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Guide to using YubiKey for GPG and SSH
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README.md Grammar and lint. Fix #73. 2018-11-28 21:38:35 -08:00

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.

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! 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. See Compare YubiKeys.

Consider purchasing a pair of YubiKeys, programming both, and storing one in a safe secondary location, in case of loss or damage to the first key.

Live image

It is recommended to generate cryptographic keys and configure YubiKey from a secure environment. One way to do that is by downloading and booting to a Debian Live or Tails image loaded from a USB drive into memory.

Download the latest image and verify its integrity:

$ curl -LfO https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/debian-live-9.6.0-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

$ gpg --verify SHA512SUMS.sign SHA512SUMS
[...]
gpg: Good signature from "Debian CD signing key <debian-cd@lists.debian.org>" [unknown]
[...]

$ grep $(sha512sum debian-live-9.6.0-amd64-xfce.iso) SHA512SUMS
e35dd65fe1b078f71fcf04fa749a05bfefe4aa11a9e80f116ceec0566d65636a4ac84a9aff22aa3f7a8eeb10289d0c2f54dfe7c599d8aa16663e4f9a74f3eec5 debian-live-9.6.0-amd64-xfce.iso

Mount a USB drive and copy the image over to it:

$ sudo dd if=debian-live-9.6.0-amd64-xfce.iso of=/dev/sdc bs=4M
$ sync

Shut down the computer and disconnect any hard drives and unnecessary peripherals.

Plug in the USB drive and boot to the live image. Configure networking to continue.

Required software

Install several packages required for the following steps:

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

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

macOS Download and install Homebrew and the following Brew packages - gnupg yubikey-personalization hopenpgp-tools ykman pinentry-mac

Windows Download and install Gpg4Win and PuTTY.

Entropy

Generating keys will require a lot of randomness. To check the available bits of entropy available on Linux:

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

Optional A hardware random number generator like OneRNG will increase the speed of entropy generation and possibly its quality. To install and configure OneRNG:

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

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

$ sha256sum onerng_3.6-1_all.deb
a9ccf7b04ee317dbfc91518542301e2d60ebe205d38e80563f29aac7cd845ccb

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

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

$ sudo service rng-tools restart

If the service fails to start, kick off atd and try again:

$ sudo atd ; sudo service rng-tools restart

Plug in the OneRNG and empty /dev/random - the light on the device should dim briefly. Verify the available entropy pool is re-seeded.

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

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

Creating keys

Create a temporary directory which will be deleted on reboot:

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

Create a hardened configuration for GPG with the following options or by downloading my recommended version directly:

$ curl -Lfo $GNUPGHOME/gpg.conf https://raw.githubusercontent.com/drduh/config/master/gpg.conf

$ cat $GNUPGHOME/gpg.conf
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
require-cross-certification
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. This master key should be kept offline at all times and only accessed to revoke or issue new sub-keys.

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

Generate a new key with GPG, selecting (4) RSA (sign only) and 4096 bit keysize. Do not set the key to expire - see Note #3.

$ gpg --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)
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

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>

As of GPG version 2.1, a revocation certificate is automatically generated at this time.

Export the key ID as a variable 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: SC  
    trust: ultimate      validity: ultimate
[ultimate] (1). Dr Duh <doc@duh.to>

Use 4096-bit keysize - or 2048-bit on NEO.

Use a 1 year expiration - it can always be renewed using the offline Master certification key.

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: SC  
    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: SC  
    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: SC  
    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 keys

List the generated secret keys and verify the output:

$ 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] [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]

To verify with OpenPGP key checks, 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".

Export keys

The Master 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 keys

Once keys are moved to hardware, they cannot be extracted again, so make sure you have made an encrypted backup before proceeding. An encrypted USB drive or container can be made using VeraCrypt.

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

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

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

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

$ sudo fdisk -l /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 usb
Enter passphrase for /dev/sde1:

Create a filesystem:

$ sudo mkfs.ext4 /dev/mapper/usb -L usb
mke2fs 1.43.4 (31-Jan-2017)
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 mount /dev/mapper/usb /mnt

Backup all GPG files to it:

$ sudo cp -avi $GNUPGHOME /mnt

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

$ sudo cryptsetup luksClose usb

Configure YubiKey

Plug in YubiKey and configure it with the ykpersonalize utility:

$ sudo ykpersonalize -m82
Firmware version 4.3.7 Touch level 527 Program sequence 1

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.

Note YubiKey NEO shipped after November 2015 have all modes enabled; so this configuration may be skipped. 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 SmartCard.

Windows Use the YubiKey NEO Manager to enable CCID functionality.

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 ...: 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 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

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

gpg/card> quit

Transfer keys

Transferring keys to YubiKey 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: 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: SC
    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: SC
    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: SC
    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 moved to YubiKey as indicated by ssb>:

$ 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] [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]

Export public key

Mount another USB drive to copy the public key, or save it somewhere where you can easily access later.

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 the YubiKey for SSH.

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

On Windows:

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

Optionally, the public key may be uploaded to a public keyserver:

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

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

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 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

You can reboot back into the Live image to test YubiKey.

Install required programs:

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

Create a hardened configuration for GPG with the following options or by downloading my recommended version directly:

$ mkdir ~/.gnupg ; curl -Lfo ~/.gnupg/gpg.conf https://raw.githubusercontent.com/drduh/config/master/gpg.conf

$ cat ~/.gnupg/gpg.conf
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
require-cross-certification
use-agent

$ chmod 600 ~/.gnupg/gpg.conf

Import public key

To import the public key from a file on an encrypted USB drive:

$ sudo cryptsetup luksOpen /dev/sdd1 usb
Enter passphrase for /dev/sdd1:

$ sudo mount /dev/mapper/usb /mnt

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

To download the public key 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

If you get the error gpgkeys: HTTP fetch error 1: unsupported protocol - this means you need to install a special version of curl which supports GPG:

$ sudo apt-get install -y gnupg-curl

Trust master key

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

$ gpg --edit-key 0xFF3E7D88647EBCDB

Secret key is available.

gpg> trust
pub  4096R/0xFF3E7D88647EBCDB  created: 2016-05-24  expires: never       usage: SC
                               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: SC
                               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> save

Insert YubiKey

Re-connect 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.

Encryption

$ echo "test message string" | gpg --encrypt --armor --recipient 0xFF3E7D88647EBCDB
-----BEGIN PGP MESSAGE-----
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=pzkv
-----END PGP MESSAGE-----

Decryption

$ 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]

test message string

Signing

$ echo "test message string" | gpg --armor --clearsign --default-key 0xBECFA3C1AE191D15
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

test message string
-----BEGIN PGP SIGNATURE-----
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=+ylJ
-----END PGP SIGNATURE-----

Verifying signature

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

test message string
-----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

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 with the following options or by downloading my recommended version directly:

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

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

Alternatively, you may want to use /usr/bin/pinentry-gnome3 to use a GUI manager. 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 your shell rc file:

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

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

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

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 -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

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 connection, use the YubiKey Manager and 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 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.

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 your YubiKey and prompt you for your PIN.

To authenticate:

Windows Run the following command:

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

You can then change your repository url to git@github.com:USERNAME/repository and 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 YubiKey plugged in and try the git command again.

OpenBSD

Install pcsc-tools and enable with doas rcctl enable pcscd, then reboot in order to recognize YubiKey.

Windows

Export the SSH key from GPG:

$ gpg --export-ssh-key $USERID

Copy this key to a file 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 YubiKey, 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 public key verification, PuTTY will forward the request to GPG, which will prompt you for your PIN and authorize the login using your YubiKey.

Troubleshooting

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

  • If you encounter problems connecting to YubiKey with GPG - 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 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.

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 two configurations - 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.

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