On Samstag, 12. Dezember 2020 16:29:55 CET Nicolas Boullis wrote:
> Since the smartcard that held all my subkeys died, I have to replace my
> subkeys, and I’m willing to store them on several smartcards, just in
> case I am unlucky again…
>
> I wonder whether I should the same subkey or different subkeys on
> different smartcards.
I see little reason for setting up a second smartcard for subkeys as disaster
recovery backup for the first smartcard. Reasoning below:
> As far as I understand it, for encryption, if I have several encryption
> subkeys, people who send me encrypted messages will encrypt for single
> subkey.
Yes, unless they explicitly specify multiple subkeys gpg will use the most
recently created valid encryption subkey.
> Hence, if I want to be able to decrypt the message with any
> smartcard, then I have to use a single subkey that is held by all
> smartcards.
Yes, but why a second smartcard? The reason for making a backup of the
encryption (sub)key is that this will allow you to decrypt messages even if
the smartcard dies. But if your second smartcard also dies, then all is lost
... unless you have an off-card backup of the private encryption key. But this
off-card backup would allow you to create a new smartcard _after_ the first
smartcard dies. So, instead of setting up two smartcards with identical
encryption keys I suggest to save the money for the second smartcard and
instead create an off-card backup of the private encryption key that you store
somewhere safe (see Erich's mail).
> As for signature subkeys, as I understand it, there is no problem with
> using several distinct subkeys, so I can sign with the one that is
> available, and people who verify the signature will accept any subkey.
> Moreover, if a smartcard is lost/stolen, I can revoke its signature
> subkey.
Yes, but what purpose serves the second smartcard? Again, you can easily setup
a replacement smartcard _after_ the first smartcard dies. Typically, you would
create the signature (sub)key on-card to avoid any risk of compromise of the
private key.
> As for the authentication subkeys (that I use for SSH connection), it
> behaves like the signature subkeys, except that I have to explicitly
> allow each subkey on all machine that I want to connect to.
Yes, for the authentication use case setting up a second smartcard does make
sense because otherwise you'd lose access to the machines if the first
smartcard dies. But you could as well keep an off-card backup of the
authentication key. Or you could create a second "master" authentication key
off-card and store it somewhere safe. One argument for using a second
smartcard instead is to avoid any risk of compromise of the private
authentication keys.
> As a bonus question: given that my “master” private key is also stored
> on a smartcard, is there a way to ask GnuPG to generate a signature
> subkey on a second smartcard, while signing it with the first smartcard?
Yes, but only with the unreleased development version of GnuPG. With 2.2.25
trying to add a subkey from an existing key from card failed here. There have
been quite some fixes with regard to smartcard support in the development
version in the last few weeks.
What I did:
Remove the first smartcard. Insert the second smartcard. Then
$ gpg --edit-key mas...@example.net
gpg (GnuPG) 2.3.0-beta1490; Copyright (C) 2020 Free Software Foundation, Inc.
[...]
sec ed25519/B16F599516474ABA
created: 2020-08-03 expires: 2022-08-03 usage: SC
card-no: [s/n of first card]
trust: ultimate validity: ultimate
sub cv25519/2BBE9540CAF56DC9
created: 2020-08-03 expires: never usage: E
[ultimate] (1). mas...@example.net
gpg> addkey
Secret parts of primary key are stored on-card.
Please select what kind of key you want:
(3) DSA (sign only)
(4) RSA (sign only)
(5) Elgamal (encrypt only)
(6) RSA (encrypt only)
(10) ECC (sign only)
(12) ECC (encrypt only)
(14) Existing key from card
Your selection? 14
Serial number of the card: D27600012401000609074582
Available keys:
(1) 930509C5F5BD42B2ABCB4C7BDC2FD64BE59086CE OPENPGP.1 rsa2048 (cert,sign*)
(2) AEA62514505EBDDB4C2FF8AF27B02F221ECAFBCE OPENPGP.2 rsa2048 (encr*)
(3) 56EBCBEEA72DBF1BFAFE583F7FF36CBCB895C265 OPENPGP.3 rsa2048 (sign,auth*)
Your selection? 1
Please specify how long the key should be valid.
0 = key does not expire
= key expires in n days
w = key expires in n weeks
m = key expires in n months
y = key expires in n years
Key is valid for? (0)
Key does not expire at all
Is this correct? (y/N) y
Really create? (y/N) y
-> "Please insert the card with serial number: [s/n of first card]"
-> "Please unlock the card" [s/n of first card]
-> "Please unlock the card" [s/n of second card]
sec ed25519/B16F599516474ABA
created: 2020-08-03 expires: 2022-08-03 usage: SC
card-no: [s/n of first card]
trust: ultimate validity: ultimate
