Offline Ed25519 OpenPGP key with subkeys on FST-01G running Gnuk

Below I describe how to generate an OpenPGP key and import it to a FST-01G device running Gnuk. See my earlier post on planning for my new OpenPGP key and the post on preparing the FST-01G to run Gnuk. For comparison with a RSA/YubiKey based approach, you can read about my setup from 2014.

Most of the steps below are covered by the Gnuk manual. The primary complication for me is the use of a offline machine and storing GnuPG directory stored on a USB memory device.

Offline machine

I use a laptop that is not connected to the Internet and boot it from a read-only USB memory stick. Finding a live CD that contains the necessary tools for using GnuPG with smartcards (gpg-agent, scdaemon, pcscd) is significantly harder than it should be. Using a rarely audited image begs the question of whether you can trust it. A patched kernel/gpg to generate poor randomness would be an easy and hard to notice hack. I’m using the PGP/PKI Clean Room Live CD. Recommendations on more widely used and audited alternatives would be appreciated. Select “Advanced Options” and “Run Shell” to escape the menus. Insert a new USB memory device, and prepare it as follows:

pgp@pgplive:/home/pgp$ sudo wipefs -a /dev/sdX
pgp@pgplive:/home/pgp$ sudo fdisk /dev/sdX
# create a primary partition of Linux type
pgp@pgplive:/home/pgp$ sudo mkfs.ext4 /dev/sdX1
pgp@pgplive:/home/pgp$ sudo mount /dev/sdX1 /mnt
pgp@pgplive:/home/pgp$ sudo mkdir /mnt/gnupghome
pgp@pgplive:/home/pgp$ sudo chown pgp.pgp /mnt/gnupghome
pgp@pgplive:/home/pgp$ sudo chmod go-rwx /mnt/gnupghome

GnuPG configuration

Set your GnuPG home directory to point to the gnupghome directory on the USB memory device. You will need to do this in every terminal windows you open that you want to use GnuPG in.

pgp@pgplive:/home/pgp$ export GNUPGHOME=/mnt/gnupghome
pgp@pgplive:/home/pgp$

At this point, you should be able to run gpg --card-status and get output from the smartcard.

Create master key

Create a master key and make a backup copy of the GnuPG home directory with it, together with an export ASCII version.

pgp@pgplive:/home/pgp$ gpg --quick-gen-key "Simon Josefsson <simon@josefsson.org>" ed25519 sign 216d
gpg: keybox '/mnt/gnupghome/pubring.kbx' created
gpg: /mnt/gnupghome/trustdb.gpg: trustdb created
gpg: key D73CF638C53C06BE marked as ultimately trusted
gpg: directory '/mnt/gnupghome/openpgp-revocs.d' created
gpg: revocation certificate stored as '/mnt/gnupghome/openpgp-revocs.d/B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE.rev'
pub   ed25519 2019-03-20 [SC] [expires: 2019-10-22]
      B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE
      B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE
uid                      Simon Josefsson <simon@josefsson.org>

pgp@pgplive:/home/pgp$ gpg -a --export-secret-keys B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE > $GNUPGHOME/masterkey.txt
pgp@pgplive:/home/pgp$ sudo cp -a $GNUPGHOME $GNUPGHOME-backup-masterkey
pgp@pgplive:/home/pgp$ 

Create subkeys

Create subkeys and make a backup of them too, as follows.

pgp@pgplive:/home/pgp$ gpg --quick-add-key B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE cv25519 encr 216d
pgp@pgplive:/home/pgp$ gpg --quick-add-key B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE ed25519 auth 216d
pgp@pgplive:/home/pgp$ gpg --quick-add-key B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE ed25519 sign 216d
pgp@pgplive:/home/pgp$ gpg -a --export-secret-keys B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE > $GNUPGHOME/mastersubkeys.txt
pgp@pgplive:/home/pgp$ gpg -a --export-secret-subkeys B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE > $GNUPGHOME/subkeys.txt
pgp@pgplive:/home/pgp$ sudo cp -a $GNUPGHOME $GNUPGHOME-backup-mastersubkeys
pgp@pgplive:/home/pgp$ 

Move keys to card

Prepare the card by setting Admin PIN, PIN, your full name, sex, login account, and key URL as you prefer, following the Gnuk manual on card personalization.

Move the subkeys from your GnuPG keyring to the FST01G using the keytocard command.

Take a final backup — because moving the subkeys to the card modifes the local GnuPG keyring — and create a ASCII armored version of the public key, to be transferred to your daily machine.

pgp@pgplive:/home/pgp$ gpg --list-secret-keys
/mnt/gnupghome/pubring.kbx
--------------------------
sec   ed25519 2019-03-20 [SC] [expires: 2019-10-22]
      B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE
uid           [ultimate] Simon Josefsson <simon@josefsson.org>
ssb>  cv25519 2019-03-20 [E] [expires: 2019-10-22]
ssb>  ed25519 2019-03-20 [A] [expires: 2019-10-22]
ssb>  ed25519 2019-03-20 [S] [expires: 2019-10-22]

pgp@pgplive:/home/pgp$ gpg -a --export-secret-keys B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE > $GNUPGHOME/masterstubs.txt
pgp@pgplive:/home/pgp$ gpg -a --export-secret-subkeys B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE > $GNUPGHOME/subkeysstubs.txt
pgp@pgplive:/home/pgp$ gpg -a --export B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE > $GNUPGHOME/publickey.txt
pgp@pgplive:/home/pgp$ cp -a $GNUPGHOME $GNUPGHOME-backup-masterstubs
pgp@pgplive:/home/pgp$ 

Transfer to daily machine

Copy publickey.txt to your day-to-day laptop and import it and create stubs using --card-status.

jas@latte:~$ gpg --import < publickey.txt 
gpg: key D73CF638C53C06BE: public key "Simon Josefsson <simon@josefsson.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1
jas@latte:~$ gpg --card-status

Reader ...........: Free Software Initiative of Japan Gnuk (FSIJ-1.2.14-67252015) 00 00
Application ID ...: D276000124010200FFFE672520150000
Version ..........: 2.0
Manufacturer .....: unmanaged S/N range
Serial number ....: 67252015
Name of cardholder: Simon Josefsson
Language prefs ...: sv
Sex ..............: male
URL of public key : https://josefsson.org/key-20190320.txt
Login data .......: jas
Signature PIN ....: not forced
Key attributes ...: ed25519 cv25519 ed25519
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 0
Signature key ....: A3CC 9C87 0B9D 310A BAD4  CF2F 5172 2B08 FE47 45A2
      created ....: 2019-03-20 23:40:49
Encryption key....: A9EC 8F4D 7F1E 50ED 3DEF  49A9 0292 3D7E E76E BD60
      created ....: 2019-03-20 23:40:26
Authentication key: CA7E 3716 4342 DF31 33DF  3497 8026 0EE8 A9B9 2B2B
      created ....: 2019-03-20 23:40:37
General key info..: sub  ed25519/51722B08FE4745A2 2019-03-20 Simon Josefsson <simon@josefsson.org>
sec   ed25519/D73CF638C53C06BE  created: 2019-03-20  expires: 2019-10-22
ssb>  cv25519/02923D7EE76EBD60  created: 2019-03-20  expires: 2019-10-22
                                card-no: FFFE 67252015
ssb>  ed25519/80260EE8A9B92B2B  created: 2019-03-20  expires: 2019-10-22
                                card-no: FFFE 67252015
ssb>  ed25519/51722B08FE4745A2  created: 2019-03-20  expires: 2019-10-22
                                card-no: FFFE 67252015
jas@latte:~$ 

Before the key can be used after the import, you must update the trust database for the secret key.

Now you should have a offline master key with subkey stubs. Note in the output below that the master key is not available (sec#) and the subkeys are stubs for smartcard keys (ssb>).

jas@latte:~$ gpg --list-secret-keys
sec#  ed25519 2019-03-20 [SC] [expires: 2019-10-22]
      B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE
uid           [ultimate] Simon Josefsson <simon@josefsson.org>
ssb>  cv25519 2019-03-20 [E] [expires: 2019-10-22]
ssb>  ed25519 2019-03-20 [A] [expires: 2019-10-22]
ssb>  ed25519 2019-03-20 [S] [expires: 2019-10-22]

jas@latte:~$

If your environment variables are setup correctly, SSH should find the authentication key automatically.

jas@latte:~$ ssh-add -L
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILzCFcHHrKzVSPDDarZPYqn89H5TPaxwcORgRg+4DagE cardno:FFFE67252015
jas@latte:~$ 

GnuPG and SSH are now ready to be used with the new key. Thanks for reading!

Installing Gnuk on FST-01G running NeuG

The FST-01G device that you order from the FSF shop runs NeuG. To be able to use the device as a OpenPGP smartcard, you need to install Gnuk. While Niibe covers this on his tutorial, I found the steps a bit complicated to follow. The following guides you from buying the device to getting a FST-01G running Gnuk ready for use with GnuPG.

Once you have received the device and inserted it into a USB port, your kernel log (sudo dmesg) will show something like the following:

[628772.874658] usb 1-1.5.1: New USB device found, idVendor=234b, idProduct=0004
[628772.874663] usb 1-1.5.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[628772.874666] usb 1-1.5.1: Product: Fraucheky
[628772.874669] usb 1-1.5.1: Manufacturer: Free Software Initiative of Japan
[628772.874671] usb 1-1.5.1: SerialNumber: FSIJ-0.0
[628772.875204] usb-storage 1-1.5.1:1.0: USB Mass Storage device detected
[628772.875452] scsi host6: usb-storage 1-1.5.1:1.0
[628773.886539] scsi 6:0:0:0: Direct-Access     FSIJ     Fraucheky        1.0  PQ: 0 ANSI: 0
[628773.887522] sd 6:0:0:0: Attached scsi generic sg2 type 0
[628773.888931] sd 6:0:0:0: [sdb] 128 512-byte logical blocks: (65.5 kB/64.0 KiB)
[628773.889558] sd 6:0:0:0: [sdb] Write Protect is off
[628773.889564] sd 6:0:0:0: [sdb] Mode Sense: 03 00 00 00
[628773.890305] sd 6:0:0:0: [sdb] No Caching mode page found
[628773.890314] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[628773.902617]  sdb:
[628773.906066] sd 6:0:0:0: [sdb] Attached SCSI removable disk

The device comes up as a USB mass storage device. Conveniently, it contain documentation describing what it is, and you identify the version of NeuG it runs as follows.

jas@latte:~/src/gnuk$ head /media/jas/Fraucheky/README 
NeuG - a true random number generator implementation (for STM32F103)

							  Version 1.0.7
							     2018-01-19
						           Niibe Yutaka
				      Free Software Initiative of Japan

To convert the device into the serial-mode that is required for the software upgrade, use the eject command for the device (above it came up as /dev/sdb): sudo eject /dev/sdb. The kernel log will now contain something like this:

[628966.847387] usb 1-1.5.1: reset full-speed USB device number 27 using ehci-pci
[628966.955723] usb 1-1.5.1: device firmware changed
[628966.956184] usb 1-1.5.1: USB disconnect, device number 27
[628967.115322] usb 1-1.5.1: new full-speed USB device number 28 using ehci-pci
[628967.233272] usb 1-1.5.1: New USB device found, idVendor=234b, idProduct=0001
[628967.233277] usb 1-1.5.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[628967.233280] usb 1-1.5.1: Product: NeuG True RNG
[628967.233283] usb 1-1.5.1: Manufacturer: Free Software Initiative of Japan
[628967.233286] usb 1-1.5.1: SerialNumber: FSIJ-1.0.7-67252015
[628967.234034] cdc_acm 1-1.5.1:1.0: ttyACM0: USB ACM device

The strings NeuG True RNG and FSIJ-1.0.7 suggest it is running NeuG version 1.0.7.

Now both Gnuk itself and reGNUal needs to be built, as follows. If you get any error message, you likely don’t have the necessary dependencies installed.

jas@latte:~/src$ git clone https://salsa.debian.org/gnuk-team/gnuk/neug.git
jas@latte:~/src$ git clone https://salsa.debian.org/gnuk-team/gnuk/gnuk.git
jas@latte:~/src$ cd gnuk/src/
jas@latte:~/src/gnuk/src$ git submodule update --init
jas@latte:~/src/gnuk/src$ ./configure --vidpid=234b:0000
...
jas@latte:~/src/gnuk/src$ make
...
jas@latte:~/src/gnuk/src$ cd ../regnual/
jas@latte:~/src/gnuk/regnual$ make
jas@latte:~/src/gnuk/regnual$ cd ../../

You are now ready to flash the device, as follows.

jas@latte:~/src$ sudo neug/tool/neug_upgrade.py -f gnuk/regnual/regnual.bin gnuk/src/build/gnuk.bin 
gnuk/regnual/regnual.bin: 4544
gnuk/src/build/gnuk.bin: 113664
CRC32: 931cab51

Device: 
Configuration: 1
Interface: 1
20000e00:20005000
Downloading flash upgrade program...
start 20000e00
end   20001f00
# 20001f00: 31 : 196
Run flash upgrade program...
Wait 3 seconds...
Device: 
08001000:08020000
Downloading the program
start 08001000
end   0801bc00
jas@latte:~/src$ 

Remove and insert the device and the kernel log should contain something like this:

[629120.399875] usb 1-1.5.1: new full-speed USB device number 32 using ehci-pci
[629120.511003] usb 1-1.5.1: New USB device found, idVendor=234b, idProduct=0000
[629120.511008] usb 1-1.5.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[629120.511011] usb 1-1.5.1: Product: Gnuk Token
[629120.511014] usb 1-1.5.1: Manufacturer: Free Software Initiative of Japan
[629120.511017] usb 1-1.5.1: SerialNumber: FSIJ-1.2.14-67252015

The device can now be used with GnuPG as a smartcard device.

jas@latte:~/src/gnuk$ gpg --card-status
Reader ...........: 234B:0000:FSIJ-1.2.14-67252015:0
Application ID ...: D276000124010200FFFE672520150000
Version ..........: 2.0
Manufacturer .....: unmanaged S/N range
Serial number ....: 67252015
Name of cardholder: [not set]
Language prefs ...: [not set]
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: forced
Key attributes ...: rsa2048 rsa2048 rsa2048
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]
jas@latte:~/src/gnuk$ 

Congratulations!

Planning for a new OpenPGP key

I’m the process of migrating to a new OpenPGP key. I have been using GnuPG with keys stored on external hardware (smartcards) for a long time, and I’m firmly committed to that choice. Algorithm wise, RSA was the best choice back for me when I created my key in 2002, and I used it successfully with a non-standard key size for many years. In 2014 it was time for me to move to a new stronger key, and I still settled on RSA and a non-standard key size. My master key was 3744 bits instead of 1280 bits, and the smartcard subkeys were 2048 bits instead of 1024 bits. At that time, I had already moved from the OpenPGP smartcard to the NXP-based YubiKey NEO (version 3) that runs JavaCard applets. The primary relevant difference for me was the availability of source code for the OpenPGP implementation running on the device, in the ykneo-openpgp project. The device was still a proprietary hardware and firmware design though.

Five years later, it is time for a new key again, and I allow myself to revisit some decisions that I made last time.

GnuPG has supported Curve25519/Ed25519 for some time, and today I prefer it over RSA. Infrastructure has been gradually introducing support for it as well, to the point that I now believe I can cut the ropes to the old world with RSA. Having a offline master key is still a strong preference, so I will stick to that decision. You shouldn’t run around with your primary master key if it is possible to get by with subkeys for daily use, and that has worked well for me over the years.

Hardware smartcard support for Curve25519/Ed25519 has been behind software support. NIIBE Yutaka developed the FST-01 hardware device in 2011, and the more modern FST-01G device in 2016. He also wrote the Gnuk software implementation of the OpenPGP card specification that runs on the FST-01 hardware (and other devices). The FST-01 hardware design is open, and it only runs the Gnuk free software. You can buy the FST-01G device from the FSF. The device has not received the FSF Respects Your Freedom stamp, even though it is sold by FSF which seems a bit hypocritical. Hardware running Gnuk are the only free software OpenPGP smartcard that supports Curve25519/Ed25519 right now, to my knowledge. The physical form factor is not as slick as the YubiKey (especially the nano-versions of the YubiKey that can be emerged into the USB slot), but it is a trade-off I can live with. Niibe introduced the FST-01SZ at FOSDEM’19 but to me it does not appear to offer any feature over the FST-01G and is not available for online purchase right now.

I have always generated keys in software using GnuPG. My arguments traditionally was that I 1) don’t trust closed-source RSA key generation implementations, and 2) want to be able to reproduce my setup with a brand new device. With Gnuk the first argument doesn’t hold any longer. However, I still prefer to generate keys with GnuPG on a Linux-based Debian machine because that software stack is likely to receive more auditing than Gnuk. It is a delicated decision though, since GnuPG on Debian is many orders of complexity higher than the Gnuk software. My second argument is now the primary driver for this decision.

I prefer the SHA-2 family of hashes over SHA-1, and earlier had to configure GnuPG for this. Today I believe the defaults have been improved and this is no longer an issue.

Back in 2014, I had a goal of having a JPEG image embedded in my OpenPGP key. I never finished that process, and I have not been sorry for missing out on anything as a result. On the contrary, the size of the key with an embedded image woud have been even more problematic than the already large key holding 4 embedded RSA public keys in it.

To summarize, my requirements for my OpenPGP key setup in 2019 are:

  • Curve25519/Ed25519 algorithms.
  • Master key on USB stick.
  • USB stick only used on an offline computer.
  • Subkeys for daily use (signature, encryption and authentication).
  • Keys are generated in GnuPG software and imported to the smartcard.
  • Smartcard is open hardware and running free software.

Getting this setup up and running sadly requires quite some detailed work, which will be the topic of other posts… stay tuned!