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This guide demonstrates how to store credentials on a YubiKey. The private keys cannot be copied back out of the device; a separate offline "Certify" key is retained only to replace or renew them.

Purchase YubiKey

Choose a YubiKey model that includes the OpenPGP application. YubiKey Security Key and YubiKey Bio models do not include OpenPGP support and cannot be used with this guide.

Verify the YubiKey at yubico.com/genuine. Select Verify Device, touch the key when prompted, and allow the site to identify the device model. This can help detect some supply chain tampering.

Have at least two USB drives or microSD cards for storing encrypted offline backups in different physical locations.

Prepare setup environment

Use a dedicated, hardened environment to generate the keys and backups.

The following environments are ordered from least to most secure for this task:

  1. Public, shared or other computer owned by someone else
  2. Daily-use personal operating system with unrestricted network access
  3. Virtualized system with limited capabilities (using virt-manager, VirtualBox or VMware, for example)
  4. Dedicated and hardened Debian or OpenBSD installation
  5. Ephemeral Debian Live or Tails booted without primary storage attached
  6. Hardened hardware and firmware (e.g., Coreboot, Intel ME removed)
  7. Air-gapped system without network capabilities, preferably ARM-based Raspberry Pi or other architecturally diverse equivalent

Network isolation and dedicated hardware generally provide stronger protection than a daily-use online system. For most people, booting Debian Live from USB on a personal computer is a practical baseline.

Download and verify Debian

Download the latest Debian Live image and signature files. These commands download the checksum file, its signature, and the current 64-bit XFCE Live ISO:

imageUrl="https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/"
curl -sfL -O "$imageUrl/SHA512SUMS" -O "$imageUrl/SHA512SUMS.sign"
curl -sfLO "$imageUrl/$(awk '/xfce\.iso$/ {print $NF}' SHA512SUMS)"

Download the Debian signing public key:

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

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

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

The Debian Live signing public key is also available for import in pubkeys:

gpg --import pubkeys/debian-DA87E80D6294BE9B.asc

Verify the signature:

gpg --verify SHA512SUMS.sign SHA512SUMS

Confirm the fingerprint matches Debian's published fingerprint:

gpg: Good signature from "Debian CD signing key <debian-cd@lists.debian.org>"

Verify the image hash matches the signed sums file:

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

See Verifying authenticity of Debian CDs for more information.

Create bootable USB device

Connect a storage device and identify the device path. This guide uses /dev/sdc throughout, but this value may differ on your system.

Warning

The following dd commands erase and overwrite the selected device. Check the device path before continuing!

Linux

$ sudo dmesg | tail
usb-storage 3-2:1.0: USB Mass Storage device detected
sd 2:0:0:0: [sdc] Attached SCSI removable disk

Copy the Debian image to the device:

sudo dd if=debian-live-*-amd64-xfce.iso of=/dev/sdc bs=4M status=progress ; sync

OpenBSD

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

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

Prepare hardware

Shut down the computer that will be used to generate keys.

Disconnect internal storage and unnecessary peripherals, such as the wireless card.

Install software

Boot the computer from the Debian Live USB drive.

Configure networking to install the required software; see Network considerations for optional restrictions.

Tip

If the screen locks on Debian Live, unlock with user / live

Open a terminal and install the required software.

Debian/Ubuntu

sudo apt update
sudo apt -y upgrade
sudo apt -y install \
  wget gnupg2 gnupg-agent dirmngr \
  cryptsetup scdaemon pcscd \
  yubikey-personalization yubikey-manager

OpenBSD

doas pkg_add gnupg pcsc-tools

macOS

Download and install Homebrew and the following packages:

brew install gnupg yubikey-personalization ykman pinentry-mac wget

Or using MacPorts, install the following packages:

sudo port install gnupg2 yubikey-manager pinentry wget

NixOS

Use this repository's purpose-built live ISO instead of Debian Live. Copy the resulting ISO to a USB drive, boot from it, and keep the system offline while generating keys.

ref=$(git ls-remote https://github.com/drduh/Yubikey-Guide refs/heads/main | awk '{print $1}')
nix build --experimental-features "nix-command flakes" \
  github:drduh/YubiKey-Guide/$ref?dir=nix#nixosConfigurations.yubikeyLive.x86_64-linux.config.system.build.isoImage

If you have this repository checked out:

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

nix flake update --commit-lock-file

Build the image:

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

Copy to USB drive:

sudo cp -v result/iso/yubikeyLive.iso /dev/sdc ; sync

Skip steps to create a temporary working directory and a hardened configuration, as they are already part of the image.

Test builds using virtualization tools like QEMU. A virtual machine provides less isolation than a booted live system. To test the image in QEMU with 4 GiB of RAM, two CPUs, and KVM enabled:

qemu-system-x86_64 -enable-kvm -m 4G -smp 2 \
  -drive readonly=on,media=cdrom,format=raw,file=result/iso/yubikeyLive.iso

Arch Linux

sudo pacman -Syu --needed gnupg pcsclite ccid yubikey-personalization

Red Hat Enterprise Linux (RHEL)

sudo yum install -y gnupg2 pinentry pcsc-lite pcsc-lite-libs gnupg2-smime

Fedora

sudo dnf install --skip-unavailable \
  wget gnupg2 \
  cryptsetup gnupg2-scdaemon pcsc-lite \
  yubikey-personalization-gui yubikey-manager

Prepare GnuPG

Create a temporary directory for GnuPG configuration:

export GNUPGHOME=$(mktemp -d "${TMPDIR:-/tmp}/$(date +%Y.%m.%d)-XXXXXXXX")
printf "\nTemporary directory:\t%s\n\n" "$GNUPGHOME"

Note

This directory is cleared by reboot only when it is stored on a memory-backed filesystem such as tmpfs.

If it is stored on persistent disk, remove it after completing the guide with rm -rf "$GNUPGHOME" and then reboot.

Configuration

Download the recommended GnuPG configuration into the temporary GnuPG directory.

wget https://raw.githubusercontent.com/drduh/YubiKey-Guide/main/config/gpg.conf \
  -P $GNUPGHOME

Review the configuration before using it. It selects modern cryptographic defaults and privacy-oriented options, such as hiding recipient key IDs:

$ grep -v "^#" $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
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
require-secmem
no-symkey-cache
armor
use-agent
throw-keyids

Important

Networking should be disabled for the remainder of the setup.

Identity

Create a user ID - the public label attached to the key.

Use a random label if the key does not need to be publicly identified:

export IDENTITY="yk.$(LC_ALL=C tr -dc 'a-z0-9' < /dev/urandom | head -c 16)"
printf "\nIdentity:\t\t%s\n\n" "$IDENTITY"

If email encryption or Git verification use is anticipated, set the Identity label to a name and email address:

export IDENTITY="YubiKey User <yubikey@example.com>"

Tip

Use single quotes to wrap double quote characters ("):

export IDENTITY='My Identity (a.k.a. "YubiKey User") <yubikey@example.com>'

Key algorithm

Set the key algorithm and size - RSA 4096 is recommended:

export KEY_TYPE=rsa4096

Expiration

The Certify key is the offline primary key and should not have an expiration date.1

The signing, encryption, and authentication Subkeys will be stored on YubiKey. Assign Subkeys an expiration date so a lost or obsolete YubiKey becomes inoperable after that date. A two-year expiration for Subkeys is recommended, balancing security and usability. Longer expiration durations reduce maintenance frequency.

Note

After a Subkey expires, GnuPG can still use it to decrypt existing messages, and SSH may still use it for authentication. However, the expired Subkey cannot encrypt new data or create new signatures!

Subkeys are renewed or rotated using the Certify key. See Updating keys.

Set Subkeys to expire on a planned date:

export KEY_EXPIRATION=2028-08-01

The expiration date may also be relative, for example set to two years from today:

export KEY_EXPIRATION=2y

Passphrase

Create a passphrase for the offline Certify key. This passphrase protects the backup copy of the Certify key and is required to manage Subkeys.

A passphrase consisting only of uppercase letters and numbers is recommended: the limited character set makes handwritten storage easier to write and read.

The following commands will generate and print a strong passphrase2:

export CERTIFY_PASS=$(LC_ALL=C tr -dc "A-Z3-9" < /dev/urandom |
  tr -d "IOUS5" |
  head -c ${PASS_LENGTH:-24} |
  fold -w ${PASS_GROUPSIZE:-4} |
  paste -sd ${PASS_DELIMITER:--} -)
printf "\nCertify passphrase:\t%s\n\n" "$CERTIFY_PASS"

To change the passphrase length, delimiter character or group size, export the respective variable(s) prior to running the passphrase generation command, for example:

export PASS_GROUPSIZE=6
export PASS_DELIMITER=+
export PASS_LENGTH=48

Record the Certify key passphrase on durable media and store it separately from the encrypted backup containing the key material, or memorize it.

The included passphrase.html and passphrase.txt templates can assist with accurate transcription.

Save the html file, open in a browser to render and print. Mark the corresponding character on sequential rows for each passphrase character. passphrase.txt can also be printed for use without a browser:

lp -d Printer-Name passphrase.txt

Diceware is another popular method for creating memorable passphrases.

Create Certify key

The Certify key is the primary OpenPGP key: it authorizes the signing, encryption, and authentication Subkeys. Keep it offline and use it only in a secure environment to create, renew, revoke, or replace Subkeys.

Generate the Certify key:

printf '%s' "$CERTIFY_PASS" |
  gpg --batch --passphrase-fd 0 \
      --quick-generate-key "$IDENTITY" "$KEY_TYPE" cert never

Set and print KEY_FP (the full key fingerprint) and KEY_ID (the last 16 characters of the fingerprint - a shorter identifier commonly accepted by GnuPG):

export KEY_FP=$(gpg -k --with-colons $IDENTITY | awk -F: '/^fpr:/ { print $10; exit }')
export KEY_ID="${KEY_FP: -16}"
printf "\nKey ID/Fingerprint:\t%s\n%s\n\n" "$KEY_ID" "$KEY_FP"
Additional identities (optional)

Skip this section unless the OpenPGP key must have multiple public identities. Examples include addresses used for different languages or Git providers. Do not add both personal and professional addresses if linking those identities would reduce privacy - use separate YubiKeys instead.

Define a Bash array of additional user IDs. Put each complete user ID in quotes and separate array entries with spaces:

declare -a additional_uids
additional_uids=("Super Cool YubiKey 2026" "uid 1 <uid1@example.com>")

Add the additional UIDs to the Identity.

for uid in "${additional_uids[@]}" ; do \
  printf '%s' "$CERTIFY_PASS" |
  gpg --batch --passphrase-fd 0 \
      --pinentry-mode loopback --quick-add-uid "$KEY_FP" "$uid"
done

Set UID trust levels to ultimate:

gpg --command-fd 0 --pinentry-mode loopback --edit-key "$KEY_FP" <<EOF
uid *
trust
5
y
save
EOF

Create Subkeys

Generate Signature and Encryption Subkeys:

printf '%s' "$CERTIFY_PASS" |
  gpg --batch --pinentry-mode loopback --passphrase-fd 0 \
      --quick-add-key "$KEY_FP" "$KEY_TYPE" sign "$KEY_EXPIRATION"
printf '%s' "$CERTIFY_PASS" |
  gpg --batch --pinentry-mode loopback --passphrase-fd 0 \
      --quick-add-key "$KEY_FP" "$KEY_TYPE" encrypt "$KEY_EXPIRATION"

Generate the Authentication Subkey:

Note

Some SSH servers do not accept RSA authentication keys. To use Ed25519 instead, run the following command before creating the Authentication Subkey:

export KEY_TYPE=ed25519

printf '%s' "$CERTIFY_PASS" |
  gpg --batch --pinentry-mode loopback --passphrase-fd 0 \
      --quick-add-key "$KEY_FP" "$KEY_TYPE" auth "$KEY_EXPIRATION"

Verify keys

List available secret keys:

gpg -K

The output should display [C]ertify, [S]ignature, [E]ncryption and [A]uthentication keys:

sec   rsa4096/0xF0F2CFEB04341FB5 2026-08-01 [C]
      Key fingerprint = 4E2C 1FA3 372C BA96 A06A  C34A F0F2 CFEB 0434 1FB5
uid                   [ultimate] yk.pn8wgfx67khlhext
ssb   rsa4096/0xB3CD10E502E19637 2026-08-01 [S] [expires: 2028-08-01]
ssb   rsa4096/0x30CBE8C4B085B9F7 2026-08-01 [E] [expires: 2028-08-01]
ssb   rsa4096/0xAD9E24E1B8CB9600 2026-08-01 [A] [expires: 2028-08-01]

Backup keys

Export the public key:

gpg --output $GNUPGHOME/public-$KEY_ID-$(date +%F).asc \
    --armor --export $KEY_ID

Export the Certify key:

printf '%s' "$CERTIFY_PASS" |
  gpg --batch --pinentry-mode loopback --passphrase-fd 0 \
      --output $GNUPGHOME/secret-$KEY_ID-Certify.key \
      --armor --export-secret-keys $KEY_ID

Export the Subkeys:

printf '%s' "$CERTIFY_PASS" |
  gpg --batch --pinentry-mode loopback --passphrase-fd 0 \
      --output $GNUPGHOME/secret-$KEY_ID-Subkeys.key \
      --armor --export-secret-subkeys $KEY_ID

Important

The exported .key files contain secret keys. Anyone who obtains these files and the Certify key passphrase can create Subkeys and sign or decrypt data encrypted to those keys.

Create a backup on encrypted storage to be kept offline in a secure and durable location.

Repeat the process several times on multiple portable storage devices, as they may fail over time. As an additional backup measure, Paperkey can create a physical copy of key materials for improved durability.

Tip

ext2 file systems (without encryption) can be mounted on Linux and OpenBSD. Use FAT32 or NTFS file systems for macOS and Windows compatibility instead.

Warning

Confirm the destination (of) before issuing dd commands as they are destructive! This guide uses /dev/sdc - this value may be different on your system.

Linux

Attach a portable storage device and confirm its label - in this example /dev/sdc:

$ sudo dmesg | tail
usb-storage 3-2:1.0: USB Mass Storage device detected
sd 2:0:0:0: [sdc] Attached SCSI removable disk

$ sudo fdisk -l /dev/sdc
Disk /dev/sdc: 14.9 GiB, 15931539456 bytes, 31116288 sectors

Clear the first 4 MiB so the operating system does not retain previous partition tables:

sudo dd if=/dev/zero of=/dev/sdc bs=4M count=1

Remove and reconnect the storage device so the operating system reads the cleared partition table before the next command.

Create a new GPT partition table:

sudo fdisk /dev/sdc <<EOF
g
w
EOF

Create a small partition (at least 20 MB, to account for the LUKS header) for storing backups:

sudo fdisk /dev/sdc <<EOF
n


+20M
w
EOF

Use LUKS to encrypt the backup partition before storing the Certify key on it.

Generate another unique passphrase (different from the Certify key passphrase) to protect the encrypted volume:

export LUKS_PASS=$(LC_ALL=C tr -dc "A-Z3-9" < /dev/urandom |
  tr -d "IOUS5" |
  head -c ${PASS_LENGTH:-24} |
  fold -w ${PASS_GROUPSIZE:-4} |
  paste -sd ${PASS_DELIMITER:--} -)
printf "\nLUKS passphrase:\t%s\n\n" "$LUKS_PASS"

This passphrase unlocks the encrypted backup volume, which contains the Certify key. It is needed to renew, rotate, or recover Subkeys. Write the passphrase down or memorize it.

Confirm root privileges are available:

sudo -v

Format the partition:

printf '%s' "$LUKS_PASS" | sudo cryptsetup -q luksFormat /dev/sdc1

Unlock the encrypted partition and create the mapped device:

printf '%s' "$LUKS_PASS" | sudo cryptsetup -q luksOpen /dev/sdc1 gnupg-secrets

Create an ext2 filesystem:

sudo mkfs.ext2 /dev/mapper/gnupg-secrets -L gnupg-$(date +%F)

Mount the filesystem and copy the temporary GnuPG working directory with key materials:

sudo mkdir -p /mnt/encrypted-storage
sudo mount /dev/mapper/gnupg-secrets /mnt/encrypted-storage
sudo cp -av $GNUPGHOME /mnt/encrypted-storage/

Unmount and close the encrypted volume:

sudo umount /mnt/encrypted-storage
sudo cryptsetup luksClose gnupg-secrets

Repeat the process for any additional storage devices (at least two are recommended).

OpenBSD

Attach a USB disk and determine its label:

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

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

doas disklabel -h sd2

Create an a partition of FS type RAID and size of 25 Megabytes:

$ 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 using a unique Passphrase:

$ 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:

$ 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:

doas mkdir -p /mnt/encrypted-storage
doas mount /dev/sd3i /mnt/encrypted-storage
doas cp -av $GNUPGHOME /mnt/encrypted-storage

Unmount and remove the encrypted volume:

doas umount /mnt/encrypted-storage
doas bioctl -d sd3

See OpenBSD FAQ#14 for more information.

Export public key

Important

YubiKey stores the transferred private Subkeys. Each computer also needs the public key and its metadata to identify and use them with GnuPG. Without the public key, it is not be possible to use GnuPG to decrypt and sign. However, YubiKey can still be used for SSH authentication.

Connect another portable storage device or create a new partition on the existing one.

Linux

Using the same /dev/sdc device as in the previous step, create a small (at least 20 MB is recommended) partition for storing materials:

sudo fdisk /dev/sdc <<EOF
n


+20M
w
EOF

Create a filesystem and export the public key:

sudo mkfs.ext2 /dev/sdc2
sudo mkdir -p /mnt/public
sudo mount /dev/sdc2 /mnt/public
gpg --armor --export $KEY_ID | sudo tee /mnt/public/$KEY_ID-$(date +%F).asc
sudo chmod 0444 /mnt/public/*.asc

Unmount and remove the storage device:

sudo umount /mnt/public

OpenBSD

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

Create a filesystem and export the public key to it:

doas newfs sd2b
doas mkdir -p /mnt/public
doas mount /dev/sd2b /mnt/public
gpg --armor --export $KEY_ID | doas tee /mnt/public/$KEY_ID-$(date +%F).asc

Unmount and remove the storage device:

doas umount /mnt/public

Configure YubiKey

Connect YubiKey and confirm its status:

gpg --card-status

If the YubiKey is locked, follow Reset YubiKey to permanently delete any existing keys stored on it.

Change PIN

YubiKey contains several independent applications. This guide uses the OpenPGP application, whose PINs are separate from PINs used by PIV, FIDO, and other card features.

Name Default Capability
User PIN 123456 routine key operations (decrypt, sign, authenticate)
Admin PIN 12345678 manage card settings (reset PIN, change Reset Code, add keys and owner information)
Reset Code None reset PIN (more information)

The User PIN must be at least 6 characters and the Admin PIN must be at least 8 characters. A maximum of 127 ASCII characters are allowed. See Managing PINs for more information.

YubiKey limits failed PIN attempts, which reduces guessing risk, allowing the User PIN to be short and convenient for regular use.

Generate convenient numeric PINs: an 8-digit Admin PIN and a 6-digit User PIN. These meet the minimum lengths but can be increased:

PINS=$(LC_ALL=C tr -dc '0-9' < /dev/urandom | head -c 14)
export ADMIN_PIN=${PINS:0:8}
export USER_PIN=${PINS:8:6}
printf "\nAdmin PIN:\t\t%s\nUser  PIN:\t\t%s\n\n" "$ADMIN_PIN" "$USER_PIN"

Change the Admin PIN:

gpg --command-fd 0 --pinentry-mode loopback --change-pin <<EOF
3
12345678
$ADMIN_PIN
$ADMIN_PIN
q
EOF

Change the User PIN:

gpg --command-fd 0 --pinentry-mode loopback --change-pin <<EOF
1
123456
$USER_PIN
$USER_PIN
q
EOF

Remove and reinsert YubiKey.

Caution

Three incorrect User PIN entries will cause it to become blocked and must be unblocked with either the Admin PIN or Reset Code. Three incorrect Admin PIN or Reset Code entries will destroy data on YubiKey.

Increase PIN limits if the risk of accidental lockout outweighs the increased opportunity for guessing. This command sets five attempts for the User PIN, Reset Code, and Admin PIN:

ykman openpgp access set-retries 5 5 5 -f -a $ADMIN_PIN

Set attributes

Important

OpenPGP card metadata is readable without authentication by anyone with physical access to the YubiKey. This includes the card serial number, key fingerprints, public-key data, and other card attributes. Do not store a name, email address, account name, or other personal identifier in Login data, except for deliberate lost-and-found or asset-management purposes.

Set the card Login attribute. The random label generated by the following command is intentionally non-identifying:

export CARD_ATTR_LOGIN="yk.$(LC_ALL=C tr -dc 'a-z0-9' < /dev/urandom | head -c 16)"
printf "\nLogin attribute:\t%s\n\n" "$CARD_ATTR_LOGIN"

Apply the attribute:

gpg --command-fd 0 --pinentry-mode loopback --edit-card <<EOF
admin
login
$CARD_ATTR_LOGIN
$ADMIN_PIN
quit
EOF

Verify the attribute:

$ gpg --card-status | grep Login
Login data .......: yk.3i568zcrib5f

Smart card attributes can also be set with gpg --edit-card and admin mode. Use help to see available options. The login attribute is required to transfer keys.

Transfer Subkeys

Note

After each transfer, GnuPG deletes its usable local copy of the private key and retains only a record that the Subkey is on this YubiKey. The Subkey cannot be copied back from the YubiKey. Confirm that offline backups exist before continuing.

The Certify key passphrase and Admin PIN are required to transfer keys.

Signature key

Transfer the Signature key:

gpg --command-fd 0 --pinentry-mode loopback --edit-key $KEY_FP <<EOF
key 1
keytocard
1
$CERTIFY_PASS
$ADMIN_PIN
save
EOF

Encryption key

Repeat the process for the Encryption key:

gpg --command-fd 0 --pinentry-mode loopback --edit-key $KEY_FP <<EOF
key 2
keytocard
2
$CERTIFY_PASS
$ADMIN_PIN
save
EOF

Authentication key

Repeat the process for the Authentication key:

gpg --command-fd 0 --pinentry-mode loopback --edit-key $KEY_FP <<EOF
key 3
keytocard
3
$CERTIFY_PASS
$ADMIN_PIN
save
EOF

Verify transfer

Verify Subkeys are on YubiKey:

gpg -K

All three Subkeys should appear as ssb> - the > indicates the private key is on the smart card:

sec   rsa4096/0xF0F2CFEB04341FB5 2026-08-01 [C]
      Key fingerprint = 4E2C 1FA3 372C BA96 A06A  C34A F0F2 CFEB 0434 1FB5
uid                   [ultimate] yk.pn8wgfx67khlhext
ssb>  rsa4096/0xB3CD10E502E19637 2026-08-01 [S] [expires: 2028-08-01]
ssb>  rsa4096/0x30CBE8C4B085B9F7 2026-08-01 [E] [expires: 2028-08-01]
ssb>  rsa4096/0xAD9E24E1B8CB9600 2026-08-01 [A] [expires: 2028-08-01]

Finish setup

Confirm that each of the following steps was successfully completed:

  • Memorized or recorded the Certify key passphrase and stored it in a secure, durable location separate from the encrypted backup.

  • Memorized or recorded the passphrase for the encrypted backup volume and stored it separately from that volume.

  • Saved the Certify key and Subkeys to encrypted portable storage and stored the backups offline.

    • Keep at least two backups.
    • Store the backups in separate physical locations.

  • Exported a copy of the public key to a location that is easy to access later.

    • Use a separate device or a non-encrypted partition.

  • Memorized or recorded the unique User PIN and Admin PIN after changing them from their default values.

  • Transferred the Encryption, Signature, and Authentication Subkeys to the YubiKey.

    • Confirm that gpg -K shows ssb> for all three Subkeys.

  • Cleared the temporary GnuPG working directory by rebooting or by removing it.

Using YubiKey

Create or inspect the local GnuPG keyring. This command lists public keys and initializes ~/.gnupg if needed:

gpg -k

Import the recommended configuration:

cd ~/.gnupg
wget https://raw.githubusercontent.com/drduh/YubiKey-Guide/main/config/gpg.conf

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

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

Install the required packages:

Debian/Ubuntu

sudo apt update
sudo apt install -y gnupg gnupg-agent scdaemon pcscd

Arch

sudo pacman -S --needed gnupg pcsc-tools
sudo systemctl enable --now pcscd.service

macOS

brew install gnupg

Or using MacPorts

sudo port install gnupg2 pcsc-tools

OpenBSD

doas pkg_add gnupg pcsc-tools
doas rcctl enable pcscd
doas reboot

Mount the non-encrypted volume with the public key:

Debian/Ubuntu

sudo mkdir -p /mnt/public
sudo mount /dev/sdc2 /mnt/public

OpenBSD

doas mkdir -p /mnt/public
doas mount /dev/sd3i /mnt/public

Import the public key:

gpg --import /mnt/public/public-*.asc

Or download the public key from a keyserver:

gpg --recv $KEY_ID

Or with the URL on YubiKey, retrieve the public key:

printf "fetch" | gpg --command-fd 0 --pinentry-mode loopback --card-edit

Determine the key ID:

gpg -k
export KEY_ID=0xF0F2CFEB04341FB5

Assign ultimate trust by typing trust and selecting option 5 then quit:

gpg --command-fd 0 --pinentry-mode loopback --edit-key $KEY_ID <<EOF
trust
5
y
save
EOF

Remove and reinsert YubiKey.

Verify the status with gpg --card-status which will list the available Subkeys:

Reader ...........: Yubico YubiKey FIDO 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 .......: yk.3i568zcrib5f
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 ......: off
UIF setting ......: Sign=off Decrypt=off Auth=off
Signature key ....: CF5A 305B 808B 7A0F 230D  A064 B3CD 10E5 02E1 9637
      created ....: 2026-08-01 12:00:00
Encryption key....: A5FA A005 5BED 4DC9 889D  38BC 30CB E8C4 B085 B9F7
      created ....: 2026-08-01 12:00:00
Authentication key: 570E 1355 6D01 4C04 8B6D  E2A3 AD9E 24E1 B8CB 9600
      created ....: 2026-08-01 12:00:00
General key info..: sub  rsa4096/0xB3CD10E502E19637 2026-08-01 yk.pn8wgfx67khlhext
sec#  rsa4096/0xF0F2CFEB04341FB5  created: 2026-08-01  expires: never
ssb>  rsa4096/0xB3CD10E502E19637  created: 2026-08-01  expires: 2028-08-01
                                  card-no: 0006 05553211
ssb>  rsa4096/0x30CBE8C4B085B9F7  created: 2026-08-01  expires: 2028-08-01
                                  card-no: 0006 05553211
ssb>  rsa4096/0xAD9E24E1B8CB9600  created: 2026-08-01  expires: 2028-08-01
                                  card-no: 0006 05553211

sec# indicates the corresponding key is not available (the Certify key is offline).

YubiKey is now ready for use!

Encryption

Encrypt a message to yourself (useful for storing credentials or protecting backups):

printf "test message string\n" |
  gpg --encrypt --armor \
      --recipient $KEY_ID --output encrypted.txt

Decrypt the message - a prompt for the User PIN will appear:

gpg --decrypt --armor encrypted.txt

To encrypt to multiple recipients/keys, set the preferred key ID last:

printf "test message string\n" |
  gpg --encrypt --armor \
      --recipient $KEY_ID3 \
      --recipient $KEY_ID2 \
      --recipient $KEY_ID1 \
      --output encrypted.txt

Use a shell function to make encrypting files easier:

secret() {
  local file="${1}"
  local output="${file}.$(date +%s).enc"
  gpg --encrypt --armor --recipient "${KEY_ID}" \
      --output "${output}" -- "${file}" &&
    printf '%s -> %s' "${file}" "${output}"
}

reveal() {
  local file="${1}"
  local output="${file%.[0-9]*.enc}"
  gpg --decrypt --output "${output}" -- "${file}" &&
    printf '%s -> %s' "${file}" "${output}"
}

Example output:

$ secret document.pdf
document.pdf -> document.pdf.1780000000.enc

$ reveal document.pdf.1780000000.enc
gpg: encrypted with RSA key, ID 0x0000000000000000
gpg: anonymous recipient; trying secret key 0xF0F2CFEB04341FB5 ...
gpg: okay, we are the anonymous recipient.
document.pdf.1780000000.enc -> document.pdf

drduh/Purse is a secrets manager implemented with GnuPG and YubiKey.

Signature

Sign a message:

printf "test message string\n" |
  gpg --armor --clearsign > signed.txt

Verify the signature:

gpg --verify signed.txt

The output will be similar to:

gpg: Signature made Sat 01 Aug 2026 12:00:00 PM UTC
gpg:                using RSA key CF5A305B808B7A0F230DA064B3CD10E502E19637
gpg: Good signature from "yk.pn8wgfx67khlhext" [ultimate]
Primary key fingerprint: 4E2C 1FA3 372C BA96 A06A  C34A F0F2 CFEB 0434 1FB5
     Subkey fingerprint: CF5A 305B 808B 7A0F 230D  A064 B3CD 10E5 02E1 9637

Configure touch

By default, cryptographic operations do not require any action from the user after YubiKey is unlocked with the User PIN.

To require a touch for each operation, use YubiKey Manager and the Admin PIN to set key policy.

Encryption:

ykman openpgp keys set-touch dec on

Note

YubiKey Manager prior to version 5.1.0 uses enc instead of dec for encryption:

ykman openpgp keys set-touch enc on

Note

Legacy versions of YubiKey Manager use touch instead of set-touch.

Signature:

ykman openpgp keys set-touch sig on

Authentication:

ykman openpgp keys set-touch aut on

To view the configured touch policy:

ykman openpgp info

To adjust the policy, see available options with:

ykman openpgp keys set-touch --help

Cached or Cached-Fixed may be desirable for YubiKey use with email clients.

YubiKey blinks when waiting for a touch. On Linux, maximbaz/yubikey-touch-detector can also be used for indication.

SSH

Create or import a hardened configuration:

cd ~/.gnupg
wget https://raw.githubusercontent.com/drduh/YubiKey-Guide/main/config/gpg-agent.conf

Note

cache-ttl options do not apply when using YubiKey as a smart card, because the PIN is cached by the smart card itself. To clear the PIN from cache (equivalent to default-cache-ttl and max-cache-ttl), remove YubiKey, or set forcesig when editing the card to be prompted for the PIN each time.

Tip

Set pinentry-program to /usr/bin/pinentry-gnome3 for a GUI-based prompt.

macOS

Install pinentry with brew install pinentry-mac or sudo port install pinentry then edit gpg-agent.conf to set the pinentry-program path to:

  • Apple Silicon Macs: /opt/homebrew/bin/pinentry-mac
  • Intel Macs: /usr/local/bin/pinentry-mac or /opt/local/bin/pinentry (MacPorts)
  • MacGPG Suite: /usr/local/MacGPG2/libexec/pinentry-mac.app/Contents/MacOS/pinentry-mac

Then run gpgconf --kill gpg-agent for the change to take effect.

To use graphical applications on macOS, additional setup is required.

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

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

Load it:

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

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

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

Load it:

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

Reboot to activate changes.

Windows

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

Find the YubiKey label using PowerShell:

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) for more information.

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

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

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

enable-ssh-support
enable-putty-support

Restart the agent:

gpg-connect-agent killagent /bye
gpg-connect-agent /bye

Verify YubiKey details:

gpg --card-status

Import the public key and set ultimate trust:

gpg --import <path to public key file>

Retrieve the public key id:

gpg --list-public-keys

Export the SSH public key:

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

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

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

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

WSL

The goal is to configure SSH client inside WSL work together with the Windows agent, such as gpg-agent.exe.

See the WSL agent architecture illustration for an overview.

GnuPG forwarding for cryptographic operations is not supported. See vuori/weasel-pageant for more information.

One way to forward is just ssh -A (still need to eval weasel to setup local ssh-agent), and only relies on OpenSSH. In this track, ForwardAgent and AllowAgentForwarding in ssh/sshd config may be involved. However, when using ssh socket forwarding, do not enable ForwardAgent in ssh config. See SSH Agent Forwarding for more information. This requires Ubuntu 16.04 or newer for WSL and Kleopatra.

Download vuori/weasel-pageant.

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

Display the SSH key with ssh-add -l.

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

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

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

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

Reload SSH daemon:

sudo service sshd reload

Remove YubiKey and reboot. Log back into Windows, open a WSL console and enter ssh-add -l - no output should appear.

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

Connect to the remote host and use ssh-add -l to confirm forwarding works.

Agent forwarding may be chained through multiple hosts. Follow the same protocol to configure each host.

An alternate method is the usbipd-win library. If you encounter issues with accessing the YubiKey in WSL after configuring usbipd-win, you may need to add custom polkit rules to ensure proper permissions for the pcscd service. Here's an example configuration using a scard group (the group logic is optional):

Create a new rule file at /etc/polkit-1/rules.d/99-pcscd.rules:

polkit.addRule(function(action, subject) {
    if (action.id == "org.debian.pcsc-lite.access_card" &&
        subject.isInGroup("scard")) {
        return polkit.Result.YES;
    }
});

polkit.addRule(function(action, subject) {
    if (action.id == "org.debian.pcsc-lite.access_pcsc" &&
        subject.isInGroup("scard")) {
        return polkit.Result.YES;
    }
});

Replace agents

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

Add the following to the shell rc file:

export GPG_TTY=$(tty)
export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
gpgconf --launch gpg-agent
gpg-connect-agent updatestartuptty /bye > /dev/null

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

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

When using ForwardAgent for ssh-agent forwarding, SSH_AUTH_SOCK only needs to be set on the local host, where YubiKey is connected. On the remote host, ssh will set SSH_AUTH_SOCK to something like /tmp/ssh-mXzCzYT2Np/agent.7541 upon connection. Do not set SSH_AUTH_SOCK on the remote host - doing so will break SSH Agent Forwarding.

For S.gpg-agent.ssh (see 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

Tip

It is not necessary to import the GnuPG public key in order to use SSH only.

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

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

Optional Save the 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. To do this you'll need to use the command line argument -i [identity_file] or the IdentityFile and IdentitiesOnly options in .ssh/config.

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

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

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

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

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

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

Connect with public key authentication:

$ 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 enable multiple SSH connections, use the ControlMaster option.

Import SSH keys

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

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

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

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

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

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

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

$ 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.

SSH agent forwarding

Caution

SSH Agent Forwarding can add additional risk - proceed with caution!

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

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

For example, tmux does not have environment variables such as $SSH_AUTH_SOCK when connecting to remote hosts and attaching an existing session. For each shell, find the socket and export SSH_AUTH_SOCK=/tmp/ssh-agent-xxx/xxxx.socket. However, with S.gpg-agent.ssh in a fixed place, it can be used as the ssh-agent in shell rc files.

Use ssh-agent

Run ssh -A remote from the local host. On the remote host, ssh-add -l should list the public key from the YubiKey as a cardno: entry. Enable ForwardAgent yes only for explicitly trusted hosts!

Use S.gpg-agent.ssh

Before continuing, configure GnuPG agent forwarding and record the local and remote values of 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 connecting to the remote host, confirm /run/user/1000/gnupg/S.gpg-agent.ssh exists and configure SSH_AUTH_SOCK:

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

ssh-add -l should now return the correct public key.

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 forwarding

To use ssh-agent provided by OpenSSH to forward into a third host, use ssh -A third on the remote host.

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 configure sshd_config and SSH_AUTH_SOCK of third in the same way as remote, then configure SSH of remote by adding the following lines:

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

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

GitHub

Use YubiKey to sign Git commits and tags and to authenticate to GitHub over SSH. To authenticate, first add the exported SSH public key to GitHub SSH keys.

Configure the signing key:

git config --global user.signingkey $KEY_ID

Alternatively, if you are using the aforementioned IdentityFile (SSH key) for signing:

git config --global gpg.format ssh
git config --global user.signingkey ~/.ssh/id_rsa_yubikey.pub

Configure the user.name and user.email options to match the identity associated with this key:

git config --global user.name 'YubiKey User'
git config --global user.email yubikey@example.com

To sign commits or tags, use the -S option, or consider enabling commit and tag signing by default:

git config --global commit.gpgsign true
git config --global tag.gpgSign true

Windows

Configure authentication:

git config --global core.sshcommand "plink -agent"
git config --global gpg.program 'C:\Program Files (x86)\GnuPG\bin\gpg.exe'

Then update the repository URL to git@github.com:USERNAME/repository

GnuPG agent forwarding

YubiKey can sign Git commits and decrypt files on remote hosts by forwarding GnuPG agent operations. To connect through an intermediate host and then access GitHub over SSH, see SSH agent forwarding.

After forwarding is configured, GnuPG on the remote host communicates through a forwarded socket to gpg-agent on the local host. The local agent accesses the YubiKey and displays any PIN prompt locally.

On the remote host, edit /etc/ssh/sshd_config to set StreamLocalBindUnlink yes.

Optional Without root access on the remote host to edit /etc/ssh/sshd_config, socket located at gpgconf --list-dir agent-socket on the remote host will need to be removed before forwarding works. See AgentForwarding GNUPG wiki page for more information.

Import the public key on the remote host. On the local host, copy the public keyring to the remote host:

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

On modern distributions, such as Fedora 30, there is no need to set RemoteForward in ~/.ssh/config

Legacy distributions

On the local host, run:

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 some Linux distros (and macOS) it may be /home/<user>/.gnupg/S.gpg-agent.extra

Find the agent socket on the remote host:

gpgconf --list-dirs agent-socket

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

Finally, enable agent forwarding for a given host by adding the following to the local host's ~/.ssh/config (agent sockets may differ):

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

It may be necessary to edit gpg-agent.conf on the local host to add the following information:

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

Important

The pinentry program starts on the local host, not remote.

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 for more information and troubleshooting.

Chained GnuPG agent forwarding

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

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.

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

Using multiple YubiKeys

When a GnuPG key is added to YubiKey using keytocard, the credential is deleted from the keyring and a local reference ('stub') pointing to the card is added. The stub identifies the GnuPG key ID and YubiKey serial number.

When a Subkey is added to an additional YubiKey, the stub is overwritten and will now point to the latest YubiKey. GnuPG will request a specific YubiKey by serial number, as referenced by the stub, and will not recognize another YubiKey with a different serial number.

To scan an additional YubiKey and recreate the correct stub:

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

Alternatively, use a script to delete the GnuPG shadowed key, where the serial number is stored (see GnuPG #T2291):

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

gpg --card-status
EOF

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

~/scripts/remove-keygrips.sh $KEY_ID

See discussion in #19 and #112 for more information.

Email

YubiKey can decrypt and sign emails and attachments using Thunderbird and Mutt. Thunderbird supports OAuth and can be used with Gmail. See this EFF guide for more information.

Thunderbird

Follow Mozilla instructions to setup YubiKey with Thunderbird using the external GPG provider.

Note

Thunderbird will fail to decrypt messages if the armor option is configured. If you see the error gpg: [don't know]: invalid packet (ctb=2d) or message cannot be decrypted (there are unknown problems with this encrypted message), remove armor from ~/.gnupg/gpg.conf, then restart Thunderbird.

Mailvelope

Mailvelope allows YubiKey to be used with Gmail and others.

Note

Mailvelope does not work with the throw-keyids option set in gpg.conf

On macOS, install gpgme using Homebrew:

brew install gpgme

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

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

Edit the default path to allow Chrome to find GnuPG:

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

Finally, install the Mailvelope extension from the Chrome web store.

Mutt

Mutt can be used from the command line or its text-based interface. To enable OpenPGP support, copy the sample GnuPG configuration file (/usr/share/doc/mutt/samples/gpg.rc), set pgp_default_key, pgp_sign_as, and pgp_autosign in that file, then add a source line for it in the Mutt configuration file (~/.muttrc).

Note

pinentry-tty set as the pinentry program in gpg-agent.conf may cause problems with Mutt because it uses curses; use pinentry-curses or other graphic pinentry program instead.

Keyserver

Optionally, publish the public key to a keyserver so others can find it.

Select a server to send it to:

gpg --send-key $KEY_ID
gpg --keyserver keys.gnupg.net --send-key $KEY_ID
gpg --keyserver hkps://keyserver.ubuntu.com:443 --send-key $KEY_ID

Or if uploading to keys.openpgp.org:

gpg --export $KEY_ID | curl -T - https://keys.openpgp.org

A retrieval URL can also be stored on YubiKey:

URL="hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=${KEY_ID}"

Edit YubiKey with gpg --edit-card and the Admin PIN:

gpg/card> admin

gpg/card> url
URL to retrieve public key: hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF00000000000000

gpg/card> quit

Updating keys

OpenPGP does not provide forward secrecy: if an encryption key is compromised, an attacker may be able to decrypt messages encrypted to that key in the past. Renewing or replacing Subkeys limits future use of a lost or obsolete key, but it may not protect previously encrypted data. Choose an expiration and renewal schedule to meet individual risk tolerance and maintenance capacity preferences.

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

  • Renewal preserves existing Subkeys and is generally simpler to operate.
  • Replacement creates new Subkeys and may be more secure: new Subkeys will not be able to decrypt previous data, authenticate with SSH, etc. The public key will need to be updated and any encrypted data must be decrypted and re-encrypted to new Subkeys. This process is functionally equivalent to provisioning a new YubiKey.

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

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

Connect the portable storage device with the Certify key and identify the device path.

Decrypt and mount the encrypted volume:

sudo cryptsetup luksOpen /dev/sdc1 gnupg-secrets
sudo mkdir -p /mnt/encrypted-storage
sudo mount /dev/mapper/gnupg-secrets /mnt/encrypted-storage

Mount the non-encrypted public partition:

sudo mkdir -p /mnt/public
sudo mount /dev/sdc2 /mnt/public

Copy the original private key materials (after updating the encrypted storage directory name) to a temporary working directory:

export GNUPGHOME=$(mktemp -d "${TMPDIR:-/tmp}/$(date +%Y.%m.%d)-XXXXXXXX")
cp -avi /mnt/encrypted-storage/2026.*/* $GNUPGHOME/

Confirm the identity is available, set the key ID and fingerprint values:

gpg -K
export KEY_FP=$(gpg -k --with-colons $IDENTITY | awk -F: '/^fpr:/ { print $10; exit }')
export KEY_ID="${KEY_FP: -16}"
printf "\nKey ID/Fingerprint:\t%s\n%s\n\n" "$KEY_ID" "$KEY_FP"

Recall the Certify key passphrase and set it, for example:

export CERTIFY_PASS="ABCD-0123-IJKL-4567-QRST-UVWX"

Renew Subkeys

Set the updated expiration date:

export KEY_EXPIRATION=2028-09-01

Renew the Subkeys:

printf '%s' "$CERTIFY_PASS" |
  gpg --batch --pinentry-mode loopback \
      --passphrase-fd 0 --quick-set-expire "$KEY_FP" "$KEY_EXPIRATION" \
    $(gpg -K --with-colons |
      awk -F: '$1=="fpr" { if (++n > 1) printf "%s ", $10 } END { print "" }')

Export the updated public key:

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

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

gpg --import /mnt/public/*.asc

Alternatively, publish to a public key server and download it:

gpg --send-key $KEY_ID
gpg --recv $KEY_ID

The validity of the GnuPG identity will be extended, enabling encryption and signature operations.

The SSH public key does not need to be updated on remote hosts.

Rotate Subkeys

Follow the original procedure to Create Subkeys.

Previous Subkeys can be deleted from the identity.

Finish by transferring new Subkeys to YubiKey.

Copy the new temporary working directory to encrypted storage, which is still mounted:

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

Unmount and close the encrypted volume:

sudo umount /mnt/encrypted-storage
sudo cryptsetup luksClose gnupg-secrets

Export the updated public key:

sudo mkdir -p /mnt/public
sudo mount /dev/sdc2 /mnt/public
gpg --armor --export $KEY_ID | sudo tee /mnt/public/$KEY_ID-$(date +%F).asc
sudo umount /mnt/public

Remove the storage device and follow the original steps to transfer new Subkeys (4, 5 and 6) to YubiKey, replacing existing ones.

Reboot or securely erase the temporary working directory.

Reset YubiKey

If PIN attempts are exceeded, the YubiKey is locked and must be Reset and set up again using the encrypted backup.

Use YubiKey Manager to reset the OpenPGP application:

ykman openpgp reset

YubiKey can also be reset with scripts/resetCard.sh, or by copying the following commands to a file and running gpg-connect-agent -r <filename>:

/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.
/bye

Remove and reinsert the YubiKey to complete reset.

Optional hardening

The following steps may improve the security and privacy of YubiKey.

Improving entropy

Generating cryptographic keys requires high-quality randomness, measured as entropy. Most operating systems use software-based pseudorandom number generators or CPU-based hardware random number generators (HRNG).

Optionally, a device such as OneRNG may be used to increase the speed and possibly the quality of available entropy.

Before creating keys, configure rng-tools:

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:

sha256sum onerng_3.7-1_all.deb

The value must match:

b7cda2fe07dce219a95dfeabeb5ee0f662f64ba1474f6b9dddacc3e8734d8f57

Install the package:

sudo dpkg -i onerng_3.7-1_all.deb
echo "HRNGDEVICE=/dev/ttyACM0" | sudo tee /etc/default/rng-tools

Insert the device and restart rng-tools:

sudo atd
sudo service rng-tools restart

Enable KDF

Important

This feature is not compatible with legacy GnuPG versions, especially mobile clients, and may cause the PIN to always be rejected.

This step must be completed before changing PINs or moving keys or an error will occur: gpg: error for setup KDF: Conditions of use not satisfied

A key derivation function (KDF) derives a value from the PIN before it is sent to the card, reducing its exposure.

Enable KDF using the default Admin PIN of 12345678:

gpg --command-fd 0 --pinentry-mode loopback --card-edit <<EOF
admin
kdf-setup
12345678
EOF

Network considerations

This section is primarily focused on Debian/Ubuntu systems, but the concepts apply to any system connected to a network.

Whether using a virtual machine, installing on dedicated hardware, or running a Live OS temporarily, start without a network connection and disable any unnecessary services listening on all interfaces before connecting to the network.

Services such as cups and avahi may listen on the network by default, increasing potential attack surface. For this workflow, treat every network as untrusted.

Disable listening services

Ensure only essential network services are running. If a service does not exist, a "Failed to stop" warning may appear:

sudo systemctl stop bluetooth exim4 cups avahi avahi-daemon sshd

Firewall

Enable a basic firewall policy of deny inbound, allow outbound.

Debian does not enable a default host-firewall policy by default.

On Ubuntu, ufw is built-in and can be enabled with:

sudo ufw enable

On systems without ufw, nftables is replacing iptables. The nftables wiki has examples for a baseline deny inbound, allow outbound policy. The fw.inet.basic policy covers both IPv4 and IPv6.

Download this README and any other resources to another external drive when creating the bootable media, to have this information ready to use offline.

Regardless of which policy is configured, write the contents to a file (e.g., nftables.conf) and apply the policy with the following command:

sudo nft -f ./nftables.conf

Review system state

NetworkManager should be the only listening service on port 68/udp to obtain a DHCP lease (and 58/icmp6 if you have IPv6).

To examine a process's command line arguments, use ps axjf to print a process tree.

# Dump network state information
sudo ss -anp -A inet

# List all processes in a process tree
ps axjf

# BSD syntax, list all processes but no process tree
ps aux

If you find any additional processes listening on the network that aren't needed, take note and disable them with one of the following:

# Stop services managed by systemctl
sudo systemctl stop <process-name>

# Terminate the process by matching its command line string
sudo pkill -f '<process-name-or-command-line-string>'

# Obtain the process ID
pgrep -f '<process-name-or-command-line-string>'

# Terminate the process ID
sudo kill <pid>

After reviewing the system state and applying any desired restrictions, connect to the network only to download required software. Disconnect again before generating keys.

Notes

  1. YubiKey has two USB configurations selected by a short or long touch. By default, a short touch may type a one-time-password string beginning with cccccccc. This behavior is separate from the OpenPGP workflow and can be moved to the second configuration or disabled with ykman config usb -d OTP.

  2. Using YubiKey for GnuPG does not prevent use of other features, such as WebAuthn and OTP.

  3. To switch between YubiKeys, remove the first YubiKey and restart gpg-agent, ssh-agent and pinentry with pkill "gpg-agent|ssh-agent|pinentry" ; eval $(gpg-agent --daemon --enable-ssh-support) then insert the other YubiKey and run gpg-connect-agent updatestartuptty /bye.

  4. To use YubiKey on multiple computers, import the corresponding public keys, then confirm YubiKey is visible with gpg --card-status. Trust the imported public keys ultimately with trust and 5, then gpg --list-secret-keys will show the correct and trusted key.

  5. To participate in keysigning parties, signing imported public keys requires first setting up a secure ephemeral environment and importing the Certify key into that enclave; a signing Subkey cannot be used to sign imported public keys.

Troubleshooting

Symptom or error Recommended action
General GnuPG questions or unfamiliar options Use man gpg to review GnuPG options and command-line flags.
Need more diagnostic detail Restart gpg-agent with verbose debug output: pkill gpg-agent; gpg-agent --daemon --no-detach -v -v --debug-level advanced --homedir ~/.gnupg
General YubiKey or GnuPG issue Remove and reinsert the YubiKey, or restart gpg-agent.
Yubikey core error: no yubikey present Confirm that the YubiKey is inserted correctly. It should blink once when plugged in.
Yubikey core error: no yubikey present persists Install a newer version of yubikey-personalization, as described in Install software.
General key info..: [none] in card-status output Import the public key.
The public key is lost Follow Recovering lost GPG public keys from your YubiKey.
gpg: decryption failed: secret key not available Install GnuPG 2.x. Also verify that the PIN is correct and that the required private Subkey is available on the YubiKey.
Yubikey core error: write error The YubiKey may be locked. Install and run yubikey-personalization-gui to unlock it.
Key does not match the card's capability Use 2048-bit RSA key sizes.
sign_and_send_pubkey: signing failed: agent refused operation Verify that ssh-agent has been replaced with gpg-agent, as described in Replace agents.
sign_and_send_pubkey: signing failed: agent refused operation persists Run gpg-connect-agent updatestartuptty /bye.
sign_and_send_pubkey: signing failed: agent refused operation persists after updating the startup TTY Set a valid pinentry program path in ~/.gnupg/gpg-agent.conf. A related error may be gpg: decryption failed: No secret key.
sign_and_send_pubkey: signing failed: agent refused operation with OpenSSH 8.9p1 or later This may be a known OpenSSH issue. Review the linked Arch Linux discussion.
The agent has no identities from ssh-add -L Install and start scdaemon.
Error connecting to agent: No such file or directory from ssh-add -L Ensure that the agent's UNIX socket is configured and available. Review Replace agents.
Permission denied (publickey) Run SSH with verbosity, for example ssh -v host, and confirm that the card's public key is offered: Offering public key: RSA SHA256:... cardno:00060123456. Also verify the user exists on the target system (different from the user on the local system).
SSH authentication still fails Increase SSH verbosity with up to three flags: ssh -vvv host.
SSH authentication still fails after reviewing verbose client output On the server, stop the background SSH service, for example with sudo systemctl stop sshd, then run /usr/sbin/sshd -eddd in the foreground with extensive debug logging to inspect the authentication attempt. Note that the server will not fork and will only process one connection and has to be restarted after every SSH connection test.
Please insert the card with serial number ... See Using multiple YubiKeys.
There is no assurance this key belongs to the named user, encryption failed: Unusable public key, or No public key Run gpg --edit-key and set owner trust to 5 = I trust ultimately.
Need the secret key to do this while setting trust Specify trust for the identity with the trust-key [key ID] directive.
pass insert reports There is no assurance this key belongs to the named user and encryption failed: Unusable public key Adjust the identity's trust level as described above.
gpg: ... skipped: Unusable public key, signing failed: Unusable secret key, or encryption failed: Unusable public key The Subkey may have expired. Follow Updating keys to renew or rotate the Subkeys.
The pinentry dialog does not appear and SSH signing fails with agent refused operation Install the dbus-user-session package, then restart the session or system.
Unexpected PIN prompts during SSH authentication Add disable-application piv to ~/.gnupg/scdaemon.conf.
gpg: selecting card failed: No such device or gpg: OpenPGP card not available: No such device The local pcscd service may lack permission to access the card. Create the Polkit rule shown below, replacing wheel if the system uses a different administrative group.

Create /etc/polkit-1/rules.d/99-pcscd.rules with the following contents:

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;
        }
});

Alternative solutions

Additional resources

Footnotes

  1. Revocation certificates should be used to revoke an identity. ↩

  2. See issue 477 for NIST guideline discussion. ↩

About

Community guide to using YubiKey for GnuPG and SSH - protect secrets with hardware crypto.

drduh.github.io/YubiKey-Guide/

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