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

Posted on 2019-03-21 by simon — 9 Comments ↓

Below I describe how to generate an OpenPGP key and import its subkeys 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!

9 Replies to “Offline Ed25519 OpenPGP key with subkeys on FST-01G running Gnuk”

Brian on 2019-03-22 at 22:12 said:

Hi,
Isn’t it better to create the master key with only certify capability [C] and not certify+sign [SC] ?
See https://github.com/lfit/itpol/blob/master/kernel-developer-pgp-guide.md#understanding-the-master-certify-key
Thanks for yours gpg related guides.

Reply ↓
- simon on 2019-03-23 at 09:19 said:
  
  Hi Brian! Thanks for the link, that’s a good walk-through!
  
  I have thought about whether to include the S bit on the master key, and my reason for doing it is purely to be able to sign statements (such as the transition statement) using my offline key. The subkeys are more likely to be compromised, and I wouldn’t want statements signed by them to be read as a key transition statement for the master key. This is a delicate point, though, and I don’t think most people reading a key transition statement thinks about this aspect and make sure it is signed by the master key rather than a subkey.
  
  So this is my reason for including the S bit on the master key. What other reasons are there for including the S bit? What reasons are there for NOT including the S bit? I’m curious, maybe it is possible to establish a best practice based on rational arguments. Or we have the situation where this depends on the use-case for the key, and the preferences of the key owner. More discussion would be useful though 🙂
  
  /Simon
  
  Reply ↓
Alexey Kopytko on 2019-04-08 at 05:20 said:

For the record, with a more-less recent GPG version you can switch capability bits on and off as you see fit, even if they’re on a key.

https://security.stackexchange.com/questions/31614/how-to-change-subkey-usage-of-a-pgp-key/206766#206766

Reply ↓
Pingback: OpenPGP smartcard under GNOME on Debian 10 Buster – Simon Josefsson's blog
Savic Miram on 2020-03-31 at 03:50 said:

Hi.

Thanks for this post.

Why the Encryption subkey uses CV25519 and, Authentication/Sign/Certify uses ED25519?

Thanks.

Reply ↓
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