Dec 5, 2017| Paddy Steed
You can use a Yubikey USB device to securely generate and store your SSH key. This can be used to load your private key on demand, protected by a PIN. Perfect for pair-programming on shared machines!
This post is part of a series on using Yubikeys to secure development whilst pair-programming on shared machines.
In this post I will show you how to use your Yubikey to generate and store an RSA key, and then configure your computer to use GPG to load that key and authenticate with remote SSH servers. Each time you authenticate you’ll need to have your Yubikey inserted, and you’ll be prompted for a PIN.
I’ll assume you have a Yubikey 4. Some of the terminal output will differ slightly depending on your operating system; these snippets are captured on Fedora.
Digital security is generally much more difficult than physical security. If I have a secret written on a piece of paper, I could hide that paper somewhere, I could keep it on my person, I could put it in safety deposit box. All of these are pretty good ways to make sure nobody reads whatever is on the paper, and all are simple to reason about. If I have that same secret on a file on my laptop, however, there is much more to think about. Every piece of software I run has permission to read that file.
My laptop has a network connection, so an attacker does not need to ever meet me to steal my secret. A Yubikey almost turns a digital security problem into a physical security one.
If your SSH key is stored on a Yubikey it cannot be copied, it cannot be stolen remotely, and the software on whatever machine you stick it into cannot read it. You can even PIN protect the key, which means that if somebody physically steals your Yubikey they would have to guess the PIN correctly to be able to use it, and even then, they would be unable to extract the key.
On 15 October 2017 a serious problem was announced in a library used by Yubikey firmware responsible for generating RSA keys. The ROCA (Revenge of Coppersmith's Attack) hack enables the computing of the private part of a RSA keypair from the public part alone.
This is fixed in all Yubikeys manufactured since June, and I am comfortable recommending generating RSA keys on your Yubikey.
gpg using your operating system’s package manager so that you can interact with your Yubikey.
gpg 2.2.23has a bug that prevents on-card key generation. Despite what’s written in the bug report, we’ve had success with version
Plug in your Yubikey, and run
gpg --change-pin to change the PIN from the default of
123456. This is the PIN you’ll be asked to enter whenever you need to access the private key, for example when doing
Despite the term PIN, it can be up to 8 alphanumeric characters.
$ gpg --change-pin 1 - change PIN 2 - unblock PIN 3 - change Admin PIN 4 - set the Reset Code Q - quit Your selection? 1 Please enter the PIN New PIN New PIN PIN changed.
If you enter your newly-set PIN incorrectly three times then the Yubikey will be locked, and you must unlock it using the admin PIN, which is
12345678 by default.
You should change the admin PIN too, by running the same command again, this time selecting option 3:
$ gpg --change-pin 1 - change PIN 2 - unblock PIN 3 - change Admin PIN 4 - set the Reset Code Q - quit Your selection? 3 gpg: 3 Admin PIN attempts remaining before card is permanently locked Please enter the Admin PIN New Admin PIN New Admin PIN Admin PIN changed.
If you later enter the admin PIN incorrectly three times you will need to factory-reset the Yubikey.
Firstly, you need to configure
ssh on the machines you’ll be working on to use
gpg-agent to handle authentication, which will in turn load an RSA key from your Yubikey - provided that you enter the correct PIN.
The below snippet appends the appropriate config to your
.bashrc (if you use zsh you will need to append these lines to
$ cat <<EOF >> ~/.bashrc export GPG_TTY=$(tty) gpg-connect-agent updatestartuptty /bye unset SSH_AGENT_PID export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket) EOF $ . ~/.bashrc
You may also want to install a GUI for entering your Yubikey’s PIN.
pinentry-curses CLI tool always uses the
tty it was started in, which gets problematic when changing terminals or when PIN entry was prompted by a background process (such as Git Radar, which we use at EngineerBetter).
To install a PIN entry GUI on macOS, run
brew install pinentry-mac. On Linux you might want to install
pinentry-mac may have additional installation steps, so you should pay attention to the
brew install output. At the time of writing, you are asked to configure
gpg-agent to use
pinentry-mac by default:
echo "pinentry-program /usr/local/bin/pinentry-mac" >> ~/.gnupg/gpg-agent.conf
I recommend generating your RSA key on the Yubikey itself, rather than generating the key on your computer and then copying it to the Yubikey. This way you know that it has never been on the filesystem, where it could be snooped upon.
$ gpg --card-edit gpg: detected reader `Yubico Yubikey 4 OTP+U2F+CCID 00 00' Application ID ...: D2760001240102010006069174660000 Version ..........: 2.1 Manufacturer .....: unknown Serial number ....: 06917466 Name of cardholder: [not set] Language prefs ...: [not set] Sex ..............: unspecified URL of public key : [not set] Login data .......: [not set] Private DO 1 .....: [not set] Private DO 2 .....: [not set] Signature PIN ....: not forced Key attributes ...: 2048R 2048R 2048R Max. PIN lengths .: 127 127 127 PIN retry counter : 3 0 3 Signature counter : 0 Signature key ....: [none] Encryption key....: [none] Authentication key: [none] General key info..: [none] gpg/card> admin Admin commands are allowed gpg/card> generate
You will be asked if you want to backup the encryption key. If you are only using this key for SSH authentication I would recommend you do not do this. It is fairly easy to go though this process again if you lose your Yubikey, and it means your key will never leave your Yubikey.
Make off-card backup of encryption key? (Y/n) n Please note that the factory settings of the PINs are PIN = `123456' Admin PIN = `12345678' You should change them using the command --change-pin gpg: gpg-agent is not available in this session Please enter the PIN What keysize do you want for the Signature key? (2048) What keysize do you want for the Encryption key? (2048) What keysize do you want for the Authentication key? (2048) Please specify how long the key should be valid. 0 = key does not expire <n> = key expires in n days <n>w = key expires in n weeks <n>m = key expires in n months <n>y = key expires in n years
By default, your key will not expire. I would recommend setting an expiry date.
Key is valid for? (0) 1y Key expires at Wed 14 Nov 2018 14:18:02 GMT Is this correct? (y/N) y You need a user ID to identify your key; the software constructs the user ID from the Real Name, Comment and Email Address in this form: "Heinrich Heine (Der Dichter) <email@example.com>" Real name: Paddy Steed Email address: firstname.lastname@example.org Comment: You selected this USER-ID: "Paddy Steed <email@example.com>" Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O gpg: generating new key gpg: 3 Admin PIN attempts remaining before card is permanently locked Please enter the Admin PIN gpg: please wait while key is being generated ... gpg: key generation completed (4 seconds) gpg: signatures created so far: 0 gpg: generating new key gpg: please wait while key is being generated ... gpg: key generation completed (6 seconds) gpg: signatures created so far: 1 gpg: signatures created so far: 2 gpg: generating new key gpg: please wait while key is being generated ... gpg: key generation completed (9 seconds) gpg: signatures created so far: 3 gpg: signatures created so far: 4 gpg: key A4D581D2 marked as ultimately trusted public and secret key created and signed. gpg: checking the trustdb gpg: public key of ultimately trusted key 53157188 not found gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 2 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 2u gpg: next trustdb check due at 2018-11-14 pub 2048R/A4D581D2 2017-11-14 [expires: 2018-11-14] Key fingerprint = 31A1 BADF 181F B053 0D06 8B45 7B4E 9486 A4D5 81D2 uid Paddy Steed <firstname.lastname@example.org> sub 2048R/0A3CA35A 2017-11-14 [expires: 2018-11-14] sub 2048R/4746682A 2017-11-14 [expires: 2018-11-14] gpg/card> quit
Your RSA key is now generated, and the public key is stored on your GPG keychain.
The public key that you generated is stored in your local keychain (eg
~/.gnupg/pubring.kbx). However, we need to share it with the world for two reasons:
You should upload your public key to one or more popular keyservers:
$ gpg --list-secret email@example.com sec> rsa2048 2017-11-22 [SC] [expires: 2021-04-29] FEEDBEEFC0C0A7D867D34ADEADD0D0CAFEDECADE Card serial no. = 0006 06917459 uid [ unknown] Paddy Steed <firstname.lastname@example.org> ssb> rsa2048 2017-11-22 [A] [expires: 2021-04-29] ssb> rsa2048 2017-11-22 [E] [expires: 2021-04-29]
Copy the long ID from the output above, and then upload it to one or more servers:
$ gpg --keyserver keys.openpgp.org --send-key FEEDBEEFC0C0A7D867D34ADEADD0D0CAFEDECADE $ gpg --keyserver keys.gnupg.net --send-key FEEDBEEFC0C0A7D867D34ADEADD0D0CAFEDECADE $ gpg --keyserver pgp.mit.edu --send-key FEEDBEEFC0C0A7D867D34ADEADD0D0CAFEDECADE $ gpg --keyserver keyserver.ubuntu.com --send-key FEEDBEEFC0C0A7D867D34ADEADD0D0CAFEDECADE
If you experience the following error
gpg: keyserver send failed: Server indicated a failure, try adding the
hkp protocol along with a port override:
$ gpg --keyserver hkp://keys.gnupg.net:80 --send-key FEEDBEEFC0C0A7D867D34ADEADD0D0CAFEDECADE
Now the public key is safely stored elsewhere, you can get back to configuring SSH.
Assuming you have configured
ssh-add -L will display the public key in SSH format (you may need to open a new shell session if your ssh-agent already has a key loaded).
$ ssh-add -L ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCy7PhCvbb+R0UIsQdBvIpWQNSBOZkfV+7E0d55Gxzppt9tvQHbWJwzi/… cardno:000606917466
You can put this in
~/.authorized_keys for any machine you want to be able to log in to, and paste it into GitHub, GitLab, and similar tools.
source ~/.bashrcif you’re using the same terminal as earlier
git pull, and see that authentication fail (you shouldn’t have any keys loaded at this point - you can list loaded keys with
git pullcomplete successfully.
Earlier in this post it was recommended that your GPG key was given an expiry date. What happens when it expires?
If your GPG key has expired, assuming that you’ve configured Git to use it for commit signing, you’ll see this when trying to commit:
error: gpg failed to sign the data fatal: failed to write commit object
You can extend the life of your key simply.
Ensure that your Yubikey is inserted, and run
gpg --list-secret <your-email-address>. This will show the key and subkeys:
gpg: checking the trustdb gpg: marginals needed: 3 completes needed: 1 trust model: pgp gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: next trustdb check due at 2023-04-29 sec> rsa2048 2017-11-22 [SC] [expires: 2023-04-29] FEEDBEEFC0C0A7D867D34ADEADD0D0CAFEDECADE Card serial no. = 0006 06917459 uid [ultimate] Daniel Jones <email@example.com> ssb> rsa2048 2017-11-22 [E] [expires: 2023-04-29] ssb> rsa2048 2017-11-22 [A] [expires: 2023-04-29]
Enter the interactive GPG key-editing prompt with
gpg --edit-key <long-id-from-above>, and you’ll see something like this:
sec rsa2048/ADD0D0CAFEDECADE created: 2017-11-22 expires: 2023-04-29 usage: SC card-no: 0006 06917459 trust: ultimate validity: ultimate ssb rsa2048/BA53B2D6734C8A8E created: 2017-11-22 expires: 2023-04-29 usage: E card-no: 0006 06917459 ssb rsa2048/AF5A4B5AA8A8C5BF created: 2017-11-22 expires: 2023-04-29 usage: A card-no: 0006 06917459 [ultimate] (1). Daniel Jones <firstname.lastname@example.org>
By default there is no subkey selected, so we’ll be operating on the main key. Use the
expire command, and you’ll be prompted to enter a number followed by a unit, such as
2y for two years. Upon pressing
return, you’ll be prompted to verify the expiration date, which you should confirm.
gpg> expire Are you sure you want to change the expiration time for multiple subkeys? (y/N) y Please specify how long the key should be valid. 0 = key does not expire <n> = key expires in n days <n>w = key expires in n weeks <n>m = key expires in n months <n>y = key expires in n years Key is valid for? (0) 2y Key expires at Sat 29 Apr 12:17:52 2023 BST Is this correct? (y/N) y
You’ll then get this hint:
gpg: WARNING: Your encryption subkey expires soon. gpg: You may want to change its expiration date too.
We now need to select the subkeys for editing too. They are 1-indexed, and are selected with the
key N command. Each selected key will have an asterisk next to it.
key 0 will deselect subkeys.
gpg> key 1 sec rsa2048/ADD0D0CAFEDECADE created: 2017-11-22 expires: 2023-04-29 usage: SC card-no: 0006 06917459 trust: ultimate validity: ultimate ssb* rsa2048/BA53B2D6734C8A8E created: 2017-11-22 expired: 2021-04-29 usage: E card-no: 0006 06917459 ssb rsa2048/AF5A4B5AA8A8C5BF created: 2017-11-22 expired: 2021-04-29 usage: A card-no: 0006 06917459 [ultimate] (1). Daniel Jones <email@example.com>
Select all subkeys, and then run the
expire command exactly as you did for the main key. Your keys are now extended, and you may now commit as normal.