This is a guide to using [YubiKey](https://www.yubico.com/products/yubikey-hardware/) 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, signing and authentication keys, which can also be used for SSH. Many of the principles in this document are applicable to other smart card devices.
Keys stored on YubiKey are [non-exportable](https://support.yubico.com/support/solutions/articles/15000010242-can-i-duplicate-or-back-up-a-yubikey-) (as opposed to file-based 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. All signing and encryption operations happen on the card, rather than in OS memory.
**Security Note**: If you followed this guide before Jan 2021, your PUK (Pin Unblock Key) may be set to its default value of `12345678`. An attacker can use this to reset your PIN and use your Yubikey. Please see the [Change PUK](#change-puk) section for details on how to change your PUK.
All YubiKeys except the blue "security key" model are compatible with this guide. NEO models are limited to 2048-bit RSA keys. Compare YubiKeys [here](https://www.yubico.com/products/yubikey-hardware/compare-products-series/). Yubico have also just released a press release and blog post about supporting resident ssh keys on their Yubikeys including blue "security key 5 NFC" with OpenSSH 8.2 or later, see [here](https://www.yubico.com/blog/github-now-supports-ssh-security-keys/) for details.
To verify a YubiKey is genuine, open a [browser with U2F support](https://support.yubico.com/support/solutions/articles/15000009591-how-to-confirm-your-yubico-device-is-genuine-with-u2f) to [https://www.yubico.com/genuine/](https://www.yubico.com/genuine/). Insert a Yubico device, and select *Verify Device* to begin the process. Touch the YubiKey when prompted, and if asked, allow it to see the make and model of the device. If you see *Verification complete*, the device is authentic.
This website verifies YubiKey device attestation certificates signed by a set of Yubico certificate authorities, and helps mitigate [supply chain attacks](https://media.defcon.org/DEF%20CON%2025/DEF%20CON%2025%20presentations/DEF%20CON%2025%20-%20r00killah-and-securelyfitz-Secure-Tokin-and-Doobiekeys.pdf).
To create cryptographic keys, a secure environment that can be reasonably assured to be free of adversarial control is recommended. Here is a general ranking of environments most to least likely to be compromised:
1. Dedicated air-gapped system with no networking capabilities
This guide recommends using a bootable "live" Debian Linux image to provide such an environment, however, depending on your threat model, you may want to take fewer or more steps to secure it.
Shut down the computer and disconnect internal hard drives and all unnecessary peripheral devices. If being run within a VM, this part can be skipped as no such devices should be attached to the VM since the image will still be run as a "live image".
On NixOS, ensure that you have `pinentry-program /run/current-system/sw/bin/pinentry-curses` in your `$GNUPGHOME/gpg-agent.conf` before running any `gpg` commands.
**Note** An additional Python package dependency may need to be installed to use [`ykman`](https://support.yubico.com/support/solutions/articles/15000012643-yubikey-manager-cli-ykman-user-guide) - `pip install yubikey-manager`
You may also need more recent versions of [yubikey-personalization](https://developers.yubico.com/yubikey-personalization/Releases/) and [yubico-c](https://developers.yubico.com/yubico-c/Releases/).
Most operating systems use software-based pseudorandom number generators. On newer machines there are CPU based hardware random number generators (HRNG) or you can use a separate hardware device like the White Noise or [OneRNG](https://onerng.info/onerng/) will [increase the speed](https://lwn.net/Articles/648550/) of entropy generation and possibly the quality.
From YubiKey firmware version 5.2.3 onwards - which introduces "Enhancements to OpenPGP 3.4 Support" - we can gather additional entropy from the YubiKey itself via the SmartCard interface.
## YubiKey
To feed the system's PRNG with entropy generated by the YubiKey itself, issue:
The first key to generate is the master key. It will be used for certification only: to issue sub-keys that are used for encryption, signing and authentication.
**Important** The master key should be kept offline at all times and only accessed to revoke or issue new sub-keys. Keys can also be generated on the YubiKey itself to ensure no other copies exist.
**Important** Save this credential in a permanent, secure place as it will be needed to issue new sub-keys after expiration, and to provision additional YubiKeys.
**Tip** On Linux or OpenBSD, select the password using the mouse or by double-clicking on it to copy to clipboard. Paste using the middle mouse button or `Shift`-`Insert`.
Export the key ID as a [variable](https://stackoverflow.com/questions/1158091/defining-a-variable-with-or-without-export/1158231#1158231) (`KEYID`) for use later:
Create a [signing key](https://stackoverflow.com/questions/5421107/can-rsa-be-both-used-as-encryption-and-signature/5432623#5432623) by selecting `addkey` then `(4) RSA (sign only)`:
GPG doesn't provide an authenticate-only key type, so select `(8) RSA (set your own capabilities)` and toggle the required capabilities until the only allowed action is `Authenticate`:
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". hokey may also indicate a problem (red text) with `Key expiration times: []` on the primary key (see [Note #3](#notes) about not setting an expiry for the primary key).
On Windows, note that using any extension other than `.gpg` or attempting IO redirection to a file will garble the secret key, making it impossible to import it again at a later date:
Although we will backup and store the master key in a safe place, it is best practice to never rule out the possibility of losing it or having the backup fail. Without the master key, it will be impossible to renew or rotate subkeys or generate a revocation certificate, the PGP identity will be useless.
Even worse, we cannot advertise this fact in any way to those that are using our keys. It is reasonable to assume this *will* occur at some point and the only remaining way to deprecate orphaned keys is a revocation certificate.
Once keys are moved to YubiKey, they cannot be moved again! Create an **encrypted** backup of the keyring and consider using a [paper copy](https://www.jabberwocky.com/software/paperkey/) of the keys as an additional backup measure.
**Tip** The ext2 filesystem (without encryption) can be mounted on both Linux and OpenBSD. Consider using a FAT32/NTFS filesystem for MacOS/Windows compatibility instead.
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.
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.
**Important** Without the *public* key, you will not be able to use GPG to encrypt, decrypt, nor sign messages. However, you will still be able to use YubiKey for SSH authentication.
Create another partition on the removable storage device to store the public key, or reconnect networking and upload to a key server.
After some time, the public key will 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).
Use the [YubiKey Manager](https://developers.yubico.com/yubikey-manager) application (note, this is not the similarly named older YubiKey NEO Manager) to enable CCID functionality.
The [GPG interface](https://developers.yubico.com/PGP/) is separate from other modules on a Yubikey such as the [PIV interface](https://developers.yubico.com/PIV/Introduction/YubiKey_and_PIV.html). The GPG interface has its own *PIN*, *Admin PIN*, and *Reset Code* - these should be changed from default values!
Values are valid up to 127 ASCII characters and must be at least 6 (*PIN*) or 8 (*Admin PIN*, *Reset Code*) characters. See the GnuPG documentation on [Managing PINs](https://www.gnupg.org/howtos/card-howto/en/ch03s02.html) for details.
The number of retry attempts can be changed with the following command, documented [here](https://docs.yubico.com/software/yubikey/tools/ykman/OpenPGP_Commands.html#ykman-openpgp-access-set-retries-options-pin-retries-reset-code-retries-admin-pin-retries):
**Important** Transferring keys to YubiKey using `keytocard` is a destructive, one-way operation only. Make sure you've made a backup before proceeding: `keytocard` converts the local, on-disk key into a stub, which means the on-disk copy is no longer usable to transfer to subsequent security key devices or mint additional keys.
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.
When you add a GPG key to a Yubikey using the *keytocard* command, GPG deletes the key from your keyring and adds a *stub* pointing to that exact Yubikey (the stub identifies the GPG KeyID and the Yubikey's serial number).
However, when you do this same operation for a second Yubikey, the stub in your keyring is overwritten by the *keytocard* operation and now the stub points to your second Yubikey. Adding more repeats this overwriting operation.
In other words, the stub will point ONLY to the LAST Yubikey written to.
When using GPG key operations with the GPG key you placed onto the Yubikeys, GPG will request a specific Yubikey asking that you insert a Yubikey with a given serial number (referenced by the stub). GPG will not recognise another Yubikey with a different serial number without manual intervention.
You can force GPG to scan the card and re-create the stubs to point to another Yubikey.
Having created two (or more Yubikeys) with the same GPG key (as described above) where the stubs are pointing to the second Yubikey:
Insert the first Yubikey (which has a different serial numnber) and run the following command:
PGP does not provide forward secrecy - a compromised key may be used to decrypt all past messages. Although keys stored on YubiKey are difficult to steal, it is not impossible - the key and PIN could be taken, or a vulnerability may be discovered in key hardware or the random number generator used to create them, for example. Therefore, it is good practice to occassionally rotate sub-keys.
When a sub-key expires, it can either be renewed or replaced. Both actions require access to the offline master key. Renewing sub-keys by updating their expiration date indicates you are still in possession of the offline master key and is more convenient.
Replacing keys, on the other hand, is less convenient but more secure: the new sub-keys will **not** be able to decrypt previous messages, authenticate with SSH, etc. Contacts will need to receive the updated public key and any encrypted secrets need to be decrypted and re-encrypted to new sub-keys to be usable. This process is functionally equivalent to "losing" the YubiKey and provisioning a new one. However, you will always be able to decrypt previous messages using the offline encrypted backup of the original keys.
Neither rotation method is superior and it's up to personal philosophy on identity management and individual threat model to decide which one to use, or whether to expire sub-keys at all. Ideally, sub-keys would be ephemeral: used only once for each encryption, signing and authentication event, however in practice that is not really feasible nor worthwhile with YubiKey. Advanced users may want to dedicate an offline device for more frequent key rotations and ease of provisioning.
To renew or rotate sub-keys, follow the same process as generating keys: boot to a secure environment, install required software and disconnect networking.
Connect the offline secret storage device with the master keys and identify the disk label:
Renewing sub-keys is simpler: you do not need to generate new keys, move keys to the YubiKey, or update any SSH public keys linked to the GPG key. All you need to do is to change the expiry time associated with the public key (which requires access to the master key you just loaded) and then to export that public key and import it on any computer where you wish to use the **GPG** (as distinct from the SSH) key.
To change the expiration date of all sub-keys, start by selecting all keys:
This will extend the validity of your GPG key and will allow you to use it for SSH authorization. Note that you do _not_ need to update the SSH public key located on remote servers.
Rotating keys is more a bit more involved. First, follow the original steps to generate each sub-key. Previous sub-keys may be kept or deleted from the identity.
Disconnect the storage device and follow the original steps to transfer new keys (4, 5 and 6) to the YubiKey, replacing existing ones. Reboot or securely erase the GPG temporary working directory.
Notations can be added to user ID(s) and can be used in conjunction with [Keyoxide](https://keyoxide.org) to create [OpenPGP identity proofs](https://keyoxide.org/guides/openpgp-proofs).
Adding notations requires access to the master key so we can follow the setup instructions taken from this [section](#setup-environment) of this guide.
Please note that there is no need to connect the Yubikey to the setup environment and that we do not need to generate new keys, move keys to the YubiKey, or update any SSH public keys linked to the GPG key.
After having completed the environment setup, it is possible to follow any of the guides listed under "Adding proofs" in the Keyoxide ["Guides"](https://keyoxide.org/guides/) page __up until the notation is saved using the `save` command__.
_Note that if you want to use a **YubiKey ONLY for SSH** (and don't really care about PGP/GPG), then [since OpenSSH v8.2](https://www.openssh.com/txt/release-8.2) you alternatively can simply `ssh-keygen -t ed25519-sk` (without requiring anything else from this guide!), as explained [e.g. in this guide](https://github.com/vorburger/vorburger.ch-Notes/blob/develop/security/ed25519-sk.md). Yubico also recently announced support for resident ssh keys under OpenSSH 8.2+ on their blue "security key 5 nfc" as mentioned in their [blog post](https://www.yubico.com/blog/github-now-supports-ssh-security-keys/)._
[gpg-agent](https://wiki.archlinux.org/index.php/GnuPG#SSH_agent) supports the OpenSSH ssh-agent protocol (`enable-ssh-support`), as well as Putty's Pageant on Windows (`enable-putty-support`). This means it can be used instead of the traditional ssh-agent / pageant. There are some differences from ssh-agent, notably that gpg-agent does not _cache_ keys rather it converts, encrypts and stores them - persistently - as GPG keys and then makes them available to ssh clients. Any existing ssh private keys that you'd like to keep in `gpg-agent` should be deleted after they've been imported to the GPG agent.
When importing the key to `gpg-agent`, you'll be prompted for a passphrase to protect that key within GPG's key store - you may want to use the same passphrase as the original's ssh version. GPG can both cache passphrases for a determined period (ref. `gpg-agent`'s various `cache-ttl` options), and since version 2.1 can store and fetch passphrases via the macOS keychain. Note than when removing the old private key after importing to `gpg-agent`, keep the `.pub` key file around for use in specifying ssh identities (e.g. `ssh -i /path/to/identity.pub`).
Probably the biggest thing missing from `gpg-agent`'s ssh agent support is being able to remove keys. `ssh-add -d/-D` have no effect. Instead, you need to use the `gpg-connect-agent` utility to lookup a key's keygrip, match that with the desired ssh key fingerprint (as an MD5) and then delete that keygrip. The [gnupg-users mailing list](https://lists.gnupg.org/pipermail/gnupg-users/2016-August/056499.html) has more information.
Create a hardened configuration for gpg-agent by downloading [drduh/config/gpg-agent.conf](https://github.com/drduh/config/blob/master/gpg-agent.conf):
**Important** The `cache-ttl` options do **NOT** apply when using a YubiKey as a smartcard as the PIN is [cached by the smartcard itself](https://dev.gnupg.org/T3362). Therefore, in order to clear the PIN from cache (smartcard equivalent to `default-cache-ttl` and `max-cache-ttl`), you need to unplug the YubiKey.
**Tip** Set `pinentry-program /usr/bin/pinentry-gnome3` for a GUI-based prompt. If the _pinentry_ graphical dialog doesn't show and you get this error: `sign_and_send_pubkey: signing failed: agent refused operation`, you may need to install the `dbus-user-session` package and restart the computer for the `dbus` user session to be fully inherited; this is because behind the scenes, `pinentry` complains about `No $DBUS_SESSION_BUS_ADDRESS found`, falls back to `curses` but doesn't find the expected `tty`.
On macOS, use `brew install pinentry-mac` and set the program path to `pinentry-program /usr/local/bin/pinentry-mac` or `pinentry-program /usr/local/MacGPG2/libexec/pinentry-mac.app/Contents/MacOS/pinentry-mac` if using MacGPG Suite.
On modern systems, `gpgconf --list-dirs agent-ssh-socket` will automatically set `SSH_AUTH_SOCK` to the correct value and is better than hard-coding to `run/user/$UID/gnupg/S.gpg-agent.ssh`, if available:
If you use fish, the correct lines for your `config.fish` would look like this (consider putting them into the `is-interactive` block depending on your use case):
```fish
set -x GPG_TTY (tty)
set -x SSH_AUTH_SOCK (gpgconf --list-dirs agent-ssh-socket)
Note that if you use `ForwardAgent` for ssh-agent forwarding, `SSH_AUTH_SOCK` only needs to be set on the *local* laptop (workstation), where the YubiKey is plugged in. On the *remote* server that we SSH into, `ssh` will automatically set `SSH_AUTH_SOCK` to something like `/tmp/ssh-mXzCzYT2Np/agent.7541` when we connect. We therefore do **NOT** manually set `SSH_AUTH_SOCK` on the server - doing so would break [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding).
If you use `S.gpg-agent.ssh` (see [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding) for more info), `SSH_AUTH_SOCK` should also be set on the *remote*. However, `GPG_TTY` should not be set on the *remote*, explanation specified in that section.
By default, SSH attempts to use all the identities available via the agent. It's often a good idea to manage exactly which keys SSH will use to connect to a server, for example to separate different roles or [to avoid being fingerprinted by untrusted ssh servers](https://blog.filippo.io/ssh-whoami-filippo-io/). To do this you'll need to use the command line argument `-i [identity_file]` or the `IdentityFile` and `IdentitiesOnly` options in `.ssh/config`.
The argument provided to `IdentityFile` is traditionally the path to the _private_ key file (for example `IdentityFile ~/.ssh/id_rsa`). For the YubiKey - indeed, in general for keys stored in an ssh agent - `IdentityFile` should point to the _public_ key file, `ssh` will select the appropriate private key from those available via the ssh agent. To prevent `ssh` from trying all keys in the agent use the `IdentitiesOnly yes` option along with one or more `-i` or `IdentityFile` options for the target host.
To reiterate, with `IdentitiesOnly yes`, `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.
**Tip** 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).
If there are existing SSH keys that you wish to make available via `gpg-agent`, you'll need to import them. You should then remove the original private keys. When importing the key, `gpg-agent` uses the key's filename as the key's label; this makes it easier to follow where the key originated from. In this example, we're starting with just the YubiKey's key in place and importing `~/.ssh/id_rsa`:
When invoking `ssh-add`, it will prompt for the SSH key's passphrase if present, then the `pinentry` program will prompt and confirm for a new passphrase to use to encrypt the converted key within the GPG key store.
When using the key `pinentry` will be invoked to request the key's passphrase. The passphrase will be cached for up to 10 minutes idle time between uses, to a maximum of 2 hours.
The latter one may be more insecure as raw socket is just forwarded (not like `S.gpg-agent.extra` with only limited functionality; if `ForwardAgent` implemented by OpenSSH is just forwarding the raw socket, then they are insecure to the same degree). But for the latter one, one convenience is that one may forward once and use this agent everywhere in the remote. So again, proceed with caution!
For example, `tmux` does not have some environment variables like `$SSH_AUTH_SOCK` when you ssh into remote and attach an old `tmux` session. In this case if you use `ForwardAgent`, you need to find the socket manually and `export SSH_AUTH_SOCK=/tmp/ssh-agent-xxx/xxxx.socket` for each shell. But with `S.gpg-agent.ssh` in fixed place, one can just use it as ssh-agent in their shell rc file.
You should now be able use `ssh -A remote` on the _local_ machine to log into _remote_, and should then be able to use YubiKey as if it were connected to the remote machine. For example, using e.g. `ssh-add -l` on that remote machine should show the public key from the YubiKey (note `cardno:`). (If you don't want to have to remember to use `ssh -A`, you can use `ForwardAgent yes` in `~/.ssh/config`. As a security best practice, always use `ForwardAgent yes` only for a single `Hostname`, never for all servers.)
First you need to go through [Remote Machines (GPG Agent Forwarding)](#remote-machines-gpg-agent-forwarding), know the conditions for gpg-agent forwarding and know the location of `S.gpg-agent.ssh` on both the local and the remote.
If you use `ssh-agent` provided by OpenSSH and want to forward it into a *third* box, you can just `ssh -A third` on the *remote*.
Meanwhile, if you use `S.gpg-agent.ssh`, assume you have gone through the steps above and have `S.gpg-agent.ssh` on the *remote*, and you would like to forward this agent into a *third* box, first you may need to configure `sshd_config` and `SSH_AUTH_SOCK` of *third* in the same way as *remote*, then in the ssh config of *remote*, add the following lines
You can use YubiKey to sign GitHub commits and tags. It can also be used for GitHub SSH authentication, allowing you to push, pull, and commit without a password.
**Note** If you encounter the error `gpg: signing failed: No secret key` - run `gpg --card-status` with YubiKey plugged in and try the git command again.
Windows can already have some virtual smartcard readers installed, like the one provided for Windows Hello. To ensure your YubiKey is the correct one used by scdaemon, you should add it to its configuration. You will need your device's full name. To find your device's full name, plug in your YubiKey and open PowerShell to run the following command:
The name slightly differs according to the model. Thanks to [Scott Hanselman](https://www.hanselman.com/blog/HowToSetupSignedGitCommitsWithAYubiKeyNEOAndGPGAndKeybaseOnWindows.aspx) for sharing this information.
* Retrieve the public key id: `> gpg --list-public-keys`
* Export the SSH key from GPG: `> gpg --export-ssh-key <public key id>`
Copy this key to a file for later use. It represents the public SSH key corresponding to the secret key on the YubiKey. You can upload this key to any server you wish to SSH into.
Create a shortcut that points to `gpg-connect-agent /bye` and place it in the startup folder `shell:startup` to make sure the agent starts after a system shutdown. Modify the shortcut properties so it starts in a "Minimized" window, to avoid unnecessary noise at startup.
Now you can use PuTTY for public key SSH authentication. When the server asks for public key verification, PuTTY will forward the request to GPG, which will prompt you for a PIN and authorize the login using YubiKey.
The goal here is to make the SSH client inside WSL work together with the Windows agent you are using (gpg-agent.exe in our case). Here is what we are going to achieve:
**Note** this works only for SSH agent forwarding. Real GPG forwarding (encryption/decryption) is actually not supported. See the [weasel-pageant](https://github.com/vuori/weasel-pageant) readme for further information.
One way to forward is just `ssh -A` (still need to eval weasel to setup local ssh-agent), and only relies on OpenSSH. In this track, `ForwardAgent` and `AllowAgentForwarding` in ssh/sshd config may be involved; However, if you use the other way (gpg ssh socket forwarding), you should not enable `ForwardAgent` in ssh config. See [SSH Agent Forwarding](#remote-machines-ssh-agent-forwarding) for more info.
Download or clone [weasel-pageant](https://github.com/vuori/weasel-pageant).
Add `eval $(/mnt/c/<path of extraction>/weasel-pageant -r -a /tmp/S.weasel-pageant)` to shell rc file. Use a named socket here so it can be used in the `RemoteForward` directive of `~/.ssh/config`. Source it with `source ~/.bashrc`.
Display the SSH key with `$ ssh-add -l`
Edit `~/.ssh/config` to add the following for each host you want to use agent forwarding:
**Note** Agent forwarding may be chained through multiple hosts - just follow the same [protocol](#remote-host-configuration) to configure each host. You may also read this part on [chained ssh agent forwarding](#chained-ssh-agent-forwarding).
To use YubiKey to sign a git commit on a remote host, or signing email/decrypt files on a remote host, configure and use GPG Agent Forwarding. To ssh through another network, especially to push to/pull from GitHub using ssh, see [Remote Machines (SSH Agent forwarding)](#remote-machines-ssh-agent-forwarding) for more info.
To do this, you need access to the remote machine and the YubiKey has to be set up on the host machine.
After gpg-agent forwarding, it is nearly the same as if YubiKey was inserted in the remote. Hence configurations except `gpg-agent.conf` for the remote can be the same as those for the local.
**Important** `gpg-agent.conf` for the remote is of no use, hence `$GPG_TTY` is of no use too for the remote. The mechanism is that after forwarding, remote `gpg` directly communicates with `S.gpg-agent` without *starting*`gpg-agent` on the remote.
On the remote machine, edit `/etc/ssh/sshd_config` to set `StreamLocalBindUnlink yes`
**Optional** If you do not have root access to the remote machine to edit `/etc/ssh/sshd_config`, you will need to remove the socket (located at `gpgconf --list-dir agent-socket`) on the remote machine before forwarding works. For example, `rm /run/user/1000/gnupg/S.gpg-agent`. Further information can be found on the [AgentForwarding GNUPG wiki page](https://wiki.gnupg.org/AgentForwarding).
Import public keys to the remote machine. This can be done by fetching from a keyserver. On the local machine, copy the public keyring to the remote machine:
On modern distributions, such as Fedora 30, there is typically no need to also set `RemoteForward` in `~/.ssh/config` as detailed in the next chapter, because the right thing happens automatically.
If any error happens (or there is no `gpg-agent.socket` in the remote) for modern distributions, you may go through the configuration steps in the next section.
## Steps for older distributions
On the local machine, run:
```console
$ gpgconf --list-dirs agent-extra-socket
```
This should return a path to agent-extra-socket - `/run/user/1000/gnupg/S.gpg-agent.extra` - though on older Linux distros (and macOS) it may be `/home/<user>/.gnupg/S/gpg-agent.extra`
Find the agent socket on the **remote** machine:
```console
$ gpgconf --list-dirs agent-socket
```
This should return a path such as `/run/user/1000/gnupg/S.gpg-agent`
Finally, enable agent forwarding for a given machine by adding the following to the local machine's ssh config file `~/.ssh/config` (your agent sockets may be different):
**Note** The pinentry program starts on *local* machine, not remote. Hence when there are needs to enter the pin you need to find the prompt on the local machine.
**Important** Any pinentry program except `pinentry-tty` or `pinentry-curses` may be used. This is because local `gpg-agent` may start headlessly (By systemd without `$GPG_TTY` set locally telling which tty it is on), thus failed to obtain the pin. Errors on the remote may be misleading saying that there is *IO Error*. (Yes, internally there is actually an *IO Error* since it happens when writing to/reading from tty while finding no tty to use, but for end users this is not friendly.)
Assume you have gone through the steps above and have `S.gpg-agent` on the *remote*, and you would like to forward this agent into a *third* box, first you may need to configure `sshd_config` of *third* in the same way as *remote*, then in the ssh config of *remote*, add the following lines:
Alternatively, use a script to delete the GnuPG shadowed key, where the card serial number is stored (see [GnuPG #T2291](https://dev.gnupg.org/T2291)):
See discussion in Issues [#19](https://github.com/drduh/YubiKey-Guide/issues/19) and [#112](https://github.com/drduh/YubiKey-Guide/issues/112) for more information and troubleshooting steps.
By default, YubiKey will perform encryption, signing and authentication operations without requiring any action from the user, after the key is plugged in and first unlocked with the PIN.
To require a touch for each key operation, install [YubiKey Manager](https://developers.yubico.com/yubikey-manager/) and recall the Admin PIN:
Depending on how the YubiKey is going to be used, you may want to look at the policy options for each of these and adjust the above commands accordingly. They can be viewed with the following command:
The touch policy is used to require user interaction for all operations using the private key on the YubiKey. The touch policy is set individually for each key slot. To see the current touch policy, run
YubiKey will blink when it is waiting for a touch. On Linux you can also use [yubikey-touch-detector](https://github.com/maximbaz/yubikey-touch-detector) to have an indicator or notification that YubiKey is waiting for a touch.
GPG keys on YubiKey can be used with ease to encrypt and/or sign emails and attachments using [Thunderbird](https://www.thunderbird.net/), [Enigmail](https://www.enigmail.net) and [Mutt](http://www.mutt.org/). Thunderbird supports OAuth 2 authentication and can be used with Gmail. See [this guide](https://ssd.eff.org/en/module/how-use-pgp-linux) from EFF for detailed instructions. Mutt has OAuth 2 support since version 2.0.
Finally, install the [Mailvelope extension](https://chrome.google.com/webstore/detail/mailvelope/kajibbejlbohfaggdiogboambcijhkke) from the Chrome app store.
Mutt has both CLI and TUI interfaces, and the latter provides powerful functions for daily email processing. In addition, PGP can be integrated such that signing/encryption/verifying/decryption can be done without leaving TUI.
To enable GnuPG support, one can just use the config file `gpg.rc` provided by mutt, usually located at `/usr/share/doc/mutt/samples/gpg.rc` after installation. One only needs to edit the file on options like `pgp_default_key`, `pgp_sign_as` and `pgp_autosign`. After editting one can `source` this rcfile in their main `muttrc` to use it.
**Important** If one uses `pinentry-tty` as one's pinentry program in `gpg-agent.conf`, it would mess with one's Mutt TUI, as reported. This is because Mutt TUI uses curses while tty output may harm the format. It is recommended to use `pinentry-curses` or other graphic pinentry program.
If PIN attempts are exceeded, the card is locked and must be [reset](https://developers.yubico.com/ykneo-openpgp/ResetApplet.html) and set up again using the encrypted backup.
1. 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 older YubiKey NEO Manager).
1. Programming YubiKey for GPG keys still lets you use its other configurations - [U2F](https://en.wikipedia.org/wiki/Universal_2nd_Factor), [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.
1. 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.
1. To switch between two or more identities on different keys - unplug the first key and restart gpg-agent, ssh-agent and pinentry with `pkill gpg-agent ; pkill ssh-agent ; pkill pinentry ; eval $(gpg-agent --daemon --enable-ssh-support)`, then plug in the other key and run `gpg-connect-agent updatestartuptty /bye` - then it should be ready for use.
1. To use yubikeys on more than one computer with gpg: After the initial setup, import the public keys on the second workstation. Confirm gpg can see the card via `gpg --card-status`, Trust the public keys you imported ultimately (as above). At this point `gpg --list-secret-keys` should show your (trusted) key.
- To get more information on potential errors, restart the `gpg-agent` process with debug output to the console with `pkill gpg-agent; gpg-agent --daemon --no-detach -v -v --debug-level advanced --homedir ~/.gnupg`.
- If you receive the error, `gpg: decryption failed: secret key not available` - you likely need to install GnuPG version 2.x. Another possibility is that there is a problem with the PIN, e.g. it is too short or blocked.
- 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 [Required software](#required-software).
- If you receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - 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` - [run the command](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=835394) `gpg-connect-agent updatestartuptty /bye`
- If you still receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - edit `~/.gnupg/gpg-agent.conf` to set a valid `pinentry` program path, e.g. `pinentry-program /usr/local/bin/pinentry-mac` on macOS.
- If you receive the error, `The agent has no identities` from `ssh-add -L`, make sure you have installed and started `scdaemon`.
- 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"`. Also see that `gpgconf --list-dirs agent-ssh-socket` is returning single path, to existing `S.gpg-agent.ssh` socket.
- 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.
- If it still fails, it may be useful to stop the background `sshd` daemon process service on the server (e.g. using `sudo systemctl stop sshd`) and instead start it in the foreground with extensive debugging output, using `/usr/sbin/sshd -eddd`. Note that the server will not fork and will only process one connection, therefore has to be re-started after every `ssh` test.
- If you receive the error, `There is no assurance this key belongs to the named user` or `encryption failed: Unusable public key` use `gpg --edit-key` to set `trust` to `5 = I trust ultimately`.
- If you receive the error, `gpg: 0x0000000000000000: skipped: Unusable public key` or `encryption failed: Unusable public key` the sub-key may be expired and can no longer be used to encrypt nor sign messages. It can still be used to decrypt and authenticate, however.
- Refer to Yubico article [Troubleshooting Issues with GPG](https://support.yubico.com/hc/en-us/articles/360013714479-Troubleshooting-Issues-with-GPG) for additional guidance.
