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Add gpg-agent information.
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README.md typos Jul 19, 2018

README.md

This is a guide to using YubiKey as a SmartCard for storing GPG encryption and signing keys.

An authentication key can also be created for SSH and used with gpg-agent.

Keys stored on a smartcard like YubiKey are non-exportable (as opposed to 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 - and all signing and encryption operations happen on the card, rather than in OS memory.

Programming YubiKey for GPG keys still lets you use its two slots - OTP and static password modes, for example.

New! Purse is a password manager which can integrate with GPG on YubiKey.

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

  1. Purchase YubiKey
  2. Install required software
    2.1 Install - Linux
    2.2 Install - macOS
    2.3 Install - Windows
  3. Creating keys
    3.1 Create temporary working directory for GPG
    3.2 Create configuration
    3.3 Create master key
    3.4 Save Key ID
    3.5 Create subkeys
    3.5a Signing key
    3.5b Encryption key
    3.5c Authentication key
    3.6 Check your work
    3.7 Export keys
    3.7a Linux/macOS
    3.7b Windows
    3.8 Backup everything
    3.8a Linux/macOS
    3.8b Windows
    3.9 Configure YubiKey
    3.9a Linux/macOS
    3.9b Windows
    3.10 Configure smartcard
    3.10a Change PINs
    3.10b Set card information
    3.11 Transfer keys
    3.11a Signature key
    3.11b Encryption key
    3.11c Authentication key
    3.12 Check your work
    3.13 Export public key
    3.14 Finish
  4. Using keys
    4.1 Create GPG configuration
    4.2 Import public key
    4.3 Insert YubiKey
    4.4 GnuPG
    4.4a Trust master key
    4.4b Encryption
    4.4c Decryption
    4.4d Signing
    4.4e Verifying signature
    4.5 SSH - Linux/Mac
    4.5a A Note on GPG Agent's SSH Agent
    4.5b Update configuration
    4.5c Replace ssh-agent with gpg-agent
    4.5d Copy public key to server
    4.5e Connect with public key authentication
    4.5f (Optional) Import SSH Keys to gpg-agent
    4.6 SSH - Windows
    4.6a GitHub
    4.7 Requiring touch to authenticate
    4.8 OpenBSD
  5. Troubleshooting
    5.1 Yubikey OTP Mode and cccccccc....
  6. References and other work

1. Purchase YubiKey

https://www.yubico.com/products/yubikey-hardware/

Consider purchasing a pair (or more) and programming both in case of loss or damage to one of them.

2. Install required software

These instructions are current to Debian 9 using YubiKey 4 - with support for 4096 bit RSA keys - in OTP+CCID mode, using GPG version 2.2. Note, older YubiKeys like the Neo are limited to 2048 bit RSA keys.

For improved security, use a live GNU/Linux distribution like Tails or Debian Live - with no connection to outgoing Internet.

2.1 Install - Linux

You will need to install the following software:

$ sudo apt-get install -y \
    gnupg2 gnupg-agent pinentry-curses scdaemon pcscd yubikey-personalization libusb-1.0-0-dev

You may also need to download and install more recent versions of yubikey-personalization and yubico-c:

    $ curl -LfsOv https://developers.yubico.com/yubikey-personalization/Releases/ykpers-1.19.0.tar.gz

    $ !!.sig
    curl -LfsOv https://developers.yubico.com/yubikey-personalization/Releases/ykpers-1.19.0.tar.gz.sig

    $ gpg yk*sig
    gpg: assuming signed data in 'ykpers-1.19.0.tar.gz'
    gpg: Signature made Tue Apr 24 01:29:05 2018 PDT
    gpg:                using RSA key 0xBCA00FD4B2168C0A
    gpg: Can't check signature: No public key

    $ gpg --recv 0xBCA00FD4B2168C0A
    gpg: key 0xBCA00FD4B2168C0A: public key "Klas Lindfors <klas@yubico.com>" 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 yk*sig
    gpg: assuming signed data in 'ykpers-1.19.0.tar.gz'
    gpg: Signature made Tue Apr 24 01:29:05 2018 PDT
    gpg:                using RSA key 0xBCA00FD4B2168C0A
    gpg: Good signature from "Klas Lindfors <klas@yubico.com>" [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: 0A3B 0262 BCA1 7053 07D5  FF06 BCA0 0FD4 B216 8C0A

    $ curl -LfsOv https://developers.yubico.com/yubico-c/Releases/libyubikey-1.13.tar.gz

    $ !!.sig
    curl -LfsOv https://developers.yubico.com/yubico-c/Releases/libyubikey-1.13.tar.gz.sig

    $ gpg libyubi*sig
    gpg: assuming signed data in 'libyubikey-1.13.tar.gz'
    gpg: Signature made Thu Mar  5 03:51:51 2015 PST
    gpg:                using RSA key 0xBCA00FD4B2168C0A
    gpg: Good signature from "Klas Lindfors <klas@yubico.com>" [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: 0A3B 0262 BCA1 7053 07D5  FF06 BCA0 0FD4 B216 8C0A

    $ tar xf libyubikey-1.13.tar.gz

    $ cd libyubikey-1.13

    $ ./configure && make && sudo make install

    $ cd ..

    $ tar xf ykpers-1.19.0.tar.gz

    $ cd ykpers-1.19.0

    $ ./configure && make && sudo make install

    $ sudo ldconfig

If on Tails, you also need to install libykpers-1-1 from the testing repository. This is a temporary fix suggested on a securedrop issue:

$ sudo apt-get install -t testing libykpers-1-1

2.2 Install - macOS

You will need to install Homebrew and the following brew packages:

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

2.3 Install - windows

Download and install Gpg4Win. If you are interested in using your YubiKey for SSH authentication you should also install PuTTY.

Skip to 3.3

3. Creating keys

3.1 Create temporary working directory for GPG

Create a directory in /tmp which won't survive a reboot:

$ export GNUPGHOME=$(mktemp -d) ; echo $GNUPGHOME
/tmp/tmp.aaiTTovYgo

3.2 Create configuration

Paste the following text into a terminal window to create a recommended GPG configuration:

$ cat << EOF > $GNUPGHOME/gpg.conf
use-agent
personal-cipher-preferences AES256 AES192 AES CAST5
personal-digest-preferences SHA512 SHA384 SHA256 SHA224
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 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
EOF

3.3 Create master key

A note on security: for optimal security you should consider performing these actions on a bootable USB that you securely erase after completing the guide. Alternatively you should disable network connectivity on your computer and make sure you securely delete all secret keys and revocation certificates.

A note on key expiry: 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.

Generate a new key with GPG, selecting RSA (sign only) and the appropriate key-size:

% gpg --full-generate-key
gpg (GnuPG) 2.2.1; Copyright (C) 2017 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

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)
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) 0
Key does not expire at all
Is this correct? (y/N) y

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

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

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

You'll be prompted to enter and verify a passphrase. Keep the passphrase handy as you'll need it throughout.

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.

Note that this key cannot be used for encryption.  You may want to use
the command "--edit-key" to generate a subkey for this purpose.
pub   rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [SC]
      Key fingerprint = 011C E16B D45B 27A5 5BA8  776D FF3E 7D88 647E BCDB
uid                              Dr Duh <doc@duh.to>

Note that as of v2.1, gpg automatically generates a revocation certificate.

3.4 Save Key ID

Export the key ID as a variable for use throughout:

$ export KEYID=0xFF3E7D88647EBCDB

3.5 Create subkeys

Note: If using a Yubikey 4, please use 4096 bit as the size for the subkeys; if using a YubiKey Neo, please use 2048 bit as the size for the subkeys.

Edit the key to add subkeys:

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

Secret key is available.

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

3.5a Signing key

First, create a signing key, selecting 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) 0
Key does not expire at all
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: SC  
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S   
[ultimate] (1). Dr Duh <doc@duh.to>

3.5b Encryption key

Next, create an encryption key, selecting 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) 0
Key does not expire at all
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: SC  
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S   
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: never       usage: E   
[ultimate] (1). Dr Duh <doc@duh.to>

3.5c Authentication key

Finally, create an authentication key.

GPG doesn't provide a 'RSA (authenticate only)' key type out of the box, so select 'RSA (set your own capabilities)' and toggle the required capabilities to end up with an Authenticate-only key:

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) 0
Key does not expire at all
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: SC  
    trust: ultimate      validity: ultimate
ssb  rsa4096/0xBECFA3C1AE191D15
    created: 2017-10-09  expires: never       usage: S   
ssb  rsa4096/0x5912A795E90DD2CF
    created: 2017-10-09  expires: never       usage: E   
ssb  rsa4096/0x3F29127E79649A3D
    created: 2017-10-09  expires: never       usage: A   
[ultimate] (1). Dr Duh <doc@duh.to>

gpg> save

3.6 Check your work

List your new secret keys:

$ gpg --list-secret-keys
/tmp.FLZC0xcM/pubring.kbx
-------------------------------------------------------------------------
sec   rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [SC]
      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]
ssb   rsa4096/0x5912A795E90DD2CF 2017-10-09 [E]
ssb   rsa4096/0x3F29127E79649A3D 2017-10-09 [A]

Verify with OpenPGP key checks:

If you're on Linux or macOS, use the automated key best practice checker:

$ sudo apt-get install hopenpgp-tools
$ 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".

3.7 Export keys

3.7a Linux/macOS

Save a copy of your keys:

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

The exported (primary) key will still have the passphrase in place.

In addition to the backup below, you might want to keep a separate copy of the revocation certificate in a safe place - $GNUPGHOME/openpgp-revocs.d/<key fingerprint>.rev

3.7b Windows

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

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

The exported (primary) key will still have the passphrase in place.

In addition to the back up detailed in the next step, you should note the location of your revocation certificate from the terminal output and copy it to a secure location. Careful, anyone that has this certificate can revoke your key!

3.8 Backup everything

3.8a Linux/macOS

Once keys are moved to hardware, they cannot be extracted again, so make sure you have made an encrypted backup before proceeding.

Also consider using a paper copy of the keys as an additional backup measure.

You can also use the pre-compiled executables available at veracrypt.fr

To format and encrypt a USB drive on Linux, first attach it and check its label:

$ dmesg | tail
[ 7667.607011] scsi8 : usb-storage 2-1:1.0
[ 7667.608766] usbcore: registered new interface driver usb-storage
[ 7668.874016] scsi 8:0:0:0: USB 0: 0 ANSI: 6
[ 7668.874242] sd 8:0:0:0: Attached scsi generic sg4 type 0
[ 7668.874682] sd 8:0:0:0: [sde] 62980096 512-byte logical blocks: (32.2 GB/30.0 GiB)
[ 7668.875022] sd 8:0:0:0: [sde] Write Protect is off
[ 7668.875023] sd 8:0:0:0: [sde] Mode Sense: 43 00 00 00
[ 7668.877939]  sde: sde1
[ 7668.879514] sd 8:0:0:0: [sde] Attached SCSI removable disk

Check the size to make sure it's the right drive:

$ sudo fdisk -l | grep /dev/sde
Disk /dev/sde: 30 GiB, 32245809152 bytes, 62980096 sectors
/dev/sde1        2048 62980095 62978048  30G  6 FAT16

Erase and create a new partition table:

$ sudo fdisk /dev/sde

Welcome to fdisk (util-linux 2.25.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

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.

Remove and reinsert the USB drive, then create a new partition, selecting defaults:

$ sudo fdisk /dev/sde

Welcome to fdisk (util-linux 2.25.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

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

Created a new partition 1 of type 'Linux' and of size 30 GiB.
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/sde1

WARNING!
========
This will overwrite data on /dev/sde1 irrevocably.

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

Mount the partition:

$ sudo cryptsetup luksOpen /dev/sde1 encrypted-usb
Enter passphrase for /dev/sde1:

Create a filesystem:

$ sudo mkfs.ext4 /dev/mapper/encrypted-usb -L encrypted-usb
mke2fs 1.42.12 (29-Aug-2014)
Creating filesystem with 7871744 4k blocks and 1970416 inodes
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
        4096000

Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

Mount the filesystem:

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

Copy files to it:

$ sudo cp -avi $GNUPGHOME /mnt/usb
‘/tmp/tmp.aaiTTovYgo’ -> ‘/mnt/usb/tmp.aaiTTovYgo’
‘/tmp/tmp.aaiTTovYgo/revoke.txt’ -> ‘/mnt/usb/tmp.aaiTTovYgo/revoke.txt’
‘/tmp/tmp.aaiTTovYgo/gpg.conf’ -> ‘/mnt/usb/tmp.aaiTTovYgo/gpg.conf’
‘/tmp/tmp.aaiTTovYgo/trustdb.gpg’ -> ‘/mnt/usb/tmp.aaiTTovYgo/trustdb.gpg’
‘/tmp/tmp.aaiTTovYgo/random_seed’ -> ‘/mnt/usb/tmp.aaiTTovYgo/random_seed’
‘/tmp/tmp.aaiTTovYgo/master.key’ -> ‘/mnt/usb/tmp.aaiTTovYgo/master.key’
‘/tmp/tmp.aaiTTovYgo/secring.gpg’ -> ‘/mnt/usb/tmp.aaiTTovYgo/secring.gpg’
‘/tmp/tmp.aaiTTovYgo/mastersub.key’ -> ‘/mnt/usb/tmp.aaiTTovYgo/mastersub.key’
‘/tmp/tmp.aaiTTovYgo/sub.key’ -> ‘/mnt/usb/tmp.aaiTTovYgo/sub.key’
‘/tmp/tmp.aaiTTovYgo/pubring.gpg~’ -> ‘/mnt/usb/tmp.aaiTTovYgo/pubring.gpg~’
‘/tmp/tmp.aaiTTovYgo/pubring.gpg’ -> ‘/mnt/usb/tmp.aaiTTovYgo/pubring.gpg’

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 USB drive:

$ sudo umount /mnt/usb
$ sudo cryptsetup luksClose encrypted-usb

3.8b Windows

An encrypted flash drive or container can be made using VeraCrypt.

3.9 Configure YubiKey

3.9a Linux/macOS

YubiKey NEOs shipped after November 2015 have all modes enabled, skip to the next step.

Older versions of the YubiKey NEO may need to be reconfigured as a composite USB device (HID + CCID) which allows OTPs to be emitted while in use as a smart card.

Plug in your YubiKey and configure it:

$ ykpersonalize -m82
Firmware version 4.2.7 Touch level 527 Program sequence 4

The USB mode will be set to: 0x82

Commit? (y/n) [n]: y

The -m option is the mode command. To see the different modes, enter ykpersonalize –help. Mode 82 (in hex) enables the YubiKey NEO as a composite USB device (HID + CCID). Once you have changed the mode, you need to re-boot the YubiKey – so remove and re-insert it.

On YubiKey NEO with firmware version 3.3 or higher you can enable composite USB device with -m86 instead of -m82.

https://www.yubico.com/2012/12/yubikey-neo-openpgp/ https://www.yubico.com/2012/12/yubikey-neo-composite-device/

3.9b Windows

Use the YubiKey NEO Manager to enable CCID functionality.

3.10 Configure smartcard

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 ...: rsa4096 rsa4096 rsa4096
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 0
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]

3.10a Change PINs

The default PIN codes are 12345678 for the Admin PIN (aka PUK) and 123456 for the PIN. The CCID-mode PINs can be up to 127 ASCII characters long.

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

3.10b Set card 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> (Press Enter)

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 3 3
Signature counter : 0
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]

gpg/card> quit

3.11 Transfer keys

Transferring keys to YubiKey hardware using keytocard is a one-way operation only, so make sure you've made a backup before proceeding.

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

3.11a Signature key

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

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

3.11b Encryption key

Type key 1 again to deselect and key 2 to select the next key:

gpg> key 1

gpg> key 2

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

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

3.11c Authentication key

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

gpg> key 2

gpg> key 3

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

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

Save and quit:

gpg> save

3.12 Check your work

ssb> indicates a stub to the private key on smartcard:

% gpg --list-secret-keys
/tmp.FLZC0xcM/pubring.kbx
-------------------------------------------------------------------------
sec   rsa4096/0xFF3E7D88647EBCDB 2017-10-09 [SC]
      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]
ssb>  rsa4096/0x5912A795E90DD2CF 2017-10-09 [E]
ssb>  rsa4096/0x3F29127E79649A3D 2017-10-09 [A]

3.13 Export public key

This file should be publicly shared:

Linux/macOS

$ gpg --armor --export $KEYID > /mnt/public-usb-key/pubkey.txt

Windows

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

Optionally, it may be uploaded to a public keyserver:

$ gpg --send-key $KEYID
gpg: sending key 0xFF3E7D88647EBCDB to hkps server hkps.pool.sks-keyservers.net
[...]

After a little while, it ought to propagate to other servers.

3.14 Finish

If all went well, you should now reboot or securely delete $GNUPGHOME.

If you are using Windows, the easiest way to remove the secret keys is to purge them from your GPG keyring.

$ gpg --delete-secret-key $KEYID

Make sure you backup up your key prior to doing this as the action is irreversible. You may also want to consider securely deleting the revocation certificate from your hard drive.

4. Using keys

4.1 Create GPG configuration

Skip this section if you are on Windows

Paste the following text into a terminal window to create a recommended GPG configuration:

$ cat << EOF > ~/.gnupg/gpg.conf
auto-key-locate keyserver
keyserver hkps://hkps.pool.sks-keyservers.net
keyserver-options no-honor-keyserver-url
keyserver-options no-honor-keyserver-url
personal-cipher-preferences AES256 AES192 AES CAST5
personal-digest-preferences SHA512 SHA384 SHA256 SHA224
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed
cert-digest-algo SHA512
s2k-cipher-algo AES256
s2k-digest-algo SHA512
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
use-agent
require-cross-certification
EOF

Ensure you change to correct rights of that file to at least avoid a warning message about incorrect file rights

chmod 600 ~/.gnupg/gpg.conf

4.2 Import public key

Import it from a file:

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

Or download from a keyserver:

$ gpg --recv 0xFF3E7D88647EBCDB
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  (RSA: 1)

Linux/macOS: You may get an error gpgkeys: HTTP fetch error 1: unsupported protocol -- this means you need to install a special version of curl which supports gnupg:

$ sudo apt-get install gnupg-curl

4.3 Insert YubiKey

Unplug and replug the Yubikey. Check the card's 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: 2016-05-24  expires: never
                      card-no: 0006 05553211
ssb>  4096R/0x5912A795E90DD2CF  created: 2016-05-24  expires: never
                      card-no: 0006 05553211
ssb>  4096R/0x3F29127E79649A3D  created: 2016-05-24  expires: never
                      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, first import your public key using the previous step.

4.4 GnuPG

4.4a Trust master key

Edit the imported key to assign it ultimate trust:

$ gpg --edit-key 0xFF3E7D88647EBCDB

Secret key is available.

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

gpg> trust
pub  4096R/0xFF3E7D88647EBCDB  created: 2016-05-24  expires: never       usage: SC
                               trust: unknown       validity: unknown
sub  4096R/0xBECFA3C1AE191D15  created: 2016-05-24  expires: never       usage: S
sub  4096R/0x5912A795E90DD2CF  created: 2016-05-24  expires: never       usage: E
sub  4096R/0x3F29127E79649A3D  created: 2016-05-24  expires: never       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: SC
                               trust: ultimate      validity: unknown
sub  4096R/0xBECFA3C1AE191D15  created: 2016-05-24  expires: never       usage: S
sub  4096R/0x5912A795E90DD2CF  created: 2016-05-24  expires: never       usage: E
sub  4096R/0x3F29127E79649A3D  created: 2016-05-24  expires: never       usage: A
[ unknown] (1). Dr Duh <doc@duh.to>
Please note that the shown key validity is not necessarily correct
unless you restart the program.

gpg> quit

4.4b Encryption

Encrypt some sample text:

Note for Windows users: Replace echo "$(uname -a)" with echo "Test123"

$ echo "$(uname -a)" | gpg --encrypt --armor --recipient 0xFF3E7D88647EBCDB
-----BEGIN PGP MESSAGE-----
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=pzkv
-----END PGP MESSAGE-----

4.4c Decryption

Decrypt the sample text by pasting it:

$ gpg --decrypt --armor
-----BEGIN PGP MESSAGE-----
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=pzkv
-----END PGP MESSAGE-----
gpg: encrypted with 4096-bit RSA key, ID 0x5912A795E90DD2CF, created
2016-05-24
      "Dr Duh <doc@duh.to>"

(Press Control-D)

Linux workstation 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08) x86_64 GNU/Linux

4.4d Signing

Sign some sample text using the signing subkey:

$ echo "$(uname -a)" | gpg --armor --clearsign --default-key 0xBECFA3C1AE191D15
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Linux workstation 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08) x86_64 GNU/Linux
-----BEGIN PGP SIGNATURE-----
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=+ylJ
-----END PGP SIGNATURE-----

4.4e Verifying signature

Verify the previous signature:

$ gpg
gpg: Go ahead and type your message ...
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Linux workstation 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08) x86_64 GNU/Linux
-----BEGIN PGP SIGNATURE-----
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=+ylJ
-----END PGP SIGNATURE-----

(Press Control-D)

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

4.5 SSH - Linux/macOS

4.5a A Note on GPG Agent's SSH Agent

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.

4.5b Update configuration

Paste the following text into a terminal window to create a recommended GPG agent configuration:

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

If you are using Linux on the desktop, you may want to use /usr/bin/pinentry-gnome3 to use a GUI manager. For macOS, try brew install pinentry-mac, and adjust the pinentry-program setting to suit.

4.5c Replace ssh-agent with gpg-agent

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

Depending on how your environment is set up, you might need to add these to your shell rc file:

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

Note On some systems, for example Arch Linux-based distributions, you may need to replace the second and the third line with:

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

4.5d Copy public key to server

There is a -L option of ssh-add that lists public key parameters of all identities currently represented by the agent. Copy and paste the following output to the server 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 ssh agent. It's often a good idea to manage exactly which key(s) 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 ssh command line argument -l [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

4.5e 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.

4.5f (Optional) Import SSH Keys to gpg-agent

If you have 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.

4.6 SSH - Windows

Begin by exporting your SSH key from GPG:

$ gpg --export-ssh-key $USERID

Copy this key to a file and keep it for later use. It represents the public SSH key corresponding to the secret key on your YubiKey. You can upload this key to any server you wish to SSH into.

To authenticate SSH sessions via our YubiKey we need to enable Gpg4Win's PuTTY integration. Create a file named gpg-agent.conf and place it in the directory C:\%APPDATA%\gnupg. The file should contain the line enable-putty-support.

Then, open a terminal and run the following commands:

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

Create a shortcut that points to gpg-connect-agent /bye and place it in your startup folder to make sure the agent starts after a system shutdown.

Now you can use PuTTY for public key SSH authentication. When the server asks for publickey verification, PuTTY will foward the request to GPG, whcih will prompt you for your PIN and authorize the login using your YubiKey.

4.6a GitHub

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

Log into GitHub and upload your SSH and PGP public keys.

Signing

Then run the following commands:

> git config --global user.signingkey $KEYID

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

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

Authentication

Run the following commands (only for Windows):

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

You can then change your repository url to: git@github.com:USERNAME/repository. Any authenticated commands will be authorized by your YubiKey.

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

4.7 Requiring touch to authenticate

Note: this is only possible on the Yubikey 4 line.

By default the Yubikey will perform key operations without requiring a touch from the user. To require a touch for every SSH connection, use the Yubikey Manager (you'll need the Admin PIN):

ykman openpgp touch aut on

To require a touch for the signing and encrypting keys as well:

ykman openpgp touch sig on
ykman openpgp touch enc on

The Yubikey will blink when it's waiting for the touch.

4.8 OpenBSD

On OpenBSD, you will need to install pcsc-tools and enable with sudo rcctl enable pcscd, then reboot in order to recognize the key.

5. Troubleshooting

  • If you don't understand some option, read man gpg.

  • If you encounter problems connecting to YubiKey with GPG, simply try unplugging and re-inserting your 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 Install 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 - you probably have ssh-agent running. 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 - On Debian, try gpg-connect-agent updatestartuptty /bye

  • 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"

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

5.1 Yubikey OTP Mode and cccccccc....

The 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).

6. References and similar work