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Free software developer security hygiene

Updated: 2017-12-01

Target audience

This document is aimed at developers working on free software projects. It covers the following topics:

  1. PGP key best practices
  2. Basic introduction to PGP and Git
  3. Basic workstation security best practices

We use the term "Free" as in "Freedom," but this guide can also be used for developing non-free or source-available ("Open Source") software. If you write code that goes into public source repositories, you can benefit from getting acquainted with and following this guide.

Topics NOT covered

This is not a "how to write secure software" guide. Please check the resources on secure coding best practices that are available for the programming languages, libraries, and development environments used by your free software project.

Structure

Each section is split into two areas:

  • The checklist that can be adapted to your project's needs
  • Free-form list of considerations that explain what dictated these decisions, together with configuration instructions

Checklist priority levels

The items in each checklist include the priority level, which we hope will help guide your decision:

  • (ESSENTIAL) items should definitely be high on the consideration list. If not implemented, they will introduce high risks to the code that gets committed to the open-source project.
  • (NICE) to have items will improve the overall security, but will affect how you interact with your work environment, and probably require learning new habits or unlearning old ones.
  • (PARANOID) is reserved for items we feel will significantly improve your security, but will require making equally significant adjustments to the way you interact with your operating system.

Remember, these are only guidelines. If you feel these priority levels do not reflect your project's commitment to security, you should adjust them as you see fit.

Basic PGP concepts and tools

Checklist

  • Understand the role of PGP in Free Software Development (ESSENTIAL)
  • Understand the basics of Public Key Cryptography (ESSENTIAL)
  • Understand PGP Encryption vs. Signatures (ESSENTIAL)
  • Understand PGP key identities (ESSENTIAL)
  • Understand PGP key validity (ESSENTIAL)
  • Install GnuPG utilities (version 2.x) (ESSENTIAL)

Considerations

The Free Software community has long relied on PGP for assuring the authenticity and integrity of software products it produced. You may not be aware of it, but whether you are a Linux, Mac or Windows user, you have previously relied on PGP to ensure the integrity of your computing environment:

  • Linux distributions rely on PGP to ensure that binary or source packages have not been altered between when they have been produced and when they are installed by the end-user.
  • Free Software projects usually provide detached PGP signatures to accompany released software archives, so that downstream projects can verify the integrity of downloaded releases before integrating them into their own distributed downloads.
  • Free Software projects routinely rely on PGP signatures within the code itself in order to track provenance and verify integrity of code committed by project developers.

This is very similar to developer certificates/code signing mechanisms used by programmers working on proprietary platforms. In fact, the core concepts behind these two technologies are very much the same -- they differ mostly in the technical aspects of the implementation and the way they delegate trust. PGP does not rely on centralized Certification Authorities, but instead lets each user assign their own trust to each certificate.

Our goal is to get your project on board using PGP for code provenance and integrity tracking, following best practices and observing basic security precautions.

Extremely Basic Overview of PGP operations

You do not need to know the exact details of how PGP works -- understanding the core concepts is enough to be able to use it successfully for our purposes. PGP relies on Public Key Cryptography to convert plain text into encrypted text. This process requires two distinct keys:

  • A public key that is known to everyone
  • A private key that is only known to the owner

Encryption

For encryption, PGP uses the public key of the owner to create a message that is only decryptable using the owner's private key:

  1. the sender generates a random encryption key ("session key")
  2. the sender encrypts the contents using the session key
  3. the sender encrypts the session key using the recipient's public PGP key
  4. the sender sends both the encrypted contents and the encrypted session key to the recipient

To decrypt:

  1. the recipient decrypts the session key using their private PGP key
  2. the recipient uses the session key to decrypt the contents of the message

Signatures

For creating signatures, the private/public PGP keys are used the opposite way:

  1. the signer generates the checksum hash of the contents
  2. the signer uses their own private PGP key to encrypt that checksum
  3. the signer provides the encrypted checksum alongside the contents

To verify the signature:

  1. the verifier generates their own checksum hash of the contents
  2. the verifier uses the signer's public PGP key to decrypt the provided checksum
  3. if the checksums match, the integrity of the contents is verified

Combined usage

Frequently, encrypted messages are also signed with the sender's own PGP key. This should be the default whenever using encrypted messaging, as encryption without authentication is not very meaningful (unless you are a whistleblower or a secret agent and need plausible deniability).

Understanding Key Identities

Each PGP key must have one or multiple Identities associated with it. Usually, an "Identity" is the person's full name and email address in the following format:

Alice Engineer <alice.engineer@example.com>

Sometimes it will also contain a comment in brackets, to tell the end-user more about that particular key:

Bob Designer (obsolete 1024-bit key) <bob.designer@example.com>

Since people can be associated with multiple professional and personal entities, they can have multiple identities on the same key:

Alice Engineer <alice.engineer@example.com>
Alice Engineer <aengineer@personalmail.example.org>
Alice Engineer <webmaster@girlswhocode.example.net>

When multiple identities are used, one of them would be marked as the "primary identity" to make searching easier.

Understanding Key Validity

To be able to use someone else's public key for encryption or verification, you need to be sure that it actually belongs to the right person (Alice) and not to an impostor (Eve). In PGP, this certainty is called "key validity:"

  • Validity: full -- means we are pretty sure this key belongs to Alice
  • Validity: marginal -- means we are somewhat sure this key belongs to Alice
  • Validity: uknown -- means there is no assurance at all that this key belongs to Alice

Web of Trust (WoT) vs. Trust on First Use (TOFU)

PGP incorporates a trust delegation mechanism known as the "Web of Trust." At its core, this is an attempt to replace the need for centralized Certification Authorities of the HTTPS/TLS world. Instead of various software makers dictating who should be your trusted certifying entity, PGP leaves this responsibility to each user.

Unfortunately, very few people understand how the Web of Trust works, and even fewer bother to keep it going. It remains an important aspect of the OpenPGP specification, but recent versions of GnuPG (2.2 and above) have implemented an alternative mechanism called "Trust on First Use" (TOFU).

You can think of TOFU as "the SSH-like approach to trust." With SSH, the first time you connect to a remote system, its key fingerprint is recorded and remembered. If the key changes in the future, the SSH client will alert you and refuse to connect, forcing you to make a decision on whether you choose to trust the changed key or not.

Similarly, the first time you import someone's PGP key, it is assumed to be trusted. If at any point in the future GnuPG comes across another key with the same identity, both the previously imported key and the new key will be marked as invalid and you will need to manually figure out which one to keep.

In this guide, we will be using the TOFU trust model.

Installing OpenPGP software

First, it is important to understand the distinction between PGP, OpenPGP, GnuPG and gpg:

  • PGP ("Pretty Good Privacy") is the name of the original commercial software
  • OpenPGP is the IETF standard compatible with the original PGP tool
  • GnuPG ("Gnu Privacy Guard") is free software that implements the OpenPGP standard
  • The command-line tool for GnuPG is called "gpg"

Today, the term "PGP" is almost universally used to mean "the OpenPGP standard," not the original commercial software, and therefore "PGP" and "OpenPGP" are interchangeable. The terms "GnuPG" and "gpg" should only be used when referring to the tools, not to the output they produce or OpenPGP features they implement. For example:

  • PGP (not GnuPG or GPG) key
  • PGP (not GnuPG or GPG) signature
  • PGP (not GnuPG or GPG) keyserver

Understanding this should protect you from an inevitable pedantic "actually" from other PGP users you come across.

Installing GnuPG

If you are using Linux, you should already have GnuPG installed. On a Mac, you should install GPG-Suite or you can use brew install gnupg2. For all other platforms, you'll need to do your own research to find the correct places to download and install GnuPG.

GnuPG 1 vs. 2

Both GnuPG v.1 and GnuPG v.2 implement the same standard, but they provide incompatible libraries and command-line tools, so many distributions ship both the legacy version 1 and the latest version 2. You need to make sure you are always using GnuPG v.2.

First, run:

$ gpg --version | head -n1

If you see gpg (GnuPG) 1.4.x, then you are using GnuPG v.1. Try the gpg2 command:

$ gpg2 --version | head -n1

If you see gpg (GnuPG) 2.x.x, then you are good to go. This guide will assume you have the version 2.2 of GnuPG (or later). If you are using version 2.0 of GnuPG, some of the commands in this guide will not work, and you should consider installing the latest 2.2 version of GnuPG.

Making sure you always use GnuPG v.2

If you have both gpg and gpg2 commands, you should make sure you are always using GnuPG v2, not the legacy version. You can make sure of this by setting the alias:

$ alias gpg=gpg2

You can put that in your .bashrc to make sure it's always loaded whenever you use the gpg commands.

Generating and protecting your master PGP key

Checklist

  • Generate the 4096-bit RSA master key (ESSENTIAL)
  • Back up the master key using paperkey (ESSENTIAL)
  • Add all relevant identities (ESSENTIAL)

Considerations

Understanding the "Master" (Certify) key

In this and next section we'll talk about the "master key" and "subkeys". It is important to understand the following:

  1. There are no technical differences between the "master key" and "subkeys."
  2. At creation time, we assign functional limitations to each key by giving it specific capabilities.
  3. A PGP key can have 4 capabilities.
    • [S] key can be used for signing
    • [E] key can be used for encryption
    • [A] key can be used for authentication
    • [C] key can be used for certifying other keys
  4. A single key may have multiple capabilities.

The key carrying the [C] (certify) capability is considered the "master" key because it is the only key that can be used to indicate relationship with other keys. Only the [C] key can be used to:

  • add or revoke other keys (subkeys) with S/E/A capabilities
  • add, change or revoke identities (uids) associated with the key
  • add or change the expiration date on itself or any subkey
  • sign other people's keys for the web of trust purposes

In the Free Software world, the [C] key is your digital identity. Once you create the key, you should take extra care to protect it and prevent it from falling into malicious hands.

Before you create the master key

Before you create your master key you need to pick your primary identity and your master passphrase.

Primary identity

An identity is basically in the same format as the "From" field in emails:

Alice Engineer <alice.engineer@example.org>

You can create new identities, revoke old ones, and change which identity is your "primary" one at any time. Since the primary identity is shown in all GnuPG operations, you should pick a name and address that are both professional and the most likely ones to be used for PGP-protected communication, such as your work address or the address you use for signing off on project commits.

Passphrase

The passphrase is used exclusively for encrypting the private key with a symmetric algorithm while it is stored on disk. If the contents of your .gnupg directory ever get leaked, a good passphrase is the last line of defense between the thief and them being able to impersonate you online, which is why it is important to set up a good passphrase.

A good guideline for a strong passphrase is 3-4 words from a rich or mixed dictionary that are not quotes from popular sources (songs, books, slogans). You'll be using this passphrase fairly frequently, so it should be both easy to type and easy to remember.

Algorithm and key strength

Even though GnuPG has had support for Elliptic Curve crypto for a while now, we'll be sticking to RSA keys, at least for a little while longer. While it is possible to start using ED25519 keys right now, it is likely that you will come across tools and hardware devices that will not be able to handle them correctly.

Generate the master key

To generate your new master key, issue the following command, putting in the right values instead of "Alice Engineer:"

$ gpg --quick-generate-key 'Alice Engineer <alice@example.org>' rsa4096 cert

A dialog will pop up asking to enter the passphrase. Then, you may need to move your mouse around or type on some keys to generate enough entropy until the command completes.

Review the output of the command, it will be something like this:

pub   rsa4096 2017-12-06 [C] [expires: 2019-12-06]
      111122223333444455556666AAAABBBBCCCCDDDD
uid                      Alice Engineer <alice@example.org>

Note the long string on the 2nd line -- that is the full fingerprint of your newly generated key. Key IDs can be represented in three different forms:

  • fingerprint, a full 40-character key identifier
  • long, last 16-characters of the fingerprint (AAAABBBBCCCCDDDD)
  • short, last 8 characters of the fingerprint (CCCCDDDD)

You should avoid using 8-character "short key IDs" as they are not sufficiently unique.

At this point, I suggest you open a text editor, copy the fingerprint of your new key and paste it there. You'll need to use it for the next few steps, so having it close by will be handy.

Back up your master key

For disaster recovery purposes -- and especially if you intend to use the Web of Trust and collect key signatures from other project developers -- you should create a hardcopy backup of your private key. This is supposed to be the "last resort" measure in case all other backup mechanisms have failed.

The best way to create a printable hardcopy of your private key is using the paperkey software written for this very purpose. Paperkey is available on all Linux distros, as well as installable via brew install paperkey on Macs.

Run the following command, replacing [fpr] with the full fingerprint of your key:

$ gpg --export-secret-key [fpr] | paperkey > /tmp/key-backup.txt

The output will be in a format that is easy to OCR or input by hand, should you ever need to recover it. Print out that file, then take a pen and write the key passphrase on the margin of the paper. This is a required step because the key printout is still encrypted with the passphrase, and if you ever change the passphrase on your key, you will not remember what it used to be when you had first created it -- guaranteed.

Put the resulting printout and the hand-written passphrase into an envelope and store in a secure and well-protected place, preferably away from your home, such as your bank vault.

NOTE ON PRINTERS: Long gone are days when printers were dumb devices connected to your computer's parallel port. These days they have full operating systems, hard drives, and cloud integration. Since the key content we send to the printer will be encrypted with the passphrase, this is a fairly safe operation, but use your best paranoid judgement.

Add relevant identities

If you have multiple relevant email addresses (personal, work, open-source project, etc), you should add them to your master key. You don't need to do this for any addresses that you don't expect to use with PGP (e.g. probably not your school alumni address).

The command is (put the full key fingerprint instead of [fpr]):

$ gpg --quick-add-uid [fpr] 'Alice Engineer <allie@example.net>'

You can review the IDs you've already added using:

$ gpg --list-key [fpr] | grep ^uid
Pick the primary UID

GnuPG will make the latest UID you add as your primary UID, so if that is different from what you want, you should fix it back:

$ gpg --quick-set-primary-uid [fpr] 'Alice Engineer <alice@example.org>'

Generating PGP subkeys

Checklist

  • Generate a 2048-bit Encryption subkey (ESSENTIAL)
  • Generate a 2048-bit Signing subkey (ESSENTIAL)
  • Generate a 2048-bit Authentication subkey (NICE)
  • Upload your public keys to a PGP keyserver (ESSENTIAL)

Considerations

Now that we've created the master key, let's create the keys you'll actually be using for day-to-day work. We create 2048-bit keys because a lot of specialized hardware (we'll discuss this further) does not handle larger keys, but also for pragmatic reasons. If we ever find ourselves in a world where 2048-bit RSA keys are not considered good enough, it will be because of fundamental problems with the RSA protocol and longer 4096-bit keys will not make much difference.

Create the subkeys

To create the subkeys, run:

$ gpg --quick-add-key [fpr] rsa2048 encr
$ gpg --quick-add-key [fpr] rsa2048 sign

You can also create the Authentication key, which will allow you to use your PGP key for ssh purposes (covered in other guides):

$ gpg --quick-add-key [fpr] rsa2048 auth

You can review your key information using gpg --list-key [fpr]:

pub   rsa4096 2017-12-06 [C] [expires: 2019-12-06]
      111122223333444455556666AAAABBBBCCCCDDDD
uid           [ultimate] Alice Engineer <alice@example.org>
uid           [ultimate] Alice Engineer <allie@example.net>
sub   rsa2048 2017-12-06 [E]
sub   rsa2048 2017-12-06 [S]

Upload your public keys to the keyserver

Your key creation is complete, so now you need to make it easier for others to find it by uploading it to one of the public keyservers. (Do not do this step if you're just messing around and aren't planning on actually using the key you've created, as this just litters keyservers with useless data.)

$ gpg --send-key [fpr]

If this command does not succeed, you can try specifying a keyserver on a port that is most likely to work:

$ gpg --keyserver hkp://pgp.mit.edu:80 --send-key [fpr]

Most keyservers communicate with each-other, so your key information will eventually synchronize to all the others.

NOTE ON PRIVACY: Keyservers are completely public and therefore, by design, leak potentially private information about you, such as your full name, nicknames, and personal or work email addresses. If you sign other people's keys or someone signs yours, keyservers will additionally become leakers of your social connections. Once such personal information makes it to the keyservers, it becomes impossible to edit or delete. Even if you revoke a signature or identity, that does not delete them from your key record, just marks them as revoked -- making them stand out even more.

That said, if you participate in software development on a public project, all of the above information is already public record, and therefore making it additionally available via a keyserver record does not result in net loss in privacy.

Upload your public key to GitHub

If you use GitHub in your development (and who doesn't?), you should upload your key following the instructions they have provided:

To generate the public key file to paste in, just run:

$ gpg --export --armor [fpr]

Moving your master key to offline storage

Checklist

  • Prepare encrypted detachable storage (ESSENTIAL)
  • Back up your GnuPG directory (ESSENTIAL)
  • Remove the master key from your home directory (NICE)
  • Remove the revocation certificate from your home directory (NICE)

Considerations

Why would you want to remove your master [C] key from your home directory? This is generally done to prevent your master key from being stolen or accidentally leaked. Private keys are tasty targets for malicious actors -- we know this from several successful malware attacks that scanned users' home directories and uploaded any private key content found there.

It would be very damaging to a developer to have their PGP keys stolen -- in the Free Software world this is often tantamount to identity theft. Removing private keys from your home directory helps protect you from such events.

Back up your GnuPG directory

!!!Do not skip this step!!!

It is important to have a readily available backup of your PGP keys should you need to recover them (this is different from the disaster-level preparedness we did with paperkey).

Prepare detachable encrypted storage

Start by getting a small USB "thumb" drive (preferably two!) that you will use for backup purposes. You will first need to encrypt them:

For the encryption passphrase, you can use the same one as on your master key.

Back up your GnuPG directory

Once the encryption process is over, re-insert the USB drive and make sure it gets properly mounted. Find out the full mount point of the device, for example by running the mount command (under Linux, external media usually gets mounted under /media/disk, under Mac it's /Volumes).

Once you know the full mount path, copy your entire GnuPG directory there:

$ cp -rp $HOME/.gnupg [/media/disk/name]/gnupg-backup

You should now test to make sure it still works:

$ gpg --homedir=[/media/disk/name]/gnupg-backup --list-key [fpr]

If you don't get any errors, then you should be good to go. Unmount the USB drive, distinctly label it so you don't blow it away next time you need to use a random USB drive, and put in a safe place -- but not too far away, because you'll need to use it every now and again for things like adding identities, extra subkeys, or adding signatures to other people's keys.

Remove the master key

Please see the previous section and make sure you have backed up your GnuPG directory in its entirety. What we are about to do will make your key useless if you do not have a usable backup!

First, identify the keygrip of your master key:

$ gpg --with-keygrip --list-key [fpr]

The output will be something like this:

pub   rsa4096 2017-12-06 [C] [expires: 2019-12-06]
      111122223333444455556666AAAABBBBCCCCDDDD
      Keygrip = AAAA999988887777666655554444333322221111
uid           [ultimate] Alice Engineer <alice@example.org>
uid           [ultimate] Alice Engineer <allie@example.net>
sub   rsa2048 2017-12-06 [E]
      Keygrip = BBBB999988887777666655554444333322221111
sub   rsa2048 2017-12-06 [S]
      Keygrip = CCCC999988887777666655554444333322221111

Find the keygrip entry that is beneath the pub line (right under the master key fingerprint). This will correspond directly to a file in your home .gnupg directory:

$ cd ~/.gnupg/private-keys-v1.d
$ ls
AAAA999988887777666655554444333322221111.key
BBBB999988887777666655554444333322221111.key
CCCC999988887777666655554444333322221111.key

All you have to do is simply remove the .key file that corresponds to the master keygrip:

$ cd ~/.gnupg/private-keys-v1.d
$ rm AAAA999988887777666655554444333322221111.key

If you issue the --list-secret-keys command, it will show that the master key is missing (the # indicates it is not available):

$ gpg --list-secret-keys
sec#  rsa4096 2017-12-06 [C] [expires: 2019-12-06]
      111122223333444455556666AAAABBBBCCCCDDDD
uid           [ultimate] Alice Engineer <alice@example.org>
uid           [ultimate] Alice Engineer <allie@example.net>
ssb   rsa2048 2017-12-06 [E]
ssb   rsa2048 2017-12-06 [S]

Remove the revocation certificate

Another file you should remove (but keep in backups) is the revocation certificate that was automatically created with your master key. A revocation certificate allows someone to permanently mark your key as revoked, meaning it can no longer be used or trusted for any purpose. You would normally use it to revoke a key that, for some reason, you can no longer control -- for example, if you had lost the passphrase.

Just as with the master key, if a revocation certificate leaks into malicious hands, it can be used to destroy your developer digital identity, so it's better to remove it from your home directory.

cd ~/.gnupg/openpgp-revocs.d
rm [fpr].rev

Move the subkeys to a hardware device

Checklist

  • Get a GnuPG-compatible hardware device (NICE)
  • Configure the device to work with GnuPG (NICE)
  • Set the user and admin PINs (NICE)
  • Move your subkeys to the device (NICE)

Considerations

Even though the master key is now safe from being leaked or stolen, the subkeys are still in the home directory. Anyone who manages to get their hands on those will be able to decrypt your communication or fake your signatures (if they know the passphrase, that is).

The best way to completely protect your keys is to move them to a specialized hardware device that is capable of smartcard operations.

The benefits of smartcards

A smartcard contains a cryptographic chip that is capable of storing private keys and performing crypto operations directly on the card itself. Because the key contents never leave the smartcard, the operating system of the computer into which you plug in the hardware device is not able to retrieve the private keys themselves. This is very different from the encrypted USB storage device we used earlier for backup purposes -- while that USB device is plugged in and decrypted, the operating system is still able to access the private key contents. Using external encrypted USB media is not a substitute to having a smartcard-capable device.

Some other benefits of smartcards:

  • they are relatively cheap and easy to obtain
  • they are small and easy to carry with you
  • they can be used with multiple devices
  • many of them are tamper-resistant (depends on manufacturer)

Available smartcard devices

Smartcards started out embedded into actual wallet-sized cards, which earned them their name. You can still buy and use GnuPG-capable smartcards, and they remain one of the cheapest available devices you can get. However, actual smartcards have one important downside: they require a smartcard reader, and very few laptops come with one.

For this reason, manufacturers have started providing small USB devices, the size of a USB thumb drive or smaller, that either have the microsim-sized smartcard pre-inserted, or that simply implement the smartcard protocol features on the internal chip. Here are a few recommendations:

  • Nitrokey Start: Open hardware and Free Software: one of the cheapest options for GnuPG use, but with fewest extra security features
  • Nitrokey Pro: Similar to the Nitrokey Start, but is tamper-resistant and offers more security features (see the U2F section of the guide)
  • Yubikey 4: Proprietary hardware and software, but cheaper than Nitrokey Pro and comes available in the USB-C form that is more useful with newer laptops; also offers additional security features such as U2F

Our recommendation is to pick a device that is capable of both smartcard functionality and U2F, which means either a Nitrokey Pro, or a Yubikey 4.

Configuring your smartcard device

Your smartcard device should Just Work (TM) the moment you plug it into any modern Linux or Mac workstation. You can verify it by running:

$ gpg --card-status

If you didn't get an error, but a full listing of the card details, then you are good to go. Unfortunately, troubleshooting all possible reasons why things may not be working for you is way beyond the scope of this guide. If you are having trouble getting the card to work with GnuPG, please seek support via your operating system's usual support channels.

PINs don't have to be numbers

Note, that despite having the name "PIN" (and implying that it must be a "number"), neither the user PIN nor the admin PIN on the card need to be numbers.

Your device will probably have default user and admin PINs set up when it arrives. For Yubikeys, these are 123456 and 12345678 respectively. If those don't work for you, please check any accompanying documentation that came with your device.

Quick setup

To configure your smartcard, you will need to use the GnuPG menu system, as there are no convenient command-line switches:

$ gpg --card-edit
...
gpg/card> admin
Admin commands are allowed
gpg/card> passwd

You should set the user PIN (1), Admin PIN (3), and the Reset Code (4). Please make sure to record and store these in a safe place -- especially the Admin PIN and the Reset Code (which allows you to completely wipe the smartcard). You so rarely need to use the Admin PIN, that you will inevitably forget what it is if you do not record it.

Getting back to the main card menu, you can also set other values (such as name, sex, login data, etc, but it's not necessary and will additionally leak information about your smartcard should you lose it).

Moving the subkeys to your smartcard

Exit the card menu (using "q") and save all changes. Next, let's move your subkeys onto the smartcard. You will need both your key passphrase and the admin PIN of the card for most operations. Remember, that [fpr] stands for the full 40-character fingerprint of your key.

$ gpg --edit-key [fpr]

Secret subkeys are available.

pub  rsa4096/AAAABBBBCCCCDDDD
     created: 2017-12-07  expires: 2019-12-07  usage: C
     trust: ultimate      validity: ultimate
ssb  rsa2048/1111222233334444
     created: 2017-12-07  expires: never       usage: E
ssb  rsa2048/5555666677778888
     created: 2017-12-07  expires: never       usage: S
[ultimate] (1). Alice Engineer <alice@example.org>
[ultimate] (2)  Alice Engineer <allie@example.net>

gpg>

Using --edit-key puts us into the menu mode again, and you will notice that the key listing is a little different. From here on, all commands are done from inside this menu mode, as indicated by gpg>.

First, let's select the key we'll be putting onto the card -- you do this by typing key 1 (it's the first one in the listing, our [E] subkey):

gpg> key 1

The output should be subtly different:

pub  rsa4096/AAAABBBBCCCCDDDD
     created: 2017-12-07  expires: 2019-12-07  usage: C
     trust: ultimate      validity: ultimate
ssb* rsa2048/1111222233334444
     created: 2017-12-07  expires: never       usage: E
ssb  rsa2048/5555666677778888
     created: 2017-12-07  expires: never       usage: S
[ultimate] (1). Alice Engineer <alice@example.org>
[ultimate] (2)  Alice Engineer <allie@example.net>

Notice the * that is next to the ssb line corresponding to the key -- it indicates that the key is currently "selected". It works as a toggle, meaning that if you type key 1 again, the * will disappear and the key will not be selected any more.

Now, let's move that key onto the smartcard:

gpg> keytocard
Please select where to store the key:
   (2) Encryption key
Your selection? 2

Since it's our [E] key, it makes sense to put it into the Encryption slot. When you submit your selection, you will be prompted first for your key passphrase, and then for the admin PIN. If the command returns without an error, your key has been moved.

Important: Now type key 1 again to unselect the first key, and key 2 to select the [S] key:

gpg> key 1
gpg> key 2
gpg> keytocard
Please select where to store the key:
   (1) Signature key
   (3) Authentication key
Your selection? 1

You can use the [S] key both for Signature and Authentication, but we want to make sure it's in the Signature slot, so choose (1). Once again, if your command returns without an error, then the operation was successful.

Finally, if you created an [A] key, you can move it to the card as well, making sure first to unselect key 2. Once you're done, choose "q":

gpg> q
Save changes? (y/N) y

Saving the changes will remove the keys you moved to the card from your home directory (but it's okay, because we have them in our backups should we need to do this again for a replacement smartcard).

Verifying that the keys were moved

If you perform --list-secret-keys now, you will see a subtle difference in the output:

$ gpg --list-secret-keys
sec#  rsa4096 2017-12-06 [C] [expires: 2019-12-06]
      111122223333444455556666AAAABBBBCCCCDDDD
uid           [ultimate] Alice Engineer <alice@example.org>
uid           [ultimate] Alice Engineer <allie@example.net>
ssb>  rsa2048 2017-12-06 [E]
ssb>  rsa2048 2017-12-06 [S]

The > in the ssb> output indicates that the subkey is only available on a smartcard. If you go back into your secret keys directory and look at the contents there, you will notice that the .key files there have been replaced with stubs:

$ cd ~/.gnupg/private-keys-v1.d
$ strings *.key

The output should contain shadowed-private-key to indicate that these files are only stubs and the actual content is on the smartcard.

Verifying that the smartcard is functioning

To verify that the smartcard is working as intended, you can create a signature:

$ echo "Hello world" | gpg --clearsign > /tmp/test.asc
$ gpg --verify /tmp/test.asc

This should ask for your smartcard PIN on your first command, and then show "Good signature" after you run gpg --verify.

Congratulations, you have successfully made it extremely difficult to steal your digital developer identity!

TODO: Extending expiration date

TODO: Revoking subkeys

TODO: Configure gpg-agent

TODO: Configure TOFU policy

Using PGP with Git

Checklist

  • Understand signed tags, commits, and pushes (ESSENTIAL)
  • Configure git to use your key (ESSENTIAL)
  • Learn how tag signing and verification works (ESSENTIAL)
  • Configure git to always sign annotated tags (NICE)
  • Learn how commit signing and verification works (ESSENTIAL)
  • Configure git to always sign commits (NICE)

Considerations

Git implements multiple levels of integration with PGP, first starting with signed tags, then introducing signed commits, and finally adding support for signed pushes.

Understanding Git Hashes

Git is a complicated beast, but you need to know what a "hash" is in order to have a good grasp on how PGP integrates with it. We'll narrow it down to two kinds of hashes: tree hashes and commit hashes.

Tree hashes

Every time you commit a change to a repository, git calculates checksum hashes of all objects in it -- contents (blobs), directories (trees), file names and permissions, etc, for each subdirectory in the repository. It only does this for trees and blobs that have changed, so as not to re-checksum the entire tree unnecessarily if only a small part of it was touched.

Then it calculates the checksum of the toplevel directory, which will inevitably be different if any part of the repository has changed.

Commit hashes

Once the tree hash has been created, git will calculate the commit hash, which will list the following information about the repository and the change being made:

  • the checksum hash of the tree
  • the checksum hash of the tree before the change (parent)
  • information about the author (name, email, time of authorship)
  • information about the committer (name, email, time of commit)
  • the commit message
Hashing function

At the time of writing, git uses the SHA1 hashing mechanism to calculate checksums, though work is under way to transition to a stronger algorithm that is more resistant to collisions. Note, that git already includes collision avoidance routines, so it is believed that a successful collision attack against git remains impractical.

Annotated tags and tag signatures

Git tags allow developers to mark specific commits in the history of each git repository. Tags can be lightweight that are more or less just a pointer at a specific commit, or they can be annotated, which becomes its own object in the git tree. An annotated tag object contains all of the following information:

  • the checksum hash of the commit being tagged
  • the tag name
  • the information about the tagger (name, email, time of tagging)
  • the tag message

A PGP-signed tag is simply an annotated tag with all these contents wrapped around in a PGP signature. When a developer signs their git tag, they effectively assure you of the following:

  • who they are (and why you should trust them)
  • what the state of their repository was at the time of signing:
    • the tag includes the hash of the commit
      • the commit hash includes the hash of the toplevel tree
        • which includes hashes of all files and subtrees
      • it also includes all information about authorship
      • including exact times when changes were made

When you clone a git repository and verify a signed tag, this gives you assurances that all contents in the repositry are exactly the same as the contents of the repository on the developer's computer at the time of signing.

Signed commits

Signed commits are very similar to signed tags, except that the contents of the commit object are PGP-signed instead of the contents of the tag object. A commit signature also gives you full verifiable information about the state of the developer's tree at the time the signature was made.

Signed pushes

This is included here for completeness' sake, since this functionality needs to be enabled on the server receiving the push before it does anything useful. As we saw above, PGP-signing a git object gives verifiable information about the developer's git tree, but not about their intent for that tree.

For example, you can be working on an experimental branch in your repository trying out a promising cool feature, but after you submit your work for review, someone finds a nasty bug in your code. Since your commits are properly signed, someone can take the branch containing your nasty bug and push it into master, introducing a vulnerability that was never intended to be in production. Since the commit is properly signed with your key, everything looks legitimate and your reputation is questioned when the bug is discovered.

Ability to enforce PGP-signatures during git push was added in order to enforce the intent of the commit, and not merely certify what the commit is.

Configure git to use your PGP key

If you only have one secret key in your keyring, then you don't really need to do anything extra, as it becomes your default key.

However, if you happen to have multiple keys, you can tell git which key should be used ([fpr] is the fingerprint of your key):

$ git config --global user.signingKey [fpr]

NOTE: If you have a distinct gpg2 command, then you should tell git to always use it instead of the legacy gpg from version 1:

$ git config --global gpg.program gpg2

How to work with signed tags

To create a signed tag, simply pass the -s switch to the tag command:

$ git tag -s [tagname]

Our recommendation is to always sign git tags, as this allows other developers to ensure that the git repository they are working with has not been maliciously altered (e.g. to introduce backdoors).

How to verify signed tags

To verify a signed tag, simply pass the -v switch to the tag command:

$ git tag -v [tagname]

If you are verifying someone else's git tag, then you will need to import their PGP key. Please refer to the "maintaining the project keyring" section below.

Verifying at pull time

If you are pulling a tag from another fork of the project repository, git should automatically verify the signature at the tip you're pulling and show you the results during the merge operation:

$ git pull [url] tags/sometag

The merge message will contain something like this:

Merge tag 'sometag' of [url]

[Tag message]

# gpg: Signature made [...]
# gpg: Good signature from [...]

Configure git to always sign annotated tags

Chances are, if you're creating an annotated tag, you'll want to sign it. To force git to always sign annotated tags, you can set a global configuration option:

$ git config --global tag.forceSignAnnotated true

Alternatively, you can just train your muscle memory to always pass the -s switch:

$ git tag -asm "Tag message" tagname

How to work with signed commits

It is easy to create signed commits, but it is much more difficult to incorporate them into your workflow. Most projects use signed commits as a cryptographically-verifiable "Committed-by:" line that records code provenance -- the commits are rarely verified by others except when tracking down project history.

To create a signed commit, you just need to pass the -S flag to the git commit command:

$ git commit -S

Our recommendation is to always sign commits and to require them of all project members, regardless of whether anyone is verifying them.

How to verify signed commits

To verify a single commit you can use verify-commit:

$ git verify-commit [hash]

You can also look at the repository log and request that all commit signatures are verified and shown:

$ git log --pretty=short --show-signatures
Verifying commits during git merge

If all members of your project sign their commits, you can enforce signature checking at merge time (and then sign the resulting merge commit itself using the -S flag):

$ git merge --verify-signatures -S merged-branch

Note, that this will break if there is even one commit that is not signed or does not pass verification. As it is often the case, technology is the easy part here, but the human side of the equation is what makes it difficult.

If your project uses mailing lists for patch management

If your project uses a mailing list for submitting and processing patches, then there is little use in signing commits, because all signature information will be lost when sent through that medium. It is still useful to sign your commits, just so others can refer to your publicly hosted git trees for reference, but the upstream developer will not benefit from it in a direct way.

You can still sign the emails containing the patches, though.