| LICENSE | ||
| README.md | ||
This is a practical 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 seem more difficult to steal than ones stored on disk, and are convenient for everyday use.
Instructions written for Debian GNU/Linux 8 (jessie) using YubiKey 4 - with support for 4096 bit RSA keys - in OTP+CCID mode, updated to GPG version 2.2.1. Some notes are included for macOS as well. Note, older YubiKeys like the Neo are limited to 2048 bit RSA keys. Please see a comparison of the different YubiKeys here.
Debian live install images are available from here and are suitable for writing to USB drives.
Programming YubiKey for GPG keys still lets you use its two slots - OTP and static password modes, for example.
If you have a comment or suggestion, please open an issue on GitHub.
- Purchase YubiKey
- Install required software
- Creating keys
- Using keys
- Troubleshooting
- References
Purchase YubiKey
https://www.yubico.com/products/yubikey-hardware/
https://www.amazon.com/Yubico/b/ref=bl_dp_s_web_10358012011?ie=UTF8&node=10358012011
Consider purchasing a pair and programming both in case of loss or damage to one of them.
Install required software
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 -sO https://developers.yubico.com/yubikey-personalization/Releases/ykpers-1.17.3.tar.gz
$ !!.sig
curl -sO https://developers.yubico.com/yubikey-personalization/Releases/ykpers-1.17.3.tar.gz.sig
$ gpg ykpers*sig
gpg: assuming signed data in `ykpers-1.17.3.tar.gz'
gpg: Signature made Mon 28 Dec 2015 11:56:41 AM UTC
gpg: using RSA key 0xBCA00FD4B2168C0A
gpg: Can't check signature: public key not found
$ gpg --recv 0xBCA00FD4B2168C0A
gpg: requesting key 0xBCA00FD4B2168C0A from hkps server hkps.pool.sks-keyservers.net
mp/2.3
[...]
gpg: key 0xBCA00FD4B2168C0A: public key "Klas Lindfors <klas@yubico.com>" imported
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
$ gpg ykpers*sig
gpg: assuming signed data in `ykpers-1.17.3.tar.gz'
gpg: Signature made Mon 28 Dec 2015 11:56:41 AM UTC
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 -sO https://developers.yubico.com/yubico-c/Releases/libyubikey-1.13.tar.gz
$ !!.sig
curl -sO 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 05 Mar 2015 11:51:51 AM UTC
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-1.17.3.tar.gz
$ cd ykpers-1.17.3
$ ./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
Install - macOS
You will need to install Homebrew and the following brew packages:
$ brew install gnupg yubikey-personalization
Creating keys
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
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
Create master key
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 keysize:
% 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.
Save Key ID
Export the key ID as a variable for use throughout:
$ export KEYID=0xFF3E7D88647EBCDB
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>
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>
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>
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
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:
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
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
Backup everything
Once keys are moved to hardware, they cannot be extracted again (otherwise, what would be the point?), 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.
To create an encrypted USB drive, 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
Finally, 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’
Make sure the correct files were copied, then unmount and disconnected the encrypted USB drive:
$ sudo umount /mnt/usb
$ sudo cryptsetup luksClose encrypted-usb
Configure YubiKey
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/
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]
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
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
Transfer keys
Transferring keys to YubiKey hardware 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>
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
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>
Move the encryption key to card:
gpg> keytocard
Please select where to store the key:
(2) Encryption key
Your selection? 2
...
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>
Move the authentication key to card:
gpg> keytocard
Please select where to store the key:
(3) Authentication key
Your selection? 3
Save and quit:
gpg> save
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]
Export public key
This file should be publicly shared:
$ gpg --armor --export $KEYID > /mnt/public-usb-key/pubkey.txt
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.
Finish
If all went well, you should now reboot or securely delete $GNUPGHOME.
Using keys
Create GPG configuration
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
keyserver-options debug
keyserver-options verbose
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
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)
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
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.
GnuPG
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
Encryption
Encrypt some sample text:
$ echo "$(uname -a)" | gpg --encrypt --armor --recipient 0xFF3E7D88647EBCDB
-----BEGIN PGP MESSAGE-----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=pzkv
-----END PGP MESSAGE-----
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
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-----
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
Putting it all together:
$ echo "$(uname -a)" | gpg --encrypt --sign --armor --default-key 0xFF3E7D88647EBCDB --recipient 0xBECFA3C1AE191D15 | gpg --decrypt --armor
gpg: encrypted with 4096-bit RSA key, ID 0x5912A795E90DD2CF, created 2016-05-24
"Dr Duh <doc@duh.to>"
Linux workstation 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08) x86_64 GNU/Linux
gpg: Signature made Wed 25 May 2016 01:00:00 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
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
write-env-file
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.
Replace ssh-agent with gpg-agent
gpg-agent provides OpenSSH agent emulation. To launch the 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
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
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.
Requiring touch to authenticate
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.
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-agentprocess. -
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, trygpg-connect-agent updatestartuptty /bye -
If you receive the error,
Error connecting to agent: No such file or directoryfromssh-add -L, the UNIX file socket that the agent uses for communication with other processes may not be set up correctly. On Debian, tryexport SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh" -
If you totally screw up, you can reset the card.
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).
References
https://developers.yubico.com/yubikey-personalization/
https://developers.yubico.com/PGP/Card_edit.html
https://blog.josefsson.org/2014/06/23/offline-gnupg-master-key-and-subkeys-on-yubikey-neo-smartcard/
https://www.esev.com/blog/post/2015-01-pgp-ssh-key-on-yubikey-neo/
https://blog.habets.se/2013/02/GPG-and-SSH-with-Yubikey-NEO
https://rnorth.org/gpg-and-ssh-with-yubikey-for-mac
https://jclement.ca/articles/2015/gpg-smartcard/
https://github.com/herlo/ssh-gpg-smartcard-config
http://www.bootc.net/archives/2013/06/09/my-perfect-gnupg-ssh-agent-setup/
https://help.riseup.net/en/security/message-security/openpgp/best-practices
https://alexcabal.com/creating-the-perfect-gpg-keypair/
https://www.void.gr/kargig/blog/2013/12/02/creating-a-new-gpg-key-with-subkeys/