't want to have to remember to use `ssh -A`, you can use `ForwardAgent yes` in `~/.ssh/config`. As a security best practice, always use `ForwardAgent yes` only for a single `Hostname`, never for all servers.) ### Use S.gpg-agent.ssh First you need to go through [Remote Machines (GPG Agent Forwarding)](#remote-machines-gpg-agent-forwarding), know the conditions for gpg-agent forwarding and know the location of `S.gpg-agent.ssh` on both the local and the remote. You may use the command: ```console $ gpgconf --list-dirs agent-ssh-socket ``` Edit `.ssh/config` to add the remote host: ``` Host Hostname remote-host.tld StreamLocalBindUnlink yes RemoteForward /run/user/1000/gnupg/S.gpg-agent.ssh /run/user/1000/gnupg/S.gpg-agent.ssh # RemoteForward [remote socket] [local socket] # Note that ForwardAgent is not wanted here! ``` After successfully ssh into the remote, you should check that you have `/run/user/1000/gnupg/S.gpg-agent.ssh` lying there. 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! ``` You should change the path according to `gpgconf --list-dirs agent-ssh-socket` on *remote* and *third*. ## 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). To configure a signing key: ```console git config --global user.signingkey $KEYID ``` The `user.email` option must match the email address associated with the PGP identity. To sign commits or tags, use the `-S` option. GPG will automatically query YubiKey and prompt for a PIN. To authenticate: **Windows** Run the following commands: ```console git config --global core.sshcommand "plink -agent" git config --global gpg.program 'C:\Program Files (x86)\GnuPG\bin\gpg.exe' ``` You can then change the repository URL to `git@github.com:USERNAME/repository` and any authenticated commands will be authorized by YubiKey. **Note** If you encounter the error `gpg: signing failed: No secret key` - run `gpg --card-status` with YubiKey plugged in and try the git command again. ## OpenBSD Install and enable tools for use with PC/SC drivers, cards, readers, then reboot to recognize YubiKey: ```console doas pkg_add pcsc-tools doas rcctl enable pcscd doas reboot ``` ## Windows Windows can already have some virtual smartcard readers installed, like the one provided for Windows Hello. To ensure YubiKey is the correct one used by scdaemon, add it to its configuration. First, find YubiKey label using PowerShell: ``` powershell PS C:\WINDOWS\system32> Get-PnpDevice -Class SoftwareDevice | Where-Object {$_.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. * Create or edit `%APPDATA%/gnupg/scdaemon.conf` to add: ``` reader-port ``` * Create or edit `%APPDATA%/gnupg/gpg-agent.conf` to add: ``` enable-ssh-support enable-putty-support ``` * Open a command console, restart the agent: ``` > gpg-connect-agent killagent /bye > gpg-connect-agent /bye ``` * Enter `> gpg --card-status` to see YubiKey details. * Import the [public key](#export-public-key): `> gpg --import ` * [Trust the Certify key](#trust-certify-key) * Retrieve the public key id: `> gpg --list-public-keys` * Export the SSH key from GPG: `> gpg --export-ssh-key ` Copy this key to a file for later use. It represents the public SSH key corresponding to the secret key on the YubiKey. You can upload this key to any server you wish to SSH into. 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 a system shutdown. Modify the shortcut properties so it starts in a "Minimized" window, to avoid unnecessary noise at startup. Now you can use PuTTY for public key SSH authentication. When the server asks for public key verification, PuTTY will forward the request to GPG, which will prompt you for a PIN and authorize the login using YubiKey. ### WSL The goal here is to make the SSH client inside WSL work together with the Windows agent you are using (gpg-agent.exe in our case). Here is what we are going to achieve: ![WSL agent architecture](media/schema_gpg.png) **Note** this works only for SSH agent forwarding. Real GPG forwarding (encryption/decryption) is actually not supported. See [weasel-pageant](https://github.com/vuori/weasel-pageant) for further information or consider using [wsl2-ssh-pageant](https://github.com/BlackReloaded/wsl2-ssh-pageant) which supports both SSH and GPG agent forwarding. #### Use ssh-agent or use S.weasel-pageant One way to forward is just `ssh -A` (still need to eval weasel to setup local ssh-agent), and only relies on OpenSSH. In this track, `ForwardAgent` and `AllowAgentForwarding` in ssh/sshd config may be involved; However, if you use the other way (gpg ssh socket forwarding), you should not enable `ForwardAgent` in ssh config. See [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding) for more info. Another way is to forward the gpg ssh socket, as described below. #### Prerequisites * Ubuntu 16.04 or newer for WSL * Kleopatra * [Windows configuration](#windows) #### WSL configuration Download or clone [weasel-pageant](https://github.com/vuori/weasel-pageant). Add `eval $(/mnt/c//weasel-pageant -r -a /tmp/S.weasel-pageant)` to shell rc file. Use a named socket here so it can be used in the `RemoteForward` directive of `~/.ssh/config`. Source it with `source ~/.bashrc`. Display the SSH key with `$ ssh-add -l` Edit `~/.ssh/config` to add the following for each host you want to use agent forwarding: ``` RemoteForward /tmp/S.weasel-pageant ``` **Note** The remote SSH socket path can be found with `gpgconf --list-dirs agent-ssh-socket` #### Remote host configuration You may have to add the following to the shell rc file. ``` export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket) ``` Add the following to `/etc/ssh/sshd_config`: ``` StreamLocalBindUnlink yes ``` And reload the SSH daemon (e.g., `sudo service sshd reload`). Unplug YubiKey, disconnect or reboot. Log back into Windows, open a WSL console and enter `ssh-add -l` - you should see nothing. Plug in YubiKey, enter the same command to display the ssh key. Log into the remote host, you should have the pinentry dialog asking for the YubiKey pin. On the remote host, type `ssh-add -l` - if you see the ssh key, that means forwarding works! **Note** Agent forwarding may be chained through multiple hosts - just follow the same [protocol](#remote-host-configuration) to configure each host. You may also read this part on [chained ssh agent forwarding](#chained-ssh-agent-forwarding). ## macOS To use gui applications on macOS, [a little bit more setup is needed](https://jms1.net/yubikey/make-ssh-use-gpg-agent.md). Create `$HOME/Library/LaunchAgents/gnupg.gpg-agent.plist` with the following contents: ``` 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 $HOME/Library/LaunchAgents/gnupg.gpg-agent-symlink.plist ``` You will need to either reboot, or log out and log back in, in order to activate these changes. # Remote Machines (GPG Agent Forwarding) This section is different from ssh-agent forwarding in [SSH](#ssh) as gpg-agent forwarding has a broader usage, not only limited to ssh. To use YubiKey to sign a git commit on a remote host, or signing email/decrypt files on a remote host, configure and use GPG Agent Forwarding. To ssh through another network, especially to push to/pull from GitHub using ssh, see [Remote Machines (SSH Agent forwarding)](#remote-machines-ssh-agent-forwarding) for more info. To do this, you need access to the remote machine and the YubiKey has to be set up on the host machine. After gpg-agent forwarding, it is nearly the same as if YubiKey was inserted in the remote. Hence configurations except `gpg-agent.conf` for the remote can be the same as those for the local. **Important** `gpg-agent.conf` for the remote is of no use, hence `$GPG_TTY` is of no use too for the remote. The mechanism is that after forwarding, remote `gpg` directly communicates with `S.gpg-agent` without *starting* `gpg-agent` on the remote. On the remote machine, edit `/etc/ssh/sshd_config` to set `StreamLocalBindUnlink yes` **Optional** If you do not have root access to the remote machine to edit `/etc/ssh/sshd_config`, you will need to remove the socket (located at `gpgconf --list-dir agent-socket`) on the remote machine before forwarding works. For example, `rm /run/user/1000/gnupg/S.gpg-agent`. Further information can be found on the [AgentForwarding GNUPG wiki page](https://wiki.gnupg.org/AgentForwarding). Import public keys to the remote machine. This can be done by fetching from a keyserver. On the local machine, copy the public keyring to the remote machine: ```console scp ~/.gnupg/pubring.kbx remote:~/.gnupg/ ``` On modern distributions, such as Fedora 30, there is typically no need to also set `RemoteForward` in `~/.ssh/config` as detailed in the next chapter, because the right thing happens automatically. If any error happens (or there is no `gpg-agent.socket` in the remote) for modern distributions, you may go through the configuration steps in the next section. ## Steps for older distributions On the local machine, run: ```console gpgconf --list-dirs agent-extra-socket ``` This should return a path to agent-extra-socket - `/run/user/1000/gnupg/S.gpg-agent.extra` - though on older Linux distros (and macOS) it may be `/home//.gnupg/S/gpg-agent.extra` Find the agent socket on the **remote** machine: ```console gpgconf --list-dirs agent-socket ``` This should return a path such as `/run/user/1000/gnupg/S.gpg-agent` Finally, enable agent forwarding for a given machine by adding the following to the local machine's ssh config file `~/.ssh/config` (your agent sockets may be different): ``` Host Hostname remote-host.tld StreamLocalBindUnlink yes RemoteForward /run/user/1000/gnupg/S.gpg-agent /run/user/1000/gnupg/S.gpg-agent.extra # RemoteForward [remote socket] [local socket] ``` If you're still having problems, it may be necessary to edit `gpg-agent.conf` file on the *local* machines to add the following information: ``` pinentry-program /usr/bin/pinentry-gtk-2 extra-socket /run/user/1000/gnupg/S.gpg-agent.extra ``` **Note** The pinentry program starts on *local* machine, not remote. Hence when there are needs to enter the pin you need to find the prompt on the local machine. **Important** Any pinentry program except `pinentry-tty` or `pinentry-curses` may be used. This is because local `gpg-agent` may start headlessly (By systemd without `$GPG_TTY` set locally telling which tty it is on), thus failed to obtain the pin. Errors on the remote may be misleading saying that there is *IO Error*. (Yes, internally there is actually an *IO Error* since it happens when writing to/reading from tty while finding no tty to use, but for end users this is not friendly.) See [Issue #85](https://github.com/drduh/YubiKey-Guide/issues/85) for more information and troubleshooting. ## Chained GPG Agent Forwarding Assume you have gone through the steps above and have `S.gpg-agent` on the *remote*, and you would like to forward this agent into a *third* box, first you may need to configure `sshd_config` of *third* in the same way as *remote*, then in the ssh config of *remote*, add the following lines: ```console Host third Hostname third-host.tld StreamLocalBindUnlink yes RemoteForward /run/user/1000/gnupg/S.gpg-agent /run/user/1000/gnupg/S.gpg-agent # RemoteForward [remote socket] [local socket] ``` You should change the path according to `gpgconf --list-dirs agent-socket` on *remote* and *third*. **Note** On *local* you have `S.gpg-agent.extra` whereas on *remote* and *third*, you only have `S.gpg-agent` # Using Multiple Keys To use a single identity with multiple YubiKeys - or to replace a lost card with another - issue this command to switch keys: ```console gpg-connect-agent "scd serialno" "learn --force" /bye ``` Alternatively, use a script to delete the GnuPG shadowed key, where the card serial number is stored (see [GnuPG #T2291](https://dev.gnupg.org/T2291)): ```console cat >> ~/scripts/remove-keygrips.sh <&2; exit 1; } KEYGRIPS=$(gpg --with-keygrip --list-secret-keys "$@" | awk '/Keygrip/ { print $3 }') for keygrip in $KEYGRIPS do rm "$HOME/.gnupg/private-keys-v1.d/$keygrip.key" 2> /dev/null done gpg --card-status EOF chmod +x ~/scripts/remove-keygrips.sh ~/scripts/remove-keygrips.sh $KEYID ``` See discussion in Issues [#19](https://github.com/drduh/YubiKey-Guide/issues/19) and [#112](https://github.com/drduh/YubiKey-Guide/issues/112) for more information and troubleshooting steps. # Adding an identity To add an identity after creating and backing up a YubiKey, first add the identity to the Certify key, and then reset YubiKey and use `keytocard` to move the Subkeys to the card again. Follow the same process as generating keys: boot to a secure environment, install required software and disconnect networking. Connect the portable storage device with the Certify key and identify the disk label: ```console $ sudo dmesg | tail mmc0: new high speed SDHC card at address a001 mmcblk0: mmc0:a001 SS16G 14.8 GiB (ro) mmcblk0: p1 p2 ``` Decrypt and mount the encrypted volume: ```console sudo cryptsetup luksOpen /dev/mmcblk0p1 secret sudo mount /dev/mapper/secret /mnt/encrypted-storage ``` Restore the backup to a temporary directory: ```console export GNUPGHOME=$(mktemp -d -t gnupg_$(date +%Y%m%d%H%M)_XXX) cp -avi /mnt/encrypted-storage/tmp.XXX/* $GNUPGHOME ``` Edit the Certify key to add the new identity: ```console $ KEYID= $ gpg --expert --edit-key $KEYID gpg> adduid Real name: Email address: Comment: Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O gpg> trust Please decide how far you trust this user to correctly verify other users' keys (by looking at passports, checking fingerprints from different sources, etc.) 1 = I don't know or won't say 2 = I do NOT trust 3 = I trust marginally 4 = I trust fully 5 = I trust ultimately m = back to the main menu Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y gpg> save ``` Export Certify and Subkeys again: ```console gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/certify.key gpg --armor --export-secret-subkeys $KEYID > $GNUPGHOME/subkeys.key ``` And the public key: ```console gpg --armor --export $KEYID | sudo tee /mnt/public/gpg-$KEYID-$(date +%F).asc ``` As before, on Windows, note that using any extension other than `.gpg` or attempting IO redirection to a file will garble the secret key, making it impossible to import it again at a later date: ```console gpg -o \path\to\dir\certify.gpg --armor --export-secret-keys $KEYID gpg -o \path\to\dir\subkeys.gpg --armor --export-secret-subkeys $KEYID gpg -o \path\to\dir\pubkey.gpg --armor --export $KEYID ``` Copy the **new** temporary working directory to encrypted storage, which should still be mounted: ```console sudo cp -avi $GNUPGHOME /mnt/encrypted-storage ``` Unmount and close the encrypted volume: ```console sudo umount /mnt/encrypted-storage sudo cryptsetup luksClose /dev/mapper/secret ``` ## Updating YubiKey Now that keys have been updated with the new identity, they will need to be loaded to YubiKey. First, [Reset](#reset) the OpenPGP applet, then follow the steps to [Configure Smartcard](#configure-smartcard) again. Next, [Transfer Keys](#transfer-keys) and reboot or securely erase the GPG temporary working directory. Finally, re-import the public key, as described in [Using Keys](#using-keys). Use `gpg -K` to verify the identity is listed. # Require touch **Note** This is not possible on YubiKey NEO. By default, YubiKey will perform encryption, signing and authentication operations without requiring any action from the user after the key is plugged in and unlocked once with the PIN. To require a touch for each key operation, install [YubiKey Manager](https://developers.yubico.com/yubikey-manager/) and recall the Admin PIN: Authentication: ```console ykman openpgp keys set-touch aut on ``` Signing: ```console ykman openpgp keys set-touch sig on ``` Encryption: ```console ykman openpgp keys set-touch dec on ``` **Note** Versions of YubiKey Manager before 5.1.0 use `enc` instead of `dec` for encryption. **Note** Older versions of YubiKey Manager use `touch` instead of `set-touch` Depending on how the YubiKey is going to be used, you may want to look at the policy options for each of these and adjust the above commands accordingly. They can be viewed with the following command: ``` $ ykman openpgp keys set-touch -h Usage: ykman openpgp keys set-touch [OPTIONS] KEY POLICY Set the touch policy for OpenPGP keys. The touch policy is used to require user interaction for all operations using the private key on the YubiKey. The touch policy is set individually for each key slot. To see the current touch policy, run the "openpgp info" subcommand. Touch policies: Off (default) no touch required On touch required Fixed touch required, can't be disabled without deleting the private key Cached touch required, cached for 15s after use Cached-Fixed touch required, cached for 15s after use, can't be disabled without deleting the private key KEY key slot to set (sig, dec, aut or att) POLICY touch policy to set (on, off, fixed, cached or cached-fixed) Options: -a, --admin-pin TEXT Admin PIN for OpenPGP -f, --force confirm the action without prompting -h, --help show this message and exit ``` If the YubiKey is going to be used within an email client that opens and verifies encrypted mail, `Cached` or `Cached-Fixed` may be desirable. YubiKey will blink when it is waiting for a touch. On Linux you can also use [yubikey-touch-detector](https://github.com/maximbaz/yubikey-touch-detector) to have an indicator or notification that YubiKey is waiting for a touch. # Email GPG keys on YubiKey can be used with ease to encrypt and/or sign emails and attachments using [Thunderbird](https://www.thunderbird.net/), [Enigmail](https://www.enigmail.net) and [Mutt](http://www.mutt.org/). Thunderbird supports OAuth 2 authentication and can be used with Gmail. See [this guide](https://ssd.eff.org/en/module/how-use-pgp-linux) from EFF for detailed instructions. Mutt has OAuth 2 support since version 2.0. ## Mailvelope [Mailvelope](https://www.mailvelope.com/en) allows GPG keys on YubiKey to be used with Gmail and others. **Important** Mailvelope [does not work](https://github.com/drduh/YubiKey-Guide/issues/178) with the `throw-keyids` option set in `gpg.conf`. On macOS, install gpgme using Homebrew: ```console brew install gpgme ``` To allow Chrome to run gpgme, edit `~/Library/Application\ Support/Google/Chrome/NativeMessagingHosts/gpgmejson.json` and add: ```json { "name": "gpgmejson", "description": "Integration with GnuPG", "path": "/usr/local/bin/gpgme-json", "type": "stdio", "allowed_origins": [ "chrome-extension://kajibbejlbohfaggdiogboambcijhkke/" ] } ``` Edit the default path to allow Chrome to find GPG: ```console sudo launchctl config user path /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin ``` Finally, install the [Mailvelope extension](https://chrome.google.com/webstore/detail/mailvelope/kajibbejlbohfaggdiogboambcijhkke) from the Chrome app store. ## Mutt Mutt has both CLI and TUI interfaces, and the latter provides powerful functions for daily email processing. In addition, PGP can be integrated such that signing/encryption/verifying/decryption can be done without leaving TUI. To enable GnuPG support, one can just use the config file `gpg.rc` provided by mutt, usually located at `/usr/share/doc/mutt/samples/gpg.rc` after installation. One only needs to edit the file on options like `pgp_default_key`, `pgp_sign_as` and `pgp_autosign`. After editting one can `source` this rcfile in their main `muttrc` to use it. **Important** If one uses `pinentry-tty` as one's pinentry program in `gpg-agent.conf`, it would mess with one's Mutt TUI, as reported. This is because Mutt TUI uses curses while tty output may harm the format. It is recommended to use `pinentry-curses` or other graphic pinentry program. # Reset If PIN attempts are exceeded, the YubiKey is locked and must be [reset](https://developers.yubico.com/ykneo-openpgp/ResetApplet.html) and set up again using the encrypted backup. Copy the following script to a file and run `gpg-connect-agent -r $file` to lock and terminate the card. Then re-insert YubiKey to reset. ```console /hex scd serialno scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40 scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40 scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40 scd apdu 00 20 00 81 08 40 40 40 40 40 40 40 40 scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40 scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40 scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40 scd apdu 00 20 00 83 08 40 40 40 40 40 40 40 40 scd apdu 00 e6 00 00 scd apdu 00 44 00 00 /echo Card has been successfully reset. ``` Or use `ykman` (sometimes in `~/.local/bin/`): ```console $ ykman openpgp reset WARNING! This will delete all stored OpenPGP keys and data and restore factory settings? [y/N]: y Resetting OpenPGP data, don't remove your YubiKey... Success! All data has been cleared and default PINs are set. PIN: 123456 Reset code: NOT SET Admin PIN: 12345678 ``` ## Recovery after reset To reinstate YubiKey from the Certify key backup (such as the one on encrypted portable storage described in [Backup](#backup)), follow [Rotating keys](#rotating-keys) to setup the environment, then [Configure Smartcard](#configure-smartcard). # Notes - YubiKey has two configurations: one invoked with a short press, and the other with a long press. By default, the short-press mode is configured for HID OTP - a brief touch will emit an OTP string starting with `cccccccc`. If you rarely use the OTP mode, you can swap it to the second configuration via the YubiKey Personalization tool. If you *never* use OTP, you can disable it entirely using the [YubiKey Manager](https://developers.yubico.com/yubikey-manager) application (note, this not the similarly named older YubiKey NEO Manager). The command to disable OTP with ykman is `ykman config usb -d OTP`. - Programming YubiKey for GPG keys still lets you use its other configurations - [U2F](https://en.wikipedia.org/wiki/Universal_2nd_Factor), [OTP](https://www.yubico.com/faq/what-is-a-one-time-password-otp/) and [static password](https://www.yubico.com/products/services-software/personalization-tools/static-password/) modes, for example. - Setting an expiry essentially forces you to manage your subkeys and announces to the rest of the world that you are doing so. Setting an expiry on a primary key is ineffective for protecting the key from loss - whoever has the primary key can simply extend its expiry period. Revocation certificates are [better suited](https://security.stackexchange.com/questions/14718/does-openpgp-key-expiration-add-to-security/79386#79386) for this purpose. It may be appropriate for your use case to set expiry dates on subkeys. - To switch between two or more identities on different keys - unplug the first YubiKey and restart gpg-agent, ssh-agent and pinentry with `pkill gpg-agent ; pkill ssh-agent ; pkill pinentry ; eval $(gpg-agent --daemon --enable-ssh-support)`, then plug in the other key and run `gpg-connect-agent updatestartuptty /bye` - then it should be ready for use. - To use YubiKey on multiple computers, import the corresponding public keys on them. Confirm gpg can see the card via `gpg --card-status`, then trust the import public keys ultimately. `gpg --list-secret-keys` should show the correct and trusted key. # Troubleshooting - Use `man gpg` to understand GPG options and command-line flags. - To get more information on potential errors, restart the `gpg-agent` process with debug output to the console with `pkill gpg-agent; gpg-agent --daemon --no-detach -v -v --debug-level advanced --homedir ~/.gnupg`. - If you encounter problems connecting to YubiKey with GPG - try unplugging and re-inserting YubiKey, and restarting the `gpg-agent` process. - If you receive the error, `gpg: decryption failed: secret key not available` - you likely need to install GnuPG version 2.x. Another possibility is that there is a problem with the PIN, e.g. it is too short or blocked. - If you receive the error, `Yubikey core error: no yubikey present` - make sure the YubiKey is inserted correctly. It should blink once when plugged in. - If you still receive the error, `Yubikey core error: no yubikey present` - you likely need to install newer versions of yubikey-personalize as outlined in [Required software](#required-software). - If you receive the error, `Yubikey core error: write error` - YubiKey is likely locked. Install and run yubikey-personalization-gui to unlock it. - If you receive the error, `Key does not match the card's capability` - you likely need to use 2048 bit RSA key sizes. - If you receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - make sure you replaced `ssh-agent` with `gpg-agent` as noted above. - If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - [run the command](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=835394) `gpg-connect-agent updatestartuptty /bye` - If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - edit `~/.gnupg/gpg-agent.conf` to set a valid `pinentry` program path. `gpg: decryption failed: No secret key` could also indicate an invalid `pinentry` path - If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - it is a [known issue](https://bbs.archlinux.org/viewtopic.php?id=274571) that openssh 8.9p1 and higher has issues with YubiKey. Adding `KexAlgorithms -sntrup761x25519-sha512@openssh.com` to `/etc/ssh/ssh_config` often resolves the issue. - If you receive the error, `The agent has no identities` from `ssh-add -L`, make sure you have installed and started `scdaemon`. - If you receive the error, `Error connecting to agent: No such file or directory` from `ssh-add -L`, the UNIX file socket that the agent uses for communication with other processes may not be set up correctly. On Debian, try `export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"`. Also see that `gpgconf --list-dirs agent-ssh-socket` is returning single path, to existing `S.gpg-agent.ssh` socket. - If you receive the error, `Permission denied (publickey)`, increase ssh verbosity with the `-v` flag and ensure the public key from the card is being offered: `Offering public key: RSA SHA256:abcdefg... cardno:00060123456`. If it is, ensure you are connecting as the right user on the target system, rather than as the user on the local system. Otherwise, be sure `IdentitiesOnly` is not [enabled](https://github.com/FiloSottile/whosthere#how-do-i-stop-it) for this host. - If SSH authentication still fails - add up to 3 `-v` flags to the `ssh` client to increase verbosity. - If it still fails, it may be useful to stop the background `sshd` daemon process service on the server (e.g. using `sudo systemctl stop sshd`) and instead start it in the foreground with extensive debugging output, using `/usr/sbin/sshd -eddd`. Note that the server will not fork and will only process one connection, therefore has to be re-started after every `ssh` test. - If you receive the error, `Please insert the card with serial number: *` see [using of multiple keys](#using-multiple-keys). - If you receive the error, `There is no assurance this key belongs to the named user` or `encryption failed: Unusable public key` use `gpg --edit-key` to set `trust` to `5 = I trust ultimately`. - If, when you try the above command, you get the error `Need the secret key to do this` - specify trust for the key in `~/.gnupg/gpg.conf` by using the `trust-key [key ID]` directive. - If, when using a previously provisioned YubiKey on a new computer with `pass`, you see the following error on `pass insert`, you need to adjust the trust associated with the key. See the note above. ``` gpg: 0x0000000000000000: There is no assurance this key belongs to the named user gpg: [stdin]: encryption failed: Unusable public key ``` - If you receive the error, `gpg: 0x0000000000000000: skipped: Unusable public key`, `signing failed: Unusable secret key`, or `encryption failed: Unusable public key` the Subkey may be expired and can no longer be used to encrypt nor sign messages. It can still be used to decrypt and authenticate, however. - If you lost your GPG public key, follow [this guide](https://www.nicksherlock.com/2021/08/recovering-lost-gpg-public-keys-from-your-yubikey/) to recover it from YubiKey. - Refer to Yubico article [Troubleshooting Issues with GPG](https://support.yubico.com/hc/en-us/articles/360013714479-Troubleshooting-Issues-with-GPG) for additional guidance. - If, when you try the above `--card-status` command, you get receive the error, `gpg: selecting card failed: No such device` or `gpg: OpenPGP card not available: No such device`, it's possible that the latest release of pcscd is now requires polkit rules to operate properly. Create the following file to allow users in the `wheel` group to use the card. Be sure to restart pcscd when you're done to allow the new rules to take effect. ``` cat << EOF > /etc/polkit-1/rules.d/99-pcscd.rules polkit.addRule(function(action, subject) { if (action.id == "org.debian.pcsc-lite.access_card" && subject.isInGroup("wheel")) { return polkit.Result.YES; } }); polkit.addRule(function(action, subject) { if (action.id == "org.debian.pcsc-lite.access_pcsc" && subject.isInGroup("wheel")) { return polkit.Result.YES; } }); EOF ``` # Alternatives * [`smlx/piv-agent`](https://github.com/smlx/piv-agent) is an SSH and GPG agent which you can use with your PIV hardware security device (e.g. a Yubikey). * [`keytotpm`](https://www.gnupg.org/documentation/manuals/gnupg/OpenPGP-Key-Management.html) is an option to use GnuPG with TPM systems. ## Create keys with batch Keys can also be generated using template files and the `batch` parameter - see [GnuPG documentation](https://www.gnupg.org/documentation/manuals/gnupg/Unattended-GPG-key-generation.html). Use the example [gen-params-rsa4096](contrib/gen-params-rsa4096) or ([gen-params-ed25519](contrib/gen-params-ed25519) template (the latter requires GnuPG v2.1.7). Generate the Certify key: ```console $ gpg --batch --generate-key gen-params-rsa4096 gpg: Generating a basic OpenPGP key gpg: key 0xEA5DE91459B80592 marked as ultimately trusted gpg: revocation certificate stored as '/tmp.FLZC0xcM/openpgp-revocs.d/D6F924841F78D62C65ABB9588B461860159FFB7B.rev' gpg: done ``` Verify the result: ```console $ gpg --list-key gpg: checking the trustdb gpg: marginals needed: 3 completes needed: 1 trust model: pgp gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u /tmp.FLZC0xcM/pubring.kbx ------------------------------- pub rsa4096/0xFF3E7D88647EBCDB 2021-08-22 [C] Key fingerprint = 011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB uid [ultimate] Dr Duh ``` The fingerprint (`011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB`) is used to create the three Subkeys for signing, authentication and encryption. Use a one or several year expiration for Subkeys - they can be renewed using the Certify key, see [rotating keys](#rotating-keys). Create a [signing subkey](https://stackoverflow.com/questions/5421107/can-rsa-be-both-used-as-encryption-and-signature/5432623#5432623): ```console gpg --quick-add-key "011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB" \ rsa4096 sign 1y ``` Now create an [encryption subkey](https://www.cs.cornell.edu/courses/cs5430/2015sp/notes/rsa_sign_vs_dec.php): ```console gpg --quick-add-key "011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB" \ rsa4096 encrypt 1y ``` Finally, create an [authentication subkey](https://superuser.com/questions/390265/what-is-a-gpg-with-authenticate-capability-used-for): ```console gpg --quick-add-key "011C E16B D45B 27A5 5BA8 776D FF3E 7D88 647E BCDB" \ rsa4096 auth 1y ``` Continue with the Verify section of this guide. # Additional resources * [Yubico - PGP](https://developers.yubico.com/PGP/) * [Yubico - PGP Card edit](https://developers.yubico.com/PGP/Card_edit.html) * [Yubico - Yubikey Personalization](https://developers.yubico.com/yubikey-personalization/) * [A Visual Explanation of GPG Subkeys (2022)](https://rgoulter.com/blog/posts/programming/2022-06-10-a-visual-explanation-of-gpg-subkeys.html) * [dhess/nixos-yubikey](https://github.com/dhess/nixos-yubikey) * [lsasolutions/makegpg](https://gitlab.com/lsasolutions/makegpg) * [Trammell Hudson - Yubikey (2020)](https://trmm.net/Yubikey) * [Yubikey forwarding SSH keys (2019)](https://blog.onefellow.com/post/180065697833/yubikey-forwarding-ssh-keys) * [GPG Agent Forwarding (2018)](https://mlohr.com/gpg-agent-forwarding/) * [Stick with security: YubiKey, SSH, GnuPG, macOS (2018)](https://evilmartians.com/chronicles/stick-with-security-yubikey-ssh-gnupg-macos) * [PGP and SSH keys on a Yubikey NEO (2015)](https://www.esev.com/blog/post/2015-01-pgp-ssh-key-on-yubikey-neo/) * [Offline GnuPG Master Key and Subkeys on YubiKey NEO Smartcard (2014)](https://blog.josefsson.org/2014/06/23/offline-gnupg-master-key-and-subkeys-on-yubikey-neo-smartcard/) * [Creating the perfect GPG keypair (2013)](https://alexcabal.com/creating-the-perfect-gpg-keypair/) * [GPG and SSH with Yubikey NEO (2013)](https://blog.habets.se/2013/02/GPG-and-SSH-with-Yubikey-NEO) * [Riseup - OpenPGP Best Practices](https://help.riseup.net/en/security/message-security/openpgp/best-practices)