This is a guide to using [YubiKey](https://www.yubico.com/products/) as a [smart card](https://security.stackexchange.com/questions/38924/how-does-storing-gpg-ssh-private-keys-on-smart-cards-compare-to-plain-usb-drives) for secure encryption, signature and authentication operations.
This is a guide to using [YubiKey](https://www.yubico.com/products/identifying-your-yubikey/) as a [smart card](https://security.stackexchange.com/questions/38924/how-does-storing-gpg-ssh-private-keys-on-smart-cards-compare-to-plain-usb-drives) for secure encryption, signature and authentication operations.
Keys stored on YubiKey are [non-exportable](https://web.archive.org/web/20201125172759/https://support.yubico.com/hc/en-us/articles/360016614880-Can-I-Duplicate-or-Back-Up-a-YubiKey-), unlike filesystem-based credentials, while remaining convenient for daily use. YubiKey can be configured to require a physical touch for cryptographic operations, reducing the risk of credential compromise.
@ -6,11 +6,11 @@ To suggest an improvement, send a pull request or open an [issue](https://github
- [Purchase YubiKey](#purchase-yubikey)
- [Prepare environment](#prepare-environment)
* [Improving entropy](#improving-entropy)
- [Install software](#install-software)
- [Prepare GnuPG](#prepare-gnupg)
* [Configuration](#configuration)
* [Identity](#identity)
* [Key](#key)
* [Expiration](#expiration)
* [Passphrase](#passphrase)
- [Create Certify key](#create-certify-key)
@ -19,7 +19,6 @@ To suggest an improvement, send a pull request or open an [issue](https://github
- [Backup keys](#backup-keys)
- [Export public key](#export-public-key)
- [Configure YubiKey](#configure-yubikey)
* [Enable KDF](#enable-kdf)
* [Change PIN](#change-pin)
* [Set attributes](#set-attributes)
- [Transfer Subkeys](#transfer-subkeys)
@ -48,10 +47,14 @@ To suggest an improvement, send a pull request or open an [issue](https://github
@ -151,51 +151,7 @@ $ doas dd if=debian-live-*-amd64-xfce.iso of=/dev/rsd2c bs=4m
1951432704 bytes transferred in 139.125 secs (14026448 bytes/sec)
```
Power off, then disconnect internal hard drives and all unnecessary devices, such as the wireless card.
## Improving entropy
Generating cryptographic keys requires high-quality [randomness](https://www.random.org/randomness/), measured as entropy.
Most operating systems use software-based pseudorandom number generators or CPU-based hardware random number generators (HRNG).
Optionally, a device such as [OneRNG](https://onerng.info/onerng/) may be used to [increase the speed](https://lwn.net/Articles/648550/) and possibly the quality of available entropy.
echo "HRNGDEVICE=/dev/ttyACM0" | sudo tee /etc/default/rng-tools
```
Insert the device and restart rng-tools:
```console
sudo atd
sudo service rng-tools restart
```
Power off, disconnect internal hard drives and all unnecessary devices, such as the wireless card.
# Install software
@ -213,15 +169,9 @@ sudo apt update
sudo apt -y upgrade
sudo apt -y install \
wget gnupg2 gnupg-agent dirmngr \
cryptsetup scdaemon pcscd \
yubikey-personalization yubikey-manager
```
**Note** Live Ubuntu images [may require modification](https://github.com/drduh/YubiKey-Guide/issues/116) to `/etc/apt/sources.list` and may need additional packages:
@ -357,17 +306,27 @@ Depending on how you plan to use GnuPG, set these values respectively:
IDENTITY="YubiKey User <yubikey@example>"
```
Or use any attribute which will uniquely identity the key:
Or use any attribute which will uniquely identity the key (this may be incompatible with certain use cases):
```console
IDENTITY="My Cool YubiKey - 2024"
```
## Key
Select the desired algorithm and key size. This guide recommends 4096-bit RSA.
Set the value:
```console
KEY_TYPE=rsa4096
```
## Expiration
Determine the desired Subkey validity duration.
Setting a key expiry forces identity and credential lifecycle management. However, setting an expiry on the primary Certify key is pointless, because it can be used to simply extend itself ([revocation certificates](https://security.stackexchange.com/questions/14718/does-openpgp-key-expiration-add-to-security/79386#79386) should be used instead).
Setting a Subkey expiry forces identity and credential lifecycle management. However, setting an expiry on the Certify key is pointless, because it can just be used to extend itself. [Revocation certificates](https://security.stackexchange.com/questions/14718/does-openpgp-key-expiration-add-to-security/79386#79386) should instead be used to revoke an identity.
This guide recommends a two year expiration for Subkeys to balance security and usability, however longer durations are possible to reduce maintenance frequency.
@ -389,25 +348,19 @@ EXPIRATION=2026-05-01
## Passphrase
Generate a passphrase, which will be used to issue the Certify key and Subkeys.
The passphrase is recommended to consist of only uppercase letters and numbers for improved readability. [Diceware](https://secure.research.vt.edu/diceware) is another method for creating strong and memorable passphrases.
Generate a passphrase for the Certify key. It will be used infrequently to manage Subkeys and should be very strong. The passphrase is recommended to consist of only uppercase letters and numbers for improved readability. [Diceware](https://secure.research.vt.edu/diceware) is another method for creating memorable passphrases.
The following commands will generate and display a strong passphrase which avoids ambiguous characters:
The following commands will generate a strong passphrase and avoid ambiguous characters:
Memorize the passphrase or write it in a secure location, ideally separate from the portable storage device used for key material.
Write the passphrase in a secure location, ideally separate from the portable storage device used for key material, or memorize it.
This repository includes a [`passphrase.html`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.html) template to help with transcription. Save the raw file, open it with a browser and print. Use a pen or permanent marker to select a letter or number on each row for each character in the passphrase.
[`passphrase.csv`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.csv) can also be printed without a browser:
This repository includes a [`passphrase.html`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.html) template to help with credential transcription. Save the raw file, open it with a browser and print. Use a pen or permanent marker to select a letter or number on each row for each character in the passphrase. [`passphrase.csv`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.csv) can also be printed without a browser:
The following command will generate Signature, Encryption and Authentication Subkeys, using the previously configured passphrase and expiration:
Use the following command to generate Signature, Encryption and Authentication Subkeys using the previously configured key type, passphrase and expiration:
Create an **encrypted** backup on portable storage to be kept offline in a secure and durable location.
The following process is recommended to be repeated several times on multiple portable storage devices, as they can fail over time. As an additional backup measure, [Paperkey](https://www.jabberwocky.com/software/paperkey/) may be used to make a physical copy of key materials for improved durability.
The following process is recommended to be repeated several times on multiple portable storage devices, as they are likely to fail over time. As an additional backup measure, [Paperkey](https://www.jabberwocky.com/software/paperkey/) can be used to make a physical copy of key materials for improved durability.
**Tip** The [ext2](https://en.wikipedia.org/wiki/Ext2) filesystem without encryption can be mounted on Linux and OpenBSD. Use [FAT32](https://en.wikipedia.org/wiki/Fat32) or [NTFS](https://en.wikipedia.org/wiki/Ntfs) filesystem for macOS and Windows compatibility instead.
@ -507,7 +457,7 @@ $ sudo fdisk -l /dev/sdc
Disk /dev/sdc: 14.9 GiB, 15931539456 bytes, 31116288 sectors
```
**Warning** Confirm the destination (`of`) before issuing the following command! This guide uses `/dev/sdc` throughout, but this value may differ on your system.
**Warning** Confirm the destination (`of`) before issuing the following command - it is destructive! This guide uses `/dev/sdc` throughout, but this value may be different on your system.
Zero the header to prepare for encryption:
@ -536,28 +486,30 @@ w
EOF
```
Use [LUKS](https://askubuntu.com/questions/97196/how-secure-is-an-encrypted-luks-filesystem) to encrypt the new partition.
Use [LUKS](https://dys2p.com/en/2023-05-luks-security.html) to encrypt the new partition.
Once again, generate a unique passphrase (different from the [Passphrase](#passphrase) used for the GnuPG identity) to protect the encrypted volume:
Generate another unique [Passphrase](#passphrase) (ideally different from the one used for the Certify key) to protect the encrypted volume:
**Note** To provision multiple YubiKeys, keep the backup mounted or remember to terminate the GnuPG process before [saving](https://lists.gnupg.org/pipermail/gnupg-users/2016-July/056353.html).
Repeat the process for any additional storage devices (at least two are recommended).
**OpenBSD**
Attach a USB disk and determine its label:
@ -664,9 +610,7 @@ doas mount /dev/sd3i /mnt/encrypted-storage
doas cp -av $GNUPGHOME /mnt/encrypted-storage
```
**Note** To set up multiple YubiKeys, keep the backup mounted or terminate GnuPG before [saving](https://lists.gnupg.org/pipermail/gnupg-users/2016-July/056353.html).
Otherwise, unmount and disconnect the encrypted volume:
Unmount and disconnect the encrypted volume:
```console
doas umount /mnt/encrypted-storage
@ -680,7 +624,7 @@ See [OpenBSD FAQ#14](https://www.openbsd.org/faq/faq14.html#softraidCrypto) for
**Important** Without the public key, it will **not** be possible to use GnuPG to decrypt nor sign messages. However, YubiKey can still be used for SSH authentication.
Create another partition on the portable storage device to store the public key, or reconnect networking and upload to a key server.
Connect another portable storage device or create a new partition on the existing one.
**Linux**
@ -707,7 +651,7 @@ sudo mount /dev/sdc2 /mnt/public
gpg --armor --export $KEYID | sudo tee /mnt/public/$KEYID-$(date +%F).asc
sudo chmod 0444 /mnt/public/0x*.asc
sudo chmod 0444 /mnt/public/*.asc
```
Unmount and remove the storage device:
@ -742,39 +686,25 @@ doas mount /dev/sd2b /mnt/public
gpg --armor --export $KEYID | doas tee /mnt/public/$KEYID-$(date +%F).asc
**Windows** 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.
## Enable KDF
Key Derived Function (KDF) enables YubiKey to store the hash of PIN, preventing the PIN from being passed as plain text.
**Note** This feature may not be compatible with older GnuPG versions, especially mobile clients. These incompatible clients will not function because the PIN will always be rejected.
Enable KDF using the default Admin pin of `12345678`:
This step must be completed before changing PINs or moving keys or an error will occur: `gpg: error for setup KDF: Conditions of use not satisfied`
If the card is locked, [Reset](#reset) it.
## Change PIN
YubiKey's PGP interface has its own PINs separate from other modules such as [PIV](https://developers.yubico.com/PIV/Introduction/YubiKey_and_PIV.html):
YubiKey's [PGP](https://developers.yubico.com/PGP/) interface has its own PINs separate from other modules such as [PIV](https://developers.yubico.com/PIV/Introduction/YubiKey_and_PIV.html):
Determine the desired PIN values. They can be shorter than the GnuPG identity passphrase due to limited brute-forcing opportunities. The User PIN should be convenient enough to remember for every-day use.
Determine the desired PIN values. They can be shorter than the Certify key passphrase due to limited brute-forcing opportunities; the User PIN should be convenient enough to remember for every-day use.
The *User PIN* must be at least 6 characters and the *Admin PIN* must be at least 8 characters. A maximum of 127 ASCII characters are allowed. See the GnuPG documentation on [Managing PINs](https://www.gnupg.org/howtos/card-howto/en/ch03s02.html) for more information.
The *User PIN* must be at least 6 characters and the *Admin PIN* must be at least 8 characters. A maximum of 127 ASCII characters are allowed. See [GnuPG - Managing PINs](https://www.gnupg.org/howtos/card-howto/en/ch03s02.html) for more information.
Set PINs manually or generate them, for example a 6 digit User PIN and 8 digit Admin PIN:
**Important** Verify a backup of Subkeys was made before proceeding. Transferring keys to YubiKey is a one-way operation: `keytocard` converts the local, on-disk key into a stub, which means the on-disk copy is no longer usable to transfer to subsequent YubiKeys.
# Transfer Subkeys
The currently selected key(s) are indicated with an `*` symbol. When transferring keys, only one subkey must be selected at a time.
**Important** Transferring keys to YubiKey is a one-way operation which converts the on-disk key into a stub making it no longer usable to transfer to subsequent YubiKeys. Ensure a backup was made before proceeding.
The Certify key passphrase and Admin PIN are required to transfer keys.
A`>` after a tag indicates the key is stored on a smart card.
The`>` after a tag indicates the key is stored on a smart card.
# Finish setup
Verify you have done the following:
- [ ] Memorized or wrote down the Certify key passphrase to a secure and durable location
- [ ] Saved the Certify key and Subkeys to encrypted portable storage, to be kept offline
* `echo $CERTIFY_PASS` to see it again; [`passphrase.html`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.html) or [`passphrase.csv`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.csv) to transcribe it
- [ ] Memorized or wrote down passphrase to encrypted volume on portable storage
* `echo $LUKS_PASS` to see it again; [`passphrase.html`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.html) or [`passphrase.csv`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.csv) to transcribe it
- [ ] Saved the Certify key and Subkeys to encrypted portable storage, to be kept offline
* At least two backups are recommended, stored at separate locations
- [ ] Exported a copy of the public key where is can be easily accessed later
- [ ] Memorized or wrote down the User Pin and Admin PIN, which are unique and changed from default values
- [ ] Moved Encryption, Signature and Authentication Subkeys to YubiKey (`gpg -K` shows `ssb>` for 3 Subkeys)
* Separate device or non-encrypted partition was used
- [ ] Memorized or wrote down the User PIN and Admin PIN, which are unique and changed from default values
* `echo $USER_PIN $ADMIN_PIN` to see them again; [`passphrase.html`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.html) or [`passphrase.csv`](https://raw.githubusercontent.com/drduh/YubiKey-Guide/master/passphrase.csv) to transcribe them
- [ ] Moved Encryption, Signature and Authentication Subkeys to YubiKey
* `gpg -K` shows `ssb>` for each of the 3 Subkeys
Reboot to clear the ephemeral environment and complete setup.
Set the following option. This avoids the problem where GnuPG will prompt, repeatedly, for the insertion of an already-inserted YubiKey:
Set the following option. This avoids the problem where GnuPG will repeatedly prompt for the insertion of an already-inserted YubiKey:
```console
touch scdaemon.conf
@ -956,22 +890,19 @@ touch scdaemon.conf
echo "disable-ccid" >>scdaemon.conf
```
> The `disable-ccid` option is only required for GnuPG versions 2.3 or later. However, setting this option does not appear to interfere with the operation of earlier versions of GnuPG so it is recommended for all installations.
By default, YubiKey will perform cryptographic operations without requiring any action from the user after the key is unlocked once with the PIN.
To require a touch for each key operation, use [YubiKey Manager](https://developers.yubico.com/yubikey-manager/) and the Admin PIN to set policy:
To require a touch for each key operation, use [YubiKey Manager](https://developers.yubico.com/yubikey-manager/) and the Admin PIN to set key policy.
Encryption:
@ -1160,7 +1102,13 @@ Encryption:
ykman openpgp keys set-touch dec on
```
**Note** Versions of YubiKey Manager before 5.1.0 use `enc` instead of `dec` for encryption. Older versions of YubiKey Manager use `touch` instead of `set-touch`
**Note** Versions of YubiKey Manager before 5.1.0 use `enc` instead of `dec` for encryption:
```console
ykman openpgp keys set-touch enc on
```
Even older versions of YubiKey Manager use `touch` instead of `set-touch`
**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.)
See [Issue #85](https://github.com/drduh/YubiKey-Guide/issues/85) for more information and troubleshooting.
See [Issue 85](https://github.com/drduh/YubiKey-Guide/issues/85) for more information and troubleshooting.
Transfer the public key to the destination host and import it:
```console
gpg --import 0x*.asc
gpg --import /mnt/public/*.asc
```
Alternatively, publish to a public key server and download it:
@ -1962,6 +1913,70 @@ Reset code: NOT SET
Admin PIN: 12345678
```
# Optional hardening
The following steps may improve the security and privacy of YubiKey.
## Improving entropy
Generating cryptographic keys requires high-quality [randomness](https://www.random.org/randomness/), measured as entropy. Most operating systems use software-based pseudorandom number generators or CPU-based hardware random number generators (HRNG).
Optionally, a device such as [OneRNG](https://onerng.info/onerng/) may be used to [increase the speed](https://lwn.net/Articles/648550/) and possibly the quality of available entropy.
Before creating keys, configure [rng-tools](https://wiki.archlinux.org/title/Rng-tools):
```console
sudo apt -y install at rng-tools python3-gnupg openssl
echo "HRNGDEVICE=/dev/ttyACM0" | sudo tee /etc/default/rng-tools
```
Insert the device and restart rng-tools:
```console
sudo atd
sudo service rng-tools restart
```
## Enable KDF
**Note** This feature may not be compatible with older GnuPG versions, especially mobile clients. These incompatible clients will not function because the PIN will always be rejected.
This step must be completed before changing PINs or moving keys or an error will occur: `gpg: error for setup KDF: Conditions of use not satisfied`
Key Derived Function (KDF) enables YubiKey to store the hash of PIN, preventing the PIN from being passed as plain text.
Enable KDF using the default Admin PIN of `12345678`:
1. YubiKey has two configurations, invoked with either a short or long press. By default, the short-press mode is configured for HID OTP; a brief touch will emit an OTP string starting with `cccccccc`. OTP mode can be swapped to the second configuration via the YubiKey Personalization tool or disabled entirely using [YubiKey Manager](https://developers.yubico.com/yubikey-manager): `ykman config usb -d OTP`
@ -1980,17 +1995,19 @@ Admin PIN: 12345678
- 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 encounter problems connecting to YubiKey with GnuPG - try unplugging and re-inserting 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. Another possibility is that there is a problem with the PIN, e.g. it is too short or blocked.
- A lot of issues can be fixed by unplugging and re-inserting YubiKey, or restarting the `gpg-agent` process.
- 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 software](#install-software).
- If you see `General key info..: [none]` in card status output - import the public key.
- 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: write error` - YubiKey is likely locked. Install and run yubikey-personalization-gui to unlock it.
- If you receive the error, `Key does not match the card's capability` - you likely need to use 2048 bit RSA key sizes.
- If you receive the error, `Key does not match the card's capability` - you likely need to use 2048-bit RSA key sizes.
- 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.
@ -2006,7 +2023,7 @@ Admin PIN: 12345678
- If you receive the error, `Permission denied (publickey)`, increase ssh verbosity with the `-v` flag and verify the public key from the card is being offered: `Offering public key: RSA SHA256:abcdefg... cardno:00060123456`. If it is, verify the correct user the target system - not 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 SSH authentication still fails - add up to 3 `-v` flags to the `ssh` client to increase verbosity.
- If SSH authentication still fails - add up to 3 `-v` flags to the `ssh` command to increase verbosity.
- 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.
drduh
1 month ago
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