Using Gnuk Token with another computer

This document describes how you can use Gnuk Token on another PC (which is not the one you generate your keys).

Note that the Token only brings your secret keys, while .gnupg directory contains keyrings and trustdb, too.

Fetch the public key and connect it to the Token

In order to use the Token, we need to put the public key and the secret key references (to the token) under .gnupg directory.

When I invoke GnuPG with --card-status option.

Reader ...........: 234B:0000:FSIJ-1.2.0-87193059:0
Application ID ...: D276000124010200FFFE871930590000
Version ..........: 2.0
Manufacturer .....: unmanaged S/N range
Serial number ....: 87193059
Name of cardholder: [not set]
Language prefs ...: [not set]
Salutation .......:
URL of public key : [not set]
Login data .......: gniibe
Signature PIN ....: not forced
Key attributes ...: ed25519 cv25519 ed25519
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 0
KDF setting ......: off
UIF setting ......: Sign=off Decrypt=off Auth=off
Signature key ....: 249C B377 1750 745D 5CDD  323C E267 B052 364F 028D
      created ....: 2015-08-12 07:10:48
Encryption key....: E228 AB42 0F73 3B1D 712D  E50C 850A F040 D619 F240
      created ....: 2015-08-12 07:10:48
Authentication key: E63F 31E6 F203 20B5 D796  D266 5F91 0521 FAA8 05B1
      created ....: 2015-08-12 07:16:14
General key info..: [none]

Here, the secret key references (to the token) are created under .gnupg/private-keys-v1.d directory. It can be also created when I do --card-status by GnuPG.

Still, it says that there is no key info related to this token on my PC ([none] for General key info), because I don’t have the public key on this PC yet.

Because I have WKD setup for my email of FSIJ, I fetch the public key by WKD using gpg –locate-key command.

$ gpg --locate-key gniibe@fsij.org
gpg: key E267B052364F028D: public key "NIIBE Yutaka <gniibe@fsij.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1
pub   ed25519 2015-08-12 [SC]
      249CB3771750745D5CDD323CE267B052364F028D
uid           [ unknown] NIIBE Yutaka <gniibe@fsij.org>
sub   cv25519 2015-08-12 [E]
sub   ed25519 2015-08-12 [A]

Good. The public key is now under .gnupg directory. We can examine by gpg --list-keys.

When I do gpg --card-status, now, I can see:

General key info..: pub  ed25519/E267B052364F028D 2015-08-12 NIIBE Yutaka <gniibe@fsij.org>
sec>  ed25519/E267B052364F028D  created: 2015-08-12  expires: never
                                card-no: FFFE 87193059
ssb>  cv25519/850AF040D619F240  created: 2015-08-12  expires: never
                                card-no: FFFE 87193059
ssb>  ed25519/5F910521FAA805B1  created: 2015-08-12  expires: never
                                card-no: FFFE 87193059

Note that, it displays the information about “General key info”.

OK, now I can use the Token on this computer.

Update trustdb for the key on Gnuk Token

Yes, I can use the Token by the public key and the secret key references to the card. More, I need to update the trustdb.

To do that, I do:

$ ./gpg --edit-key E267B052364F028D
gpg (GnuPG) 2.4.5; Copyright (C) 2024 g10 Code GmbH
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

sec  ed25519/E267B052364F028D
     created: 2015-08-12  expires: never       usage: SC
     card-no: FFFE 87193059
     trust: unknown       validity: unknown
ssb  cv25519/850AF040D619F240
     created: 2015-08-12  expires: never       usage: E
     card-no: FFFE 87193059
ssb  ed25519/5F910521FAA805B1
     created: 2015-08-12  expires: never       usage: A
     card-no: FFFE 87193059
[ unknown] (1). NIIBE Yutaka <gniibe@fsij.org>
[ unknown] (2)  NIIBE Yutaka <gniibe@debian.org>

See, the key is unknown state. Add trust for that, because it’s the key under my control.

gpg> trust
sec  ed25519/E267B052364F028D
     created: 2015-08-12  expires: never       usage: SC
     card-no: FFFE 87193059
     trust: unknown       validity: unknown
ssb  cv25519/850AF040D619F240
     created: 2015-08-12  expires: never       usage: E
     card-no: FFFE 87193059
ssb  ed25519/5F910521FAA805B1
     created: 2015-08-12  expires: never       usage: A
     card-no: FFFE 87193059
[ unknown] (1). NIIBE Yutaka <gniibe@fsij.org>
[ unknown] (2)  NIIBE Yutaka <gniibe@debian.org>

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

sec  ed25519/E267B052364F028D
     created: 2015-08-12  expires: never       usage: SC
     card-no: FFFE 87193059
     trust: ultimate      validity: unknown
ssb  cv25519/850AF040D619F240
     created: 2015-08-12  expires: never       usage: E
     card-no: FFFE 87193059
ssb  ed25519/5F910521FAA805B1
     created: 2015-08-12  expires: never       usage: A
     card-no: FFFE 87193059
[ unknown] (1). NIIBE Yutaka <gniibe@fsij.org>
[ unknown] (2)  NIIBE Yutaka <gniibe@debian.org>
Please note that the shown key validity is not necessarily correct
unless you restart the program.

gpg>

And I quit from gpg. Then, when I invoke GnuPG, it will be ultimate key. Let’s see:

$ ./gpg --edit-key E267B052364F028D
gpg (GnuPG) 2.4.5; Copyright (C) 2024 g10 Code GmbH
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

gpg: checking the trustdb
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   7  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 7u
sec  ed25519/E267B052364F028D
     created: 2015-08-12  expires: never       usage: SC
     card-no: FFFE 87193059
     trust: ultimate      validity: ultimate
ssb  cv25519/850AF040D619F240
     created: 2015-08-12  expires: never       usage: E
     card-no: FFFE 87193059
ssb  ed25519/5F910521FAA805B1
     created: 2015-08-12  expires: never       usage: A
     card-no: FFFE 87193059
[ultimate] (1). NIIBE Yutaka <gniibe@fsij.org>
[ultimate] (2)  NIIBE Yutaka <gniibe@debian.org>

gpg> quit
$

OK, all set. I’m ready to use my Gnuk Token on this PC.