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Guide to using YubiKey for GPG and SSH
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README.md

This is a guide to using YubiKey as a SmartCard 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 (as opposed to file-based keys that are stored on disk) and are convenient for everyday use. Instead of having to remember and enter passphrases to unlock SSH/GPG keys, YubiKey needs only a physical touch after being unlocked with a PIN code. All signing and encryption operations happen on the card, rather than in OS memory.

New! drduh/Purse is a password manager which uses GPG and YubiKey.

If you have a comment or suggestion, please open an Issue on GitHub.

Purchase YubiKey

All YubiKeys except the blue "security key" model are compatible with this guide. NEO models are limited to 2048-bit RSA keys. Compare YubiKeys here.

You will also need several small storage devices for booting a live image, creating backups of private and public keys.

Verify YubiKey

To verify a YubiKey is genuine, open a browser with U2F support to 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 the YubiKey's device attestation certificates signed by a set of Yubico CAs, and helps mitigate supply chain attacks.

Live image

It is recommended to generate cryptographic keys and configure YubiKey from a secure operating system and ephemeral environment, such as Debian Live, Tails, or OpenBSD.

To use Debian, download the latest live image:

$ curl -LfO https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/debian-live-9.9.0-amd64-xfce.iso

$ curl -LfO https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/SHA512SUMS

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

Verify file integrity with GPG:

$ gpg --verify SHA512SUMS.sign SHA512SUMS
gpg: Signature made Sat Apr 27 11:46:08 2019 PDT
gpg:                using RSA key DF9B9C49EAA9298432589D76DA87E80D6294BE9B
gpg: Can't check signature: No public key

$ gpg --recv DF9B9C49EAA9298432589D76DA87E80D6294BE9B
gpg: key 0xDA87E80D6294BE9B: 61 signatures not checked due to missing keys
gpg: key 0xDA87E80D6294BE9B: public key "Debian CD signing key <debian-cd@lists.debian.org>" imported
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: Total number processed: 1
gpg:               imported: 1

$ gpg --verify SHA512SUMS.sign SHA512SUMS
gpg: Signature made Sat Apr 27 11:46:08 2019 PDT
gpg:                using RSA key DF9B9C49EAA9298432589D76DA87E80D6294BE9B
gpg: Good signature from "Debian CD signing key <debian-cd@lists.debian.org>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: DF9B 9C49 EAA9 2984 3258  9D76 DA87 E80D 6294 BE9B

$ grep $(sha512sum debian-live-9.9.0-amd64-xfce.iso) SHA512SUMS
SHA512SUMS:ae064cc399126214e4aa165fdbf9659047dd2af2d3b0ca57dd5f2686d1d3730019cfe3c56ac48db2af56eb856dbca75e642fadf56bc04c538b44d3d3a2982283 debian-live-9.9.0-amd64-xfce.iso

If the key cannot be received, try changing the DNS resolver and/or specific keyserver:

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

Mount a storage device and copy the image to it:

Linux

$ 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-9.9.0-amd64-xfce.iso of=/dev/sdb bs=4M
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

$ 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-9.9.0-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)

Shut down the computer and disconnect internal hard drives and all unnecessary peripheral devices.

Consider using secure hardware like a ThinkPad X230 running Coreboot and cleaned of Intel ME.

Required software

Boot the live image and configure networking.

Note If the screen locks, unlock with user/live.

Open the terminal and install several required packages:

Debian/Ubuntu

$ sudo apt-get update && sudo apt-get install -y \
    gnupg2 gnupg-agent dirmngr \
    cryptsetup scdaemon pcscd \
    secure-delete hopenpgp-tools \
    yubikey-personalization

Arch

$ sudo pacman -Syu \
    gnupg2 pcsclite ccid hopenpgp-tools \
    yubikey-personalization

RHEL7

$ sudo yum install -y \
    gnupg2 pinentry-curses pcsc-lite pcsc-lite-libs gnupg2-smime

OpenBSD

$ doas pkg_add gnupg pcsc-tools

macOS

Download and install Homebrew and the following Brew packages:

$ brew install gnupg yubikey-personalization hopenpgp-tools ykman pinentry-mac

Windows

Download and install Gpg4Win and PuTTY.

You may also need more recent versions of yubikey-personalization and yubico-c.

Entropy

Generating cryptographic keys requires high-quality randomness, measured as entropy.

To check the available entropy available on Linux:

$ cat /proc/sys/kernel/random/entropy_avail
849

Most operating systems use software-based pseudorandom number generators. A hardware random number generator like OneRNG will increase the speed of entropy generation and possibly the quality.

Install and configure OneRNG software:

$ sudo apt-get install -y \
    at rng-tools python-gnupg openssl

$ wget https://github.com/OneRNG/onerng.github.io/raw/master/sw/onerng_3.6-1_all.deb

$ sha256sum onerng_3.6-1_all.deb
a9ccf7b04ee317dbfc91518542301e2d60ebe205d38e80563f29aac7cd845ccb  onerng_3.6-1_all.deb

$ sudo dpkg -i onerng_3.6-1_all.deb

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

Plug in the device and restart rng-tools:

$ sudo atd

$ sudo service rng-tools restart

Test by emptying /dev/random - the light on the device should dim briefly:

$ cat /dev/random >/dev/null
[Press Control-C]

Verify the available entropy pool is re-seeded:

$ cat /proc/sys/kernel/random/entropy_avail
3049

An entropy pool value greater than 2000 is sufficient.

Creating keys

Create a temporary directory which will be cleared on reboot:

$ export GNUPGHOME=$(mktemp -d)

$ cd $GNUPGHOME

Create a hardened configuration in the temporary directory with the following options:

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

$ 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
keyid-format 0xlong
list-options show-uid-validity
verify-options show-uid-validity
with-fingerprint
require-cross-certification
no-symkey-cache
throw-keyids
use-agent

Disable networking for the remainder of the setup.

Master key

The first key to generate is the master key. It will be used for certification only: to issue sub-keys that are used for encryption, signing and authentication.

Important The master key should be kept offline at all times and only accessed to revoke or issue new sub-keys. Keys can also be generated on the YubiKey itself to ensure no other copies exist.

You'll be prompted to enter and verify a passphrase - keep it handy as you'll need it multiple times later.

To generate a strong passphrase which could be written down in a hidden or secure place; or memorized:

$ gpg --gen-random -a 0 24
ydOmByxmDe63u7gqx2XI9eDgpvJwibNH

On Linux or OpenBSD, select the password with the mouse to copy it to the clipboard and 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 master key to expire - see Note #3.

$ 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
Your selection? 8

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

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

Your selection? E

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

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

Your selection? S

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

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

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

Select a name and email address - neither has to be valid nor existing.

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

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

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

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

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

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

Export the key ID as a variable (KEYID) for use later:

$ export KEYID=0xFF3E7D88647EBCDB

Sub-keys

Edit the master key to add sub-keys:

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

Secret key is available.

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

Use 4096-bit key sizes.

Use a 1 year expiration for sub-keys - they can be renewed using the offline master key. See rotating keys.

Signing

Create a signing key by selecting (4) RSA (sign only):

gpg> addkey
Key is protected.

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

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

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

Encryption

Next, create an encryption key by selecting (6) RSA (encrypt only):

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

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

Authentication

Finally, create an authentication key.

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:

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

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

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

Your selection? S

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

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

Your selection? E

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

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

Your selection? A

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

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

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

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

gpg> save

Verify

List the generated secret keys and verify the output:

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

Add any additional identities or email addresses you wish to associate using the adduid command.

Optional Verify with OpenPGP key checks, use the automated key best practice checker:

$ 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 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 about not setting an expiry for the primary key).

Export

The master key and sub-keys will be encrypted with your passphrase when exported.

Save a copy of your keys:

$ gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/mastersub.key

$ gpg --armor --export-secret-subkeys $KEYID > $GNUPGHOME/sub.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:

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

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

Backup

Once GPG keys are moved to YubiKey, they cannot be moved again! Create an encrypted backup of the keyring and consider using a paper copy of the keys as an additional backup.

Linux

Attach another external storage device and check its label:

$ sudo dmesg | tail
usb-storage 4-2:1.0: USB Mass Storage device detected
scsi host7: usb-storage 4-2:1.0
scsi 7:0:0:0: Direct-Access     TS-RDF5  SD  Transcend    TS37 PQ: 0 ANSI: 6
sd 7:0:0:0: Attached scsi generic sg1 type 0
sd 7:0:0:0: [sdb] 31116288 512-byte logical blocks: (15.9 GB/14.8 GiB)
sd 7:0:0:0: [sdb] Write Protect is off
sd 7:0:0:0: [sdb] Mode Sense: 23 00 00 00
sd 7:0:0:0: [sdb] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
sdb: sdb1
sd 7:0:0:0: [sdb] Attached SCSI removable disk

Write it with random data to prepare for encryption:

$ sudo dd if=/dev/urandom of=/dev/sdb bs=4M status=progress

Erase and create a new partition table:

$ sudo fdisk /dev/sdb
Welcome to fdisk (util-linux 2.29.2).

Command (m for help): o
Created a new DOS disklabel with disk identifier 0xeac7ee35.

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 10 Megabyte size:

$ sudo fdisk /dev/sdb
Welcome to fdisk (util-linux 2.29.2).

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): 
Partition number (1-4, default 1): 
First sector (2048-62980095, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-62980095, default 62980095): +10M

Created a new partition 1 of type 'Linux' and of size 10 MiB.

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

Use LUKS to encrypt the new partition:

$ sudo cryptsetup luksFormat /dev/sdb1

WARNING!
========
This will overwrite data on /dev/sdb1 irrevocably.

Are you sure? (Type uppercase yes): YES
Enter passphrase:
Verify passphrase:

Mount the partition:

$ sudo cryptsetup luksOpen /dev/sdb1 usb
Enter passphrase for /dev/sdb1:

Create a filesystem:

$ sudo mkfs.ext2 /dev/mapper/usb -L usb
Creating filesystem with 10240 1k blocks and 2560 inodes
Superblock backups stored on blocks:
        8193

Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

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

$ sudo mkdir /mnt/encrypted-usb

$ sudo mount /dev/mapper/usb /mnt/encrypted-usb

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

Optional Backup the OneRNG package:

$ sudo cp onerng_3.6-1_all.deb /mnt/encrypted-usb

Keep the backup mounted if you plan on setting up two or more keys as keytocard will delete the local copy on save.

Otherwise, unmount and disconnected the encrypted volume:

$ sudo umount /mnt/encrypted-usb

$ sudo cryptsetup luksClose usb

Create another partition to store the public key, or skip this step if you plan on uploading it to a key server.

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.

$ sudo fdisk /dev/sdb

Command (m for help): n
Partition type
   p   primary (1 primary, 0 extended, 3 free)
   e   extended (container for logical partitions)
Select (default p):
Partition number (2-4, default 2):
First sector (22528-31116287, default 22528):
Last sector, +sectors or +size{K,M,G,T,P} (22528-31116287, default 31116287): +10M

Created a new partition 2 of type 'Linux' and of size 10 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/sdb2
Creating filesystem with 10240 1k blocks and 2560 inodes
Superblock backups stored on blocks:
        8193

Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

$ sudo mkdir /mnt/public

$ sudo mount /dev/sdb2 /mnt/public/

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

Windows:

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

Optional Upload the public key to a public keyserver:

$ gpg --send-key $KEYID

$ gpg --keyserver pgp.mit.edu --send-key $KEYID

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

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

After some time, the public key will to propagate to other servers.

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

Initialize the disk by creating an a partition with FS type RAID and size of 10 Megabytes:

$ doas fdisk -iy sd2
Writing MBR at offset 0.

$ doas disklabel -E sd2
Label editor (enter '?' for help at any prompt)
sd2> a a
offset: [64]
size: [31101776] 10M
FS type: [4.2BSD] RAID
sd2*> w
sd2> q
No label changes

Encrypt with bioctl:

$ 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 -iy sd3
Writing MBR at offset 0.

$ 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
/dev/rsd3i: 7.8MB in 16000 sectors of 512 bytes
4 cylinder groups of 1.95MB, 125 blocks, 256 inodes each
super-block backups (for fsck -b #) at:
 32, 4032, 8032, 12032,

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

$ doas mkdir /mnt/encrypted-usb

$ doas mount /dev/sd3i /mnt/encrypted-usb

$ doas cp -avi $GNUPGHOME /mnt/encrypted-usb

Keep the backup mounted if you plan on setting up two or more keys as keytocard will delete the local copy on save.

Otherwise, unmount and disconnected the encrypted volume:

$ doas umount /mnt/encrypted-usb

$ doas bioctl -d sd3

See OpenBSD FAQ#14 for more information.

Create another partition to store the public key, or skip this step if you plan on uploading it to a key server.

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.

$ doas disklabel -E sd2
Label editor (enter '?' for help at any prompt)
sd2> a b
offset: [32130]
size: [31069710] 10M
FS type: [swap] 4.2BSD
sd2*> w
sd2> q
No label changes.

$ doas newfs sd2b
/dev/rsd2b: 15.7MB in 32096 sectors of 512 bytes
5 cylinder groups of 3.89MB, 249 blocks, 512 inodes each
super-block backups (for fsck -b #) at:
 32, 8000, 15968, 23936, 31904,

$ doas mkdir /mnt/public

$ doas mount /dev/sd2b /mnt/public

$ gpg --armor --export $KEYID | doas tee /mnt/public/$KEYID.txt

Configure Smartcard

Windows Use the YubiKey NEO Manager to enable CCID functionality.

Use GPG to configure YubiKey as a smartcard:

$ gpg --card-edit
Reader ...........: Yubico Yubikey 4 OTP U2F CCID
Application ID ...: D2760001240102010006055532110000
Version ..........: 2.1
Manufacturer .....: Yubico
Serial number ....: 05553211
Name of cardholder: [not set]
Language prefs ...: [not set]
Sex ..............: unspecified
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
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]

Change PIN

The default PIN is 123456 and default Admin PIN (PUK) is 12345678. CCID-mode PINs can be up to 127 ASCII characters.

The Admin PIN is required for some card operations and to unblock a PIN that has been entered incorrectly more than three times. See the GnuPG documentation on Managing PINs for details.

gpg/card> admin
Admin commands are allowed

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

Set information

Some fields are optional.

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 ..........: 2.1
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 ...: 2048R 2048R 2048R
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 0
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 destructive, one-way operation only. Make sure you've made a backup 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 security key devices or mint additional keys.

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.

$ gpg --edit-key $KEYID

Secret key is available.

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

Signing

Select and move the signature key. You will be prompted for the key passphrase and Admin PIN.

gpg> key 1

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

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

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

Encryption

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

gpg> key 1

gpg> key 2

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

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

[...]

Authentication

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

gpg> key 2

gpg> key 3

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

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

gpg> save

Verify card

Verify the sub-keys have been moved to YubiKey as indicated by ssb>:

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

Cleanup

Ensure you have:

  • Saved the encryption, signing and authentication sub-keys to YubiKey.
  • Saved the YubiKey PINs which you changed from defaults.
  • Saved the password to the master key.
  • Saved a copy of the master key, sub-keys and revocation certificates on an encrypted volume, to be stored offline.
  • Saved the password to that encrypted volume in a separate location.
  • Saved a copy of the public key somewhere easily accessible later.

Reboot or securely delete $GNUPGHOME and remove the secret keys from the GPG keyring:

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

$ gpg --delete-secret-key $KEYID

Important Make sure you have securely erased all generated keys and revocation certificates if a Live image was not used!

Using keys

Download drduh/config/gpg.conf:

$ cd ~/.gnupg ; wget https://raw.githubusercontent.com/drduh/config/master/gpg.conf

$ chmod 600 gpg.conf

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

Linux

$ sudo apt-get update && sudo apt-get install -y \
     gnupg2 gnupg-agent gnupg-curl scdaemon pcscd

$ sudo mount /dev/sdb2 /mnt

OpenBSD

$ doas pkg_add gnupg pcsc-tools

$ doas mount /dev/sd2b /mnt

Import the key:

$ gpg --import /mnt/pubkey.txt
gpg: key 0xFF3E7D88647EBCDB: public key "Dr Duh <doc@duh.to>" imported
gpg: Total number processed: 1
gpg:               imported: 1

Or download the public key from a keyserver:

$ gpg --recv $KEYID
gpg: requesting key 0xFF3E7D88647EBCDB from hkps server hkps.pool.sks-keyservers.net
[...]
gpg: key 0xFF3E7D88647EBCDB: public key "Dr Duh <doc@duh.to>" imported
gpg: Total number processed: 1
gpg:               imported: 1

Edit the master key to assign it ultimate trust by selecting trust then option 5:

$ export KEYID=0xFF3E7D88647EBCDB

$ gpg --edit-key $KEYID

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

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

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

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

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

gpg> quit

Remove and re-insert the YubiKey and check the status:

$ gpg --card-status
Application ID ...: D2760001240102010006055532110000
Version ..........: 2.1
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 ...: 4096R 4096R 4096R
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 0
Signature key ....: 07AA 7735 E502 C5EB E09E  B8B0 BECF A3C1 AE19 1D15
      created ....: 2016-05-24 23:22:01
Encryption key....: 6F26 6F46 845B BEB8 BDF3  7E9B 5912 A795 E90D D2CF
      created ....: 2016-05-24 23:29:03
Authentication key: 82BE 7837 6A3F 2E7B E556  5E35 3F29 127E 7964 9A3D
      created ....: 2016-05-24 23:36:40
General key info..: pub  4096R/0xBECFA3C1AE191D15 2016-05-24 Dr Duh <doc@duh.to>
sec#  4096R/0xFF3E7D88647EBCDB  created: 2016-05-24  expires: never
ssb>  4096R/0xBECFA3C1AE191D15  created: 2017-10-09  expires: 2018-10-09
                      card-no: 0006 05553211
ssb>  4096R/0x5912A795E90DD2CF  created: 2017-10-09  expires: 2018-10-09
                      card-no: 0006 05553211
ssb>  4096R/0x3F29127E79649A3D  created: 2017-10-09  expires: 2018-10-09
                      card-no: 0006 05553211

sec# indicates master key is not available (as it should be stored encrypted offline).

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 passwords and other credentials):

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

To encrypt to multiple recipients (or to multiple keys):

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

Decrypt the message:

$ 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:

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

Verify the signature:

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

Rotating keys

PGP does not provide 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 random number generator used to create them, for example. Therefore, it is good practice to occassionally rotate sub-keys.

When a sub-key expires, it can either be renewed or replaced. Both actions require access to the offline master key. Renewing sub-keys by updating their expiration date indicates you are still in possession of the offline master key and is more convenient.

Replacing keys, on the other hand, is less convenient but more secure: the new sub-keys 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 sub-keys 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 offline 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 sub-keys at all. Ideally, sub-keys would be ephemeral: used only once for each encryption, signing and authentication event, however in practice that is not really feasible or worthwhile with YubiKey. Advanced users may want to dedicate an offline device for more frequent key rotations and ease of provisioning.

To renew or rotate sub-keys, follow the same procedure to boot to a secure environment. Install required software and disconnect networking. Decrypt and mount the offline volume, then import the master key and configuration to a temporary working directory:

$ export GNUPGHOME=$(mktemp -d)

$ gpg --import /mnt/encrypted-usb/tmp.XXX/mastersub.key

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

Edit the master key:

$ export KEYID=0xFF3E7D88647EBCDB

$ gpg --edit-key $KEYID

Secret key is available
[...]

Follow the original steps to generate each sub-key. Previous sub-keys may be kept or deleted from the identity.

Finish by exporting new keys:

$ gpg --armor --export-secret-keys $KEYID > $GNUPGHOME/mastersub.key

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

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

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

There should now be at least two versions of the master and sub-keys backed up:

$ ls /mnt/encrypted-usb
lost+found  tmp.ykhTOGjR36  tmp.2gyGnyCiHs

Unmount and close the encrypted volume:

$ sudo umount /mnt/encrypted-usb

$ sudo cryptsetup luksClose /dev/mapper/usb/

Export the updated public key:

$ sudo mkdir /mnt/public

$ sudo mount /dev/sdb2 /mnt/public

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

$ sudo umount /mnt/public

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

SSH

gpg-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 has more information.

Create configuration

Create a hardened configuration for gpg-agent by downloading drduh/config/gpg-agent.conf:

$ curl -o ~/.gnupg/gpg-agent.conf https://raw.githubusercontent.com/drduh/config/master/gpg-agent.conf

$ cat ~/.gnupg/gpg-agent.conf
enable-ssh-support
default-cache-ttl 60
max-cache-ttl 120
pinentry-program /usr/bin/pinentry-curses

Alternatively, you may want to use /usr/bin/pinentry-gnome3 for a GUI-based prompt.

On macOS, use brew install pinentry-mac and adjust the program path to suit.

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:

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

On some systems, you may need to use the following instead:

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

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:

$ 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. 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:

$ 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:

$ 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).
[...]

Note To make multiple connections or securely transfer many files, consider using the ControlMaster ssh option. Also see drduh/config/ssh_config.

Touch to authenticate

Note This is not possible on YubiKey NEO.

By default, YubiKey will perform key operations without requiring a touch from the user. To require a touch for every SSH authentication, use the YubiKey Manager and Admin PIN:

$ ykman openpgp touch aut on

To require a touch for signing and encryption operations:

$ ykman openpgp touch sig on

$ ykman openpgp touch enc on

The YubiKey will blink when it's waiting for touch.

Import SSH keys

If there are existing SSH keys that you wish to make available via gpg-agent, you'll need to import them. You should then remove the original private keys. When importing the key, gpg-agent uses the key's filename as the key's label; this makes it easier to follow where the key originated from. In this example, we're starting with just the YubiKey's key 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, it will prompt for the SSH key's passphrase if present, then the pinentry program will prompt and confirm for a new passphrase to use to encrypt the converted key within the GPG key store.

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

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

Or to show the keys with 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's passphrase. The passphrase will be cached for up to 10 minutes idle time between uses, to a maximum of 2 hours.

Remote Machines (agent forwarding)

If you want to use YubiKey to sign a git commit on a remote machine, or ssh through another layer, then this is possible using Agent Forwarding.

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

On the local machine, 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 older Linux distros (and macOS) it may be /home/<user>/.gnupg/S/gpg-agent.extra

Find the agent socket on the remote machine:

$ gpgconf --list-dirs agent-socket

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

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 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.

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:

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

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
  ForwardAgent yes
  RemoteForward /run/user/1000/gnupg/S.gpg-agent /run/user/1000/gnupg/S.gpg-agent.extra
  # RemoteForward [remote socket] [local socket]

You should then be able to use YubiKey as if it were connected to the remote machine.

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

enable-ssh-support
pinentry-program /usr/bin/pinentry-curses
extra-socket /run/user/1000/gnupg/S.gpg-agent.extra

See Issue #85 for more information and troubleshooting.

GitHub

You can use YubiKey to sign GitHub commits and tags. It can also be used for GitHub SSH authentication, allowing you to push, pull, and commit without a password.

Login to GitHub and upload SSH and PGP public keys in Settings.

To configure a signing key:

> git config --global user.signingkey $KEYID

Make sure the user.email option matches the email address associated with the PGP identity.

Now, to sign commits or tags simply use the -S option. GPG will automatically query YubiKey and prompt you for a PIN.

To authenticate:

Windows

Run the following command:

> git config --global core.sshcommand 'plink -agent'

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:

$ 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 your YubiKey is the correct one used by scdaemon, you should add it to its configuration. You will need your device's full name. To find out what is your device's full name, plug your YubiKey, open the Device Manager, select "View->Show hidden devices". Go to the Software Devices list, you should see something like Yubico YubiKey OTP+FIDO+CCID 0. The name slightly differs according to the model. Thanks to Scott Hanselman for sharing this information.

  • Create or edit %APPDATA%/gnupg/scdaemon.conf, add reader-port <your yubikey device's full name>.
  • In %APPDATA%/gnupg/gpg-agent.conf, 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: > gpg --import <path to public key file>
  • Trust it: Trust master key
  • Retrieve the public key id: > gpg --list-public-keys
  • Export the SSH key from GPG: > gpg --export-ssh-key <public key id>

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

Note this works only for SSH agent forwarding. Real GPG forwarding (encryption/decryption) is actually not supported. See the weasel-pageant readme for further information.

Prerequisites

WSL configuration

Download or clone weasel-pageant.

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

Display the SSH key with $ ssh-add -l

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

ForwardAgent yes
RemoteForward <remote ssh socket path> /tmp/S.weasel-pageant

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

Remote host configuration

Add the following to the shell rc file:

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

Add the following to /etc/ssh/sshd_config:

AllowAgentForwarding yes
StreamLocalBindUnlink yes

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

Final test

  • Unplug YubiKey, disconnect or reboot.
  • Log back in to 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 in to 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 to configure each host.

Email

GPG keys on YubiKey can be used with ease to encrypt or sign email messages and attachments using Thunderbird and Enigmail. Thunderbird supports OAuth 2 authentication and can be used with Gmail. See this guide from EFF for detailed instructions.

Notes

  1. 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 application (note, this not the similarly named YubiKey NEO Manager).
  2. Programming YubiKey for GPG keys still lets you use its other configurations - U2F, OTP and static password modes, for example.
  3. 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 for this purpose. It may be appropriate for your use case to set expiry dates on subkeys.
  4. To switch between two or more identities on different keys - unplug the first key 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.

Troubleshooting

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

  • 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.

  • 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.

  • 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 gpg-connect-agent updatestartuptty /bye

  • If you still receive the error, sign_and_send_pubkey: signing failed: agent refused operation - check ~/.gnupg/gpg-agent.conf to make sure the path to pinentry is correct.

  • 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 for this host.

  • If SSH authentication stil fails - add up to 3 -v flags to increase verbosity.

  • If you totally screw up, you can reset the card.

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