This is a guide to using [YubiKey](https://www.yubico.com/faq/yubikey/) as a [SmartCard](https://security.stackexchange.com/questions/38924/how-does-storing-gpg-ssh-private-keys-on-smart-cards-compare-to-plain-usb-drives) for storing GPG encryption and signing keys.
An authentication key can also be created for SSH and used with [gpg-agent](https://unix.stackexchange.com/questions/188668/how-does-gpg-agent-work/188813#188813).
Keys stored on a smartcard like YubiKey are non-exportable (as opposed to keys that are stored on disk) and are convenient for everyday use. Instead of having to remember and enter passphrases to unlock SSH/GPG keys, YubiKey needs only a physical touch after being unlocked with a PIN code - and all signing and encryption operations happen on the card, rather than in OS memory.
These instructions are current to Debian 9 using YubiKey 4 - with support for **4096 bit** RSA keys - in OTP+CCID mode, using GPG version 2.2. Note, older YubiKeys like the Neo are [limited](https://www.yubico.com/products/yubikey-hardware/compare-yubikeys/) to **2048 bit** RSA keys. Debian live install images are available from [here](https://www.debian.org/CD/live/) and are suitable for writing to USB drives.
Programming YubiKey for GPG keys still lets you use its two slots - [OTP](https://www.yubico.com/faq/what-is-a-one-time-password-otp/) and [static password](https://www.yubico.com/products/services-software/personalization-tools/static-password/) modes, for example.
You may also need to download and install more recent versions of [yubikey-personalization](https://developers.yubico.com/yubikey-personalization/Releases/) and [yubico-c](https://developers.yubico.com/yubico-c/Releases/):
If on [Tails](https://tails.boum.org/), you also need to install `libykpers-1-1` from the testing repository. This is a temporary fix suggested on a [securedrop issue](https://github.com/freedomofpress/securedrop/issues/1035):
Paste the following [text](https://stackoverflow.com/questions/2500436/how-does-cat-eof-work-in-bash) into a terminal window to create a [recommended](https://github.com/drduh/config/blob/master/gpg.conf) GPG configuration:
> A note on security: for optimal security you should consider performing these actions on a bootable USB that you securely erase after completing the guide. Alternatively you should disable network connectivity on your computer and make sure you securely delete all secret keys and revocation certificates.
> A note on key expiry: setting an expiry essentially forces you to manage your subkeys and announces to the rest of the world that you are doing so. Setting an expiry on a primary key is ineffective for protecting the key from loss - whoever has the primary key can simply extend its expiry period. Revocation certificates are [better suited](https://security.stackexchange.com/questions/14718/does-openpgp-key-expiration-add-to-security/79386#79386) for this purpose. It may be appropriate for your use case to set expiry dates on subkeys.
Export the key ID as a [variable](https://stackoverflow.com/questions/1158091/defining-a-variable-with-or-without-export/1158231#1158231) for use throughout:
Note: If using a Yubikey 4, please use **4096 bit** as the size for the subkeys; if using a YubiKey Neo, please use **2048 bit** as the size for the subkeys.
GPG doesn't provide a 'RSA (authenticate only)' key type out of the box, so select 'RSA (set your own capabilities)' and toggle the required capabilities to end up with an Authenticate-only key:
If you're on Linux or macOS, use the automated [key best practice checker](https://riseup.net/en/security/message-security/openpgp/best-practices#openpgp-key-checks):
The output will display any problems with your key in red text. If everything is green, your key passes each of the tests. If it is red, your key has failed one of the tests.
>hokey may warn (orange text) about cross certification for the authentication key. GPG's [Signing Subkey Cross-Certification](https://gnupg.org/faq/subkey-cross-certify.html) documentation has more detail on cross certification, and gpg v2.2.1 notes "subkey <keyid> does not sign and so does not need to be cross-certified".
Please note that using any extension other than .gpg or attempting IO redirection to a file will garble your secret key, making it impossible to import it again at a later date.
The exported (primary) key will still have the passphrase in place.
In addition to the back up detailed in the next step, you should note the location of your revocation certificate from the terminal output and copy it to a secure location. Careful, anyone that has this certificate can revoke your key!
Once keys are moved to hardware, they cannot be extracted again (otherwise, what would be the point?), so make sure you have made an *encrypted* backup before proceeding.
Keep the backup mounted if you plan on setting up two or more keys (as `keytocard` will [delete](https://lists.gnupg.org/pipermail/gnupg-users/2016-July/056353.html) the local copy on save), otherwise unmount and disconnected the encrypted USB drive:
I recommend creating an encrypted flash drive or container using [VeraCrypt](https://www.veracrypt.fr/en/Downloads.html). Store your encrypted container on multiple flash drives/hard drives. You should also consider making a [paper copy](http://www.jabberwocky.com/software/paperkey/) of your keys.
YubiKey NEOs shipped after November 2015 have [all modes enabled](https://www.yubico.com/support/knowledge-base/categories/articles/yubikey-neo-manager/), skip to the next step.
Older versions of the YubiKey NEO may need to be reconfigured as a composite USB device (HID + CCID) which allows OTPs to be emitted while in use as a smart card.
> The -m option is the mode command. To see the different modes, enter `ykpersonalize –help`. Mode 82 (in hex) enables the YubiKey NEO as a composite USB device (HID + CCID). Once you have changed the mode, you need to re-boot the YubiKey – so remove and re-insert it.
The default PIN codes are `12345678` for the Admin PIN (aka PUK) and `123456` for the PIN. The CCID-mode PINs can be up to 127 ASCII characters long.
The Admin PIN is required for some card operations, and to unblock a PIN that has been entered incorrectly more than three times. See the GnuPG documentation on [Managing PINs](https://www.gnupg.org/howtos/card-howto/en/ch03s02.html) for details.
Transferring keys to YubiKey hardware using `keytocard` is a one-way operation only, so make sure you've made a backup before proceeding.
Previous gpg versions required the `toggle` command before selecting keys. The currently selected key(s) are indicated with an `*`. When moving keys only one key should be selected at a time.
Optionally, it may be uploaded to a [public keyserver](https://debian-administration.org/article/451/Submitting_your_GPG_key_to_a_keyserver):
$ gpg --send-key $KEYID
gpg: sending key 0xFF3E7D88647EBCDB to hkps server hkps.pool.sks-keyservers.net
[...]
After a little while, it ought to propagate to [other](https://pgp.key-server.io/pks/lookup?search=doc%40duh.to&fingerprint=on&op=vindex) [servers](https://pgp.mit.edu/pks/lookup?search=doc%40duh.to&op=index).
Make sure you backup up your key prior to doing this as the action is irreversible. You may also want to consider securely deleting the revocation certificate from your hard drive.
**Linux/macOS:** You may get an error `gpgkeys: HTTP fetch error 1: unsupported protocol` -- this means you need to install a special version of curl which supports gnupg:
Paste the following text into a terminal window to create a [recommended](https://github.com/drduh/config/blob/master/gpg-agent.conf) GPG agent configuration:
If you are using Linux on the desktop, you may want to use `/usr/bin/pinentry-gnome3` to use a GUI manager. For macOS, try `brew install pinentry-mac`, and adjust the `pinentry-program` setting to suit.
[gpg-agent](https://wiki.archlinux.org/index.php/GnuPG#SSH_agent) provides OpenSSH agent emulation. To launch the agent for use by ssh use the `gpg-connect-agent /bye` or `gpgconf --launch gpg-agent` commands.
Depending on how your environment is set up, you might need to add these to your shell `rc` file:
There is a `-L` option of `ssh-add` that lists public key parameters of all identities currently represented by the agent. Copy and paste the following output to the server authorized_keys file:
#### (Optional) Save public key for identity file configuration
If `IdentitiesOnly yes` is used in your `.ssh/config` (for example [to avoid being fingerprinted by untrusted ssh servers](https://blog.filippo.io/ssh-whoami-filippo-io/)), `ssh` will not automatically enumerate public keys loaded into `ssh-agent` or `gpg-agent`. This means `publickey` authentication will not proceed unless explicitly named by `ssh -i [identity_file]` or in `.ssh/config` on a per-host basis.
In the case of Yubikey usage, you do not have access to the private key, and `identity_file` can be pointed to the public key (`.pub`).
**Note** To make multiple connections or securely transfer many files, consider using the [ControlMaster](https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing) ssh option. Also see [drduh/config/ssh_config](https://github.com/drduh/config/blob/master/ssh_config).
Copy this key to a file and keep it for later use. It represents the public SSH key corresponding to the secret key on your YubiKey. You can upload this key to any server you wish to SSH into.
To authenticate SSH sessions via our YubiKey we need to enable Gpg4Win's PuTTY integration. Create a file named `gpg-agent.conf` and place it in the directory `C:\%APPDATA%\gnupg`.
The file should contain the line `enable-putty-support`.
Then, open a terminal and run the following commands:
> gpg-connect-agent killagent /bye
> gpg-connect-agent /bye
Create a shortcut that points to `gpg-connect-agent /bye` and place it in your startup folder to make sure the agent starts after a system shutdown.
Now you can use PuTTY for public key SSH authentication. When the server asks for publickey verification, PuTTY will foward the request to GPG, whcih will prompt you for your PIN and authorize the login using your YubiKey.
### 4.6a GitHub
You can use your YubiKey to sign GitHub commits and tags. It can also be used for GitHub SSH authentication, allowing you to push, pull, and commit without your GitHub password.
Log into GitHub and upload your SSH and PGP public keys.
`git@github.com:USERNAME/repository`. Any authenticated commands will be authorized by your YubiKey.
**Note:** If you encounter the error `gpg: signing failed: No secret key`, run `gpg --card-status` with your YubiKey plugged in and try the git command again.
## 4.7 Requiring touch to authenticate
Note: this is only possible on the Yubikey 4 line.
By default the Yubikey will perform key operations without requiring a touch from the user. To require a touch for every SSH connection, use the [Yubikey Manager](https://developers.yubico.com/yubikey-manager/) (you'll need the Admin PIN):
ykman openpgp touch aut on
To require a touch for the signing and encrypting keys as well:
ykman openpgp touch sig on
ykman openpgp touch enc on
The Yubikey will blink when it's waiting for the touch.
- If you don't understand some option, read `man gpg`.
- If you encounter problems connecting to YubiKey with GPG, simply try unplugging and re-inserting your YubiKey, and restarting the `gpg-agent` process.
- If you receive the error, `gpg: decryption failed: secret key not available` - you likely need to install GnuPG version 2.x.
- If you receive the error, `Yubikey core error: no yubikey present` - make sure the YubiKey is inserted correctly. It should blink once when plugged in.
- If you still receive the error, `Yubikey core error: no yubikey present` - you likely need to install newer versions of yubikey-personalize as outlined in [Install required software](#install-required-software).
- If you receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - you probably have ssh-agent running. Make sure you replaced ssh-agent with gpg-agent as noted above.
- If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - On Debian, [try](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=835394) `gpg-connect-agent updatestartuptty /bye`
- If you receive the error, `Error connecting to agent: No such file or directory` from `ssh-add -L`, the UNIX file socket that the agent uses for communication with other processes may not be set up correctly. On Debian, try `export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"`
- If you receive the error, `Permission denied (publickey)`, increase ssh verbosity with the `-v` flag and ensure the public key from the card is being offered: `Offering public key: RSA SHA256:abcdefg... cardno:00060123456`. If it is, ensure you are connecting as the right user on the target system, rather than as the user on the local system. Otherwise, be sure `IdentitiesOnly` is not [enabled](https://github.com/FiloSottile/whosthere#how-do-i-stop-it) for this host.
The Yubikey has two configurations, one invoked with a short press, and the other with a long press. By default the short-press mode is configured for HID OTP - a brief touch will emit an OTP string starting with `cccccccc`. If you rarely use the OTP mode, you can swap it to the second configuration via the Yubikey Personalization tool. If you *never* use OTP, you can disable it entirely using the [Yubikey Manager](https://developers.yubico.com/yubikey-manager) application (note, this not the similarly named Yubikey NEO Manager).