Using two yubikeys not covered under guide

[wsargent]

So, this might require more feedback, as am currently flummoxed.

If you want to use a Yubikey for Windows / MacOS / Linux login as part of a multi-factor authentication, then Yubico advises having a second Yubikey as backup. This works fine, since I can use a Yubikey Nano for the Macbook Pro, and leave the desktop one plugged in.

However, if you try copying the GPG secret key and subkeys onto a second Yubikey, "keytocard" will not let you. You'll get "secret key already stored on a card" and other similar warnings. Is it not a good idea to use two Yubikeys?

[wsargent]

It seems like the best practice is "one GPG key per device". I don't know if they're allowed to be cross signed, but I presume so.

[wsargent]

Okay, I think this is slightly more complicated. There needs to be a single KEYID, but then there need to be different subkeys for each token. Then, if a token gets stolen, you can revoke the subkey and keep the root key safe.

So this means it's not sufficient to generate a revocation certificate for the root, as is done in

https://github.com/drduh/YubiKey-Guide#create-revocation-certificate

There ALSO needs to be a revocation certificate for each subkey set tied to each token.

[wsargent]

Except... apparently having two encryption subkeys is a good way to get GPG very, very confused. Oh dear.

[wsargent]

https://tinfoil.press/t/how-and-where-to-store-your-private-pgp-key/77/6

[wsargent]

One might be tempted to have one subkey per machine so that you only need to exchange the potentially compromised subkey of that machine. In case of a single subkey used on all machines, it needs to be exchanged on all machines in case of a compromising.

But this only works for signing subkeys. If you have multiple encryption subkeys, gpg is said to encrypt only for the most recent encryption subkey and not for all known and not revoked encryption subkeys.

https://wiki.debian.org/Subkeys

[wsargent]

https://security.stackexchange.com/questions/119259/pgp-gpg-do-i-need-more-encrypton-subkeys/119262#119262

[wsargent]

After you purposely delete the secret key stubs from your keyring (otherwise it will say the keys are already associated with another card), you can even put these same keys on a different smartcard by repeating part of the process above.

https://gist.github.com/ageis/5b095b50b9ae6b0aa9bf

[wsargent]

http://www.gossamer-threads.com/lists/gnupg/users/62550

[wsargent]

This seems to assume THREE smartcards...

https://github.com/flamsmark/documentation/blob/master/gpg/smartcard-keygen.md

[wsargent]

Consider purchasing a pair and programming both in case of loss or damage to one of them.

https://github.com/drduh/YubiKey-Guide#purchase-yubikey

[wsargent]

Please select where to store the key:
   (1) Signature key
   (3) Authentication key
Your selection? 3

gpg: WARNING: such a key has already been stored on the card!

Replace existing key? (y/N) y

gpg: secret key is already stored on a card

https://lists.gnupg.org/pipermail/gnupg-users/2010-October/039592.html

So the way I've worked around this is to just delete the entire .gnupg directory and reimport from master for each key, but this seems suboptimal.

[wsargent]

I assume, although keytocard is documented as moving the key to the card, it actually copies it.

It copies, but if you then save the changes to your local disk, the original copy on local disk is deleted - so calling it a "move" operation is correct. If you want to keep a backup copy on local disk, you need to quit without saving immediately after running 'keytocard'. This behaviour is a well-known gotcha.

http://www.gossamer-threads.com/lists/gnupg/users/76764#76764

[wsargent]

But because I've already sharded my key with a previous NEO (still in my possession, no need to generate new keys), I get this response when executing the gpg> keytocard command:

gpg: secret key already stored on a card.

So now what? Do I delete the secring.gpg file on my hard drive and re-import the keys? Do I do that every time I configure a new NEO with my PGP keys?

In short, you don't need to delete whole keyring, just the key that is marked as exported to smartcard (Neo). You first need to delete the secret key from keyring with gpg --delete-secret-key. Then you import the full key (how it was before you moved it to smartcard).

Then you use the classic keytocard, etc.

http://forum.yubico.com/viewtopic.php?f=26&t=2095