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@Kranzes
Last active September 17, 2024 17:42
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SSH Resident Key Guide

Initial checks

Start by checking that there aren't any previous ssh keys inside the FIDO2 authenticator of your YubiKey. You can check if they exist by running the command below:

nix shell nixpkgs#yubikey-manager -c ykman fido credentials list

If the command above outputs a string mentioning "ssh" or "openssh", then you have already got a key generated and store on your YubiKey.

Evaluating additional authentication factors

Before generating a new ssh key to store on your YubiKey you must consider which additional required authentication factors you want to use. Below you can see a table with the available factors and their corresponding command:

Factors Description Command
No PIN or touch are required You will not be required to enter your FIDO2 PIN or touch your YubiKey each time to authenticate ssh-keygen -t ed25519-sk -O resident -O no-touch-required
PIN but no touch required Entering the PIN will be required but touching the physical key will not ssh-keygen -t ed25519-sk -O resident -O verify-required -O no-touch-required
No PIN but touch is required You will only need to touch the YubiKey to authenticate ssh-keygen -t ed25519-sk -O resident
A PIN and a touch are required (most secure) This is the most secure option, it requires both the PIN and touching to be used ssh-keygen -t ed25519-sk -O resident -O verify-required

Generating the key

Once you've decided which option fits best for your threat model you will need to run one of the commands above. Note that if using a PIN you don't need to add an additional ssh passphrase as it's redundant due to the FIDO2 PIN being used instead. I personally went with the last and most secure option so the command I used to generate the key was:

ssh-keygen -t ed25519-sk -O resident -O verify-required

Adding the new keys

Now that you have generated a key which you can use, you will need to add it to your current ssh-agent session. You can do that by first starting the agent like so:

eval "$(ssh-agent -s)"

Then add the key on the YubiKey with the command below:

ssh-add -K

You can verify that the key was added by listing all the keys available in the current ssh-agent session:

ssh-add -l

We just added our brand new ssh key temporarily to our current session. If you would like to have it permanently available on the system you can run the command:

ssh-keygen -K

This retrieves our ssh key from our YubiKey and puts the private (still protected by YubiKey) and public key in the current working directory. You must now rename them accordingly to id_ed25519_sk and id_ed25519_sk.pub and place them in your ~/.ssh directory so ssh can detect them.

Authenticating with GitHub

In order to authenticate with GitHub you will have to add your new public key to your GitHub profile over at -> github.com/settings/keys. You can retrieve the keypair by running

ssh-keygen -K

and copy the public key directly from the newly added files to the current folder, for example, id_ed25519_sk_rk.pub.

Testing authentication

Now that we've added our ssh key to GitHub we can test that the setup works correctly by running:

ssh -T git@github.com

If this worked correctly you should be greeted by a "welcoming message".

NOTE: In order to make sure that you are using the new SSH key consider moving out existing keys from the ~/.ssh directory just for this test.

@8mccm8
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8mccm8 commented Feb 27, 2024

@nmcbride
export keys with:
ssh-keygen -K

@nyanpasu64
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Can you convert an existing id_ed25519_sk into a resident key?

@joostd
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joostd commented Jun 19, 2024

No, there is no API for importing wrapped credentials in CTAP (the protocol used to communicate with security keys). I guess it could be possible in theory by using a proprietary extension, but no security key I know has such an extension.

@johanot
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johanot commented Aug 19, 2024

Is there a way to have pin entry required only once per agent session? Currently, I'm prompted for PIN at ssh-add, but also at subsequent ssh ....

@Kranzes
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Kranzes commented Aug 19, 2024

No. It's not possible.

@Toomoch
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Toomoch commented Sep 16, 2024

Hi @Kranzes, have you encountered this issue https://bugzilla.mindrot.org/show_bug.cgi?id=3572?
Currently in NixOS with a Fido2 SSH key that requires a pin it doesn't work with askPassword = "".

@Kranzes
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Kranzes commented Sep 17, 2024

Hi @Kranzes, have you encountered this issue https://bugzilla.mindrot.org/show_bug.cgi?id=3572? Currently in NixOS with a Fido2 SSH key that requires a pin it doesn't work with askPassword = "".

Hah, the "personal" user in that bug report thread is me, if you open the attached screenshot you would see "kranzes".

@Toomoch
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Toomoch commented Sep 17, 2024

Hi @Kranzes, have you encountered this issue https://bugzilla.mindrot.org/show_bug.cgi?id=3572? Currently in NixOS with a Fido2 SSH key that requires a pin it doesn't work with askPassword = "".

Hah, the "personal" user in that bug report thread is me, if you open the attached screenshot you would see "kranzes".

Oh I see, I didn't notice. I guess the only way is to use a GUI, bummer.

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