Generating keys on your host PC

In this section, we describe how to generate ECC keys.

Generating ECC keys on host PC

Here is an example session log to create newer ECC keys.

Firstly, we invoke gpg frontend with --quick-gen-key option, specifying a user ID.

$ gpg --quick-gen-key "Kunisada Chuji <chuji@gniibe.org>" default

It askes passphrase for this key on host PC (usually, by pop-up window). Note that this is a passphrase for the key on host PC. It is different thing to the passphrase of Gnuk Token. We enter two same inputs two times (once for passphrase input, and another for confirmation), <PASSWORD-KEY-ON-PC>.

Then, GnuPG generates keys (one primary key and a subkey). It emits message about random bytes, but it soon finishes.

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.
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: revocation certificate stored as '/tmp/tmp.riPmBwzyyp/openpgp-revocs.d/7AE3BF51B0CBF2F67E98C82038AC1343DBBEA961.rev'
public and secret key created and signed.

pub   ed25519 2024-09-03 [SC] [expires: 2027-09-03]
      7AE3BF51B0CBF2F67E98C82038AC1343DBBEA961
uid                      Kunisada Chuji <chuji@gniibe.org>
sub   cv25519 2024-09-03 [E]

We have the primary key with ed25519, and encryption subkey with cv25519.

Next, we add an authentication subkey which can be used with OpenSSH. As the use case is not that common, we need --expert option for GnuPG. :: We invoke gpg frontend with --edit-key, specifying the key ID.

$ gpg --expert --edit-key 7AE3BF51B0CBF2F67E98C82038AC1343DBBEA961
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  ed25519/38AC1343DBBEA961
     created: 2024-09-03  expires: 2027-09-03  usage: SC
     trust: ultimate      validity: ultimate
ssb  cv25519/52CBE6AC87EAD054
     created: 2024-09-03  expires: never       usage: E
[ultimate] (1). Kunisada Chuji <chuji@gniibe.org>

Here, it displays that there are main key and a subkey. It prompts sub-command with gpg>. We invoke addkey subcommand.

gpg> addkey

It asks a kind of key, we input 11 to select ECC for authentication.

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? 11

and then, we specify “Authenticate” capability.

Possible actions for a ECC key: Sign Authenticate
Current allowed actions: Sign

   (S) Toggle the sign capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? a

Possible actions for a ECC key: Sign Authenticate
Current allowed actions: Sign Authenticate

   (S) Toggle the sign capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? s

Possible actions for a ECC key: Sign Authenticate
Current allowed actions: Authenticate

   (S) Toggle the sign capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? q

Then, it asks which curve. We input 1 for “Curve25519”.

Please select which elliptic curve you want:
   (1) Curve 25519
   (2) Curve 448
   (3) NIST P-256
   (4) NIST P-384
   (5) NIST P-521
   (6) Brainpool P-256
   (7) Brainpool P-384
   (8) Brainpool P-512
   (9) secp256k1
Your selection? 1

It asks expiration of the key.

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)
Key does not expire at all
Is this correct? (y/N) y

And the confirmation.

Really create? (y/N) y

It goes.

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.

Then, it asks the passphrase, it is the passphrase of key on host PC. It’s the one we entered above as <PASSWORD-KEY-ON-PC>. And the subkey is added.

sec  ed25519/38AC1343DBBEA961
     created: 2024-09-03  expires: 2027-09-03  usage: SC
     trust: ultimate      validity: ultimate
ssb  cv25519/52CBE6AC87EAD054
     created: 2024-09-03  expires: never       usage: E
ssb  ed25519/AC87B245DFC7AEAC
     created: 2024-09-03  expires: never       usage: A
[ultimate] (1). Kunisada Chuji <chuji@gniibe.org>

We type save to exit form gpg.

gpg> save
$

Backup the private key

There are some ways to back up private key, such that backup .gnupg directory entirely, or use of paperkey, etc. Here, we describe backup by ASCII file. ASCII file is good, because it has less risk on transfer. Binary file has a risk to be modified on transfer.

Note that the key on host PC is protected by a passphrase (which is <PASSWORD-KEY-ON-PC> in the example above). Using the key from the backup needs this passphrase. It is common that people will forget passphrase for backup. Never forget it. You have been warned.

To make ASCII backup for private key, invokde GnuPG with --armor option and --export-secret-keys specifying the key identifier.

$ gpg --armor --output <YOUR-SECRET>.asc --export-secret-keys <YOUR-KEY-ID>

From the backup, we can recover privet key by invoking GnuPG with --import option.

$ gpg --import <YOUR-SECRET>.asc

Note that both command invocations ask you the passphrase.