Recovering From a Broken Smartcard

‘Safety regulations are written in blood’ – It turns out good practices are there for a reason. Who knew?

About a week ago, I was wrapping up a workday and just needed to commit and push to wrap things up for the day. No problem: git commit -S etc, plug in my GPG smartcard (a Nitrokey Pro 2), wait for the PIN prompt as usual, and- Huh. No PIN prompt. Strange, but maybe my USB-C dongle was acting up. Unplug the smartcard, plug into one of the builtin USB-A ports on the laptop, repeat everything and… Still no prompt. I also at this point noticed that the little red LED that normally lights up when plugged in wasn’t doing so. Oh no.

I don’t use the smartcard for much, apart from commit signing, the occasional email encryption for the couple of people that have it set up, and some very occasional per-file encryption. Commit signing I could potentially live without, but there is something pleasing about having the little green verified next to your commits. In any case, the LED not lighting up no matter which port, adapter, or OS I was using, across reboots, probably meant it had suffered a hardware fault. Which meant the subkeys were inaccessible… But of course I’d been sensible and made a backup, right? Well, yes but actually kinda. I keep my master key offline and, in a similar fashion, I’d chosen to sacrifice a bit of security for the ability to recover my subkeys in exactly this kind of situation: I originally generated the keys on a secure computer (instead of generating them directly on the smartcard) and then transferred them to both the smartcard and a couple of backup drives. Which I of course had placed somewhere clever and secure. I’m sure you can see what the problem was here… Something something “a backup is only a backup if it’s usable”…

But, I eventually managed to find one of the drives. Great! I should be able to recover my keys now. Unfortunately, past Thomas was clever and kept things safe: Having the raw subkeys on an external drive would not be the best of practices, since if the drive was lost, anyone could get the data off it, which would be disastrous for things like encryption keys. So past Thomas had encrypted things. And past Thomas had done so quite well; a bit too well in fact… After about half an hour of figuring out how exactly the encryption layers were set up, trying to remember the passwords I’d used (always a fun game), and figuring out which files were hidden where, I could finally say the classic 1337 h4xx0r phrase: “I’m in.” So. Having recovered the backups of my secret keys, type gpg --import and it should be good, right? Unfortunately, it would seem not. I must have misconfigured things when I initially exported them, so although I could recover the master key, the subkeys were still shown with a > after them, indicating that gpg was expecting to find them on the smartcard. The smartcard that I was trying to recover from losing.

At this point, I counted my losses. There is, as far as I know, no practical way to recreate the same subkeys from the master key. So I revoked the subkeys, wrote a ticket to the Nitrokey support team asking about the durability of the Pro 2 (it was < 1 year old, I felt it should have lasted a bit longer), and set about both generating new keys, but also being more sensible about everything. So without further ado:

Learning my lesson – A guide on how not to end up in my situation.

If you have looked at backups previously, you may be familiar with the ‘3-2-1’ rule: 3 backups, on at least 2 types of media, with at least 1 backup offsite. And if you have looked at gpg smartcards and/or FIDO/U2F tokens previously, you might have read that it is good practice to have at least 1 backup token. I’d only had 1 token (because hardware is expensive) and 2 backups. The 2 backups thing turned out to be very good, since without it, I would have realised that my master key was lost as well. So even though it was convoluted to recover, it was recovered. But in any case, my backups clearly needed to be (a) easier to recover from, (b) actually tested/working, and (c) on at least one extra device.

Setting things up again then:

  1. Use gpg --import in combination with your offline master key to bring your master key back online. Hopefully you remember the passphrase associated with it (in my case, I almost didn’t; always fun when you get an “Bad passphrase (try 2 of 3)” message and realise the stakes might be another bit higher than you already knew they were. Thankfully I got it on a very nervous third try).
  2. Start the key editing tool using gpg --expert --edit-key <keyid>.
  3. For each of the subkeys, use key <n> to select the subkey; verify that you have the right key selected; then check it again, is it really the subkey? are you 110% sure?; if so, then type revkey to revoke the subkey.
  4. With the subkeys revoked, create new ones using the addkey command. Depending on your old setup, you’ll want to select the same configuration. In my case, I had separate subkeys for signing, encrypting, and authenticating, so I recreated these. For each subkey, start by typing addkey. Then:
    1. For the signing and the encryption subkeys, GPG will likely have an option available that automatically sets the right uses, e.g. (4) for an RSA signing subkey, and (6) for an RSA encryption subkey. Pick the option/algorithm that you want to use.
    2. Select a keysize, e.g. 3072.
    3. Set an expiration time if you want one.
    4. Confirm that the settings are correct and that you really want to create the subkey, then unlock your masterkey with its passphrase. The subkey should be generated and you will be put back at the key editing menu when the process is complete.
    5. For the authenticating subkey, select the option with the algorithm you want and the “(set your own capabilities)” text, e.g. (8) for RSA.
    6. Type A (followed by return/enter) to toggle the ‘authenticate’ capability, then type S (followed by return/enter) and then E (followed by return/enter) to toggle the ‘sign’ and ’encrypt’ capabilities respectively. You should end up with a dialog displaying “Currently allowed actions: Authenticate”. If not, you may have accidentally enabled some capabilities. Toggle them again to disable them.
    7. Type Q (followed by return/enter) to confirm the capabilities. Then repeat steps 2-4 for this key as well.
  5. With the subkeys created, type save to save and quit the edit mode.

Confirm that your key and subkeys were saved by typing gpg -K. You should see the subkeys followed by [S], [E], and [A] for the sign, encrypt, and authenticate subkeys respectively.

With the new keys in place, time to create an actually working backup:

  1. Run
    gpg --expert --output gpg-masterkey.asc --export-secret-keys <keyid>
    
    (replacing <keyid> with the ID for your key, without the angle brackets) and enter the passphrase when prompted, to export the secret keys, including the master key to the file gpg-masterkey.asc. Adjust the filename as you want. Optionally, you can also specify --armor before --output to output the key in ASCII rather than as a binary file. This is useful if you want to print the key, but might also be a security risk since it is human-readable.
  2. Run
    gpg --expert --output gpg-subkeys.asc --export-secret-subkeys <keyid>
    
    (again, replacing etc) to export the secret subkeys to the file gpg-subkeys.asc. As with the master key, adjust the parameters as you want. (Note: --export-secret-subkeys is a GNU-specific extension to PGP and so might not work, export- or import-wise, with non GPG programs. This is why we export both.)
  3. Copy the resulting files to your secure backup drives.

Verify that your backups actually work! You should repeat these steps for the files you just created, and for each one on the backup drives to check that they were not corrupted during copying.

  1. Create a temporary directory somewhere (I tend to use $HOME/gpg_tmp and will be assuming that in the rest of this) and chmod it to permissions 700.
  2. Run
    gpg --homedir="$HOME/gpg_tmp" -K
    
    twice to have GPG set up the right files and so you can confirm that the secret keys are not in the temporary directory.
  3. Run
    gpg --homedir="$HOME/gpg_tmp" --import gpg-subkeys.asc
    
    to import the subkeys. Then verify that they’re there but, crucially, that the master key is not by running
    gpg --homedir="$HOME/gpg_tmp" -K
    
    You should see all the keys, but the master key should have a hash symbol (#) after it, indicating that it is offline.
  4. Run
    gpg --homedir="$HOME/gpg_tmp" --import gpg-masterkey.asc
    
    to import the masterkey. Then verify that the hash sign no longer appears when running
    gpg --homedir="$HOME/gpg_tmp" -K
    
    You can also verify that all the keys import when importing the gpg-masterkey.asc file by removing the directory and start again, skipping step 3.
  5. Just to be completely safe/paranoid, shred all the files in the directory by triple-checking that the command below points to the right directory and then running it
    find "$HOME/gpg_tmp" -type f -exec shred "{}" \;
    
    (this requires find and shred installed. If you don’t have these, either install them, use something similar, or don’t worry about deleting the files that thoroughly). Then, being very careful, run
    rm -rf "$HOME/gpg_tmp"
    
    to remove the temporary directory and its contents.
  6. As mentioned just before this list, repeat 1-5 for each of the copies of the files on the different backup drives.

For additional sanity, perform some operations without the --homedir flag (e.g. encrypt some files) and then try to decrypt them before and after importing the keys to the temporary directory (making sure to specify --homedir with each of the decryption commands).

Now, depending on how “last resort” you want the backups to be, I would suggest writing down the passphrase(s) for the keys and drives and storing these as well. This means that you have a way of getting back in, should everything fail, including you remembering your passphrase(s). Store these drives safely, ideally storing at least one in a physically separate location you have access to, and take them out once a year or something and re-confirm that the backups still work. Having offsite backups can be a bit trickier as a private person, but one option, if you have a cloud storage subscription, could be to create a VeraCrypt volume as a file, storing the backup files in it, and then upload the volume file to your cloud service. I’m not 100% certain, but I can’t immediately see why that shouldn’t be safe.

Finally, in terms of the smartcard/token that needs replacing, buy at least 2 new ones. I made the mistake of buying only one because I was impatient and didn’t want to wait until I could afford to buy 2. If you don’t have the money immediately, save up and buy 2 when you can do that, because it is not worth having one break at a critical moment and being without a replacement. Especially in a case like mine where the backups turned out to be slightly misconfigured. Sure, the guide above minimises the risk of error in the backups, but it can still happen and there is little gained for the much greater risk that comes with not having a backup smartcard.

Remember to update things

Given that you have just rotated all your subkeys, make sure to update anywhere you used them. In my case, this was mainly Thunderbird and GitHub (and hence my local .gitconfig), but wherever you’ve used and/or published your public key, those places now need to be updated with your new public key. You can export it as normal by running

gpg --output pubkey.asc --export <keyid>

(with --armor as the first flag, if you want it as plain ASCII), and then uploading/distributing/emailing/etc the pubkey.asc file.

Annoying little sidenote about GitHub: It doesn’t seem to have a concept of subkeys, so I had to replace the entire GPG key because it thought I was re-uploading the same one. This unfortunately means that my old commits, although still sane and signed, appear as unverified on GitHub, because it doesn’t handle revoked subkeys (see this and this for some discussion on the topic). If you need to, I believe you can verify them locally by downloading my old public key.

Summary of lessons learnt

  • There’s a reason most companies and people advise you to buy two smartcards/hardware tokens. Be smart and follow that advice, don’t think you’ll be fine with just one.
  • Keep more than one backup and make sure you know where those backups are.
  • Test your backups regularly to make sure they haven’t been corrupted and/or that you haven’t lost the memory of how they work.
  • If you’re being clever with passwords or encryption schemes, make sure you’re being clever in a way that future you will be able to work out. Hacking your past self is fun, but only for so long/when your emails+data are not on the line.
  • Optionally, make a “last resort/golden key” backup which contains all the information to completely unlock everything. Then, store this in the most secure location you have. It can be risky, but it might also be what saves you if you’ve entirely forgotten (or lost) your passwords and setup.

As always, thanks for reading. If not entertaining, I hope that it could at least be somewhat educational as to what not to do ^^;

P.S.: The wonders of technology

Nitrokey’s support team got back saying it sounded like a hardware fault, especially since the keys were supposed to last 5-10 years under daily use, but that just to be certain, I should try plugging it in to a different computer if I had one. I didn’t think it would make a difference, but for some reason it did: The red LED lit up and the card seemed to work as normal. Since this, it has been working as normal without any hickups whatsoever. Isn’t technology marvellous? -_-

This didn’t change much, since I’d already rotated keys and everything at this point, but it did at least allow me to recover some encrypted files that I thought I had lost the keys to forever. And despite reporting this back, Nitrokey still offered to replace the smartcard, just to be on the safe side, which I think is awesome! So 5/7, a perfect score, I guess: I learnt some important lessons, didn’t entirely permanently lose my data, and I found out Nitrokey makes good hardware and have a great support team.

Thomas Ekström Hansen
Thomas Ekström Hansen
PhD student in Computer Science

My research interests include low-level programming, type systems, and formal methods.

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