itpol/protecting-code-integrity.md

1438 lines
60 KiB
Markdown

# Protecting code integrity with PGP
Updated: 2021-05-13
*Status: CURRENT*
### Table of contents
- [Protecting code integrity with PGP](#protecting-code-integrity-with-pgp)
- [Table of contents](#table-of-contents)
- [Target audience](#target-audience)
- [Structure](#structure)
- [Checklist priority levels](#checklist-priority-levels)
- [Basic PGP concepts and tools](#basic-pgp-concepts-and-tools)
- [Checklist](#checklist)
- [Considerations](#considerations)
- [Extremely Basic Overview of PGP operations](#extremely-basic-overview-of-pgp-operations)
- [Encryption](#encryption)
- [Signatures](#signatures)
- [Combined usage](#combined-usage)
- [Understanding Key Identities](#understanding-key-identities)
- [Understanding Key Validity](#understanding-key-validity)
- [Web of Trust (WOT) vs. Trust on First Use (TOFU)](#web-of-trust-wot-vs-trust-on-first-use-tofu)
- [Installing OpenPGP software](#installing-openpgp-software)
- [Installing GnuPG](#installing-gnupg)
- [Generating and protecting your certification key](#generating-and-protecting-your-certification-key)
- [Checklist](#checklist-1)
- [Considerations](#considerations-1)
- [Understanding the certification key](#understanding-the-certification-key)
- [Before you create the certification key](#before-you-create-the-certification-key)
- [Primary identity](#primary-identity)
- [Passphrase](#passphrase)
- [Algorithm and key strength](#algorithm-and-key-strength)
- [Generate the certification key](#generate-the-certification-key)
- [Back up your certification key](#back-up-your-certification-key)
- [Add relevant identities](#add-relevant-identities)
- [Pick the primary UID](#pick-the-primary-uid)
- [Generating PGP subkeys](#generating-pgp-subkeys)
- [Checklist](#checklist-2)
- [Considerations](#considerations-2)
- [Create the subkeys](#create-the-subkeys)
- [Upload your public key to GitHub](#upload-your-public-key-to-github)
- [Moving your certification key to offline storage](#moving-your-certification-key-to-offline-storage)
- [Checklist](#checklist-3)
- [Considerations](#considerations-3)
- [Back up your GnuPG directory](#back-up-your-gnupg-directory)
- [Prepare detachable encrypted storage](#prepare-detachable-encrypted-storage)
- [Back up your GnuPG directory](#back-up-your-gnupg-directory-1)
- [Remove the certification key](#remove-the-certification-key)
- [Removing your certification key](#removing-your-certification-key)
- [Remove the revocation certificate](#remove-the-revocation-certificate)
- [Move the subkeys to a hardware device](#move-the-subkeys-to-a-hardware-device)
- [Checklist](#checklist-4)
- [Considerations](#considerations-4)
- [The benefits of smartcards](#the-benefits-of-smartcards)
- [Available smartcard devices](#available-smartcard-devices)
- [Configuring your smartcard device](#configuring-your-smartcard-device)
- [PINs don't have to be numbers](#pins-dont-have-to-be-numbers)
- [Quick setup](#quick-setup)
- [Moving the subkeys to your smartcard](#moving-the-subkeys-to-your-smartcard)
- [Verifying that the keys were moved](#verifying-that-the-keys-were-moved)
- [Verifying that the smartcard is functioning](#verifying-that-the-smartcard-is-functioning)
- [Other common GnuPG operations](#other-common-gnupg-operations)
- [Mounting your offline storage](#mounting-your-offline-storage)
- [Updating your regular GnuPG working directory](#updating-your-regular-gnupg-working-directory)
- [Extending key expiration date](#extending-key-expiration-date)
- [Revoking identities](#revoking-identities)
- [Using PGP with Git](#using-pgp-with-git)
- [Checklist](#checklist-5)
- [Considerations](#considerations-5)
- [Understanding Git Hashes](#understanding-git-hashes)
- [Tree hashes](#tree-hashes)
- [Commit hashes](#commit-hashes)
- [Hashing function](#hashing-function)
- [Annotated tags and tag signatures](#annotated-tags-and-tag-signatures)
- [Signed commits](#signed-commits)
- [Configure git to use your PGP key](#configure-git-to-use-your-pgp-key)
- [How to work with signed tags](#how-to-work-with-signed-tags)
- [How to verify signed tags](#how-to-verify-signed-tags)
- [Verifying at pull time](#verifying-at-pull-time)
- [Configure git to always sign annotated tags](#configure-git-to-always-sign-annotated-tags)
- [How to work with signed commits](#how-to-work-with-signed-commits)
- [How to verify signed commits](#how-to-verify-signed-commits)
- [Verifying commits during git merge](#verifying-commits-during-git-merge)
- [If your project uses mailing lists for patch management](#if-your-project-uses-mailing-lists-for-patch-management)
- [Configure git to always sign commits](#configure-git-to-always-sign-commits)
- [Configure gpg-agent options](#configure-gpg-agent-options)
- [Bonus: Using gpg-agent with ssh](#bonus-using-gpg-agent-with-ssh)
- [Protecting online accounts](#protecting-online-accounts)
- [Checklist](#checklist-6)
- [Considerations](#considerations-6)
- [Two-factor authentication with Fido U2F](#two-factor-authentication-with-fido-u2f)
- [Get a token capable of Fido U2F](#get-a-token-capable-of-fido-u2f)
- [Enable 2-factor authentication on your online accounts](#enable-2-factor-authentication-on-your-online-accounts)
- [Configure TOTP failover, if possible](#configure-totp-failover-if-possible)
- [Further reading](#further-reading)
### Target audience
This document is aimed at developers working on free software projects. It
covers the following topics:
1. PGP basics and best practices
2. How to use PGP with Git
3. How to protect your developer accounts
We use the term "Free" as in "Freedom," but this guide can also be used for
any other kind of software that relies on contributions from a distributed
team of developers. If you write code that goes into public source
repositories, you can benefit from getting acquainted with and following this
guide.
### 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.
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 that session key (using a symmetric
cipher)
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: unknown** -- 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](https://gpgtools.org) or you can use `brew
install gnupg2`. On a Windows PC, you should install
[GPG4Win](https://www.gpg4win.org), and you will probably need to adjust some
of the commands in the guide to work for you, unless you have a unix-like
environment set up. For all other platforms, you'll need to do your own
research to find the correct places to download and install GnuPG.
## Generating and protecting your certification key
### Checklist
- [ ] Generate a 4096-bit RSA certification key _(ESSENTIAL)_
- [ ] Back up the certification key using paperkey _(ESSENTIAL)_
- [ ] Add all relevant identities _(ESSENTIAL)_
### Considerations
#### Understanding the certification key
In this and next section we'll talk about the certification key and subkeys.
The certification key is often called "the master key," but it is a poor
analogy, as it doesn't act in any way like a physical master key (in the sense
that it cannot decrypt content encrypted to any of the subkeys, as one
example). For this reason, we'll stick to calling it the "certification key."
It is important to understand the following:
1. There are no technical differences between the certify key and any of the
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 used to indicate
relationship with other PGP 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 that key, you should take extra care to protect it and prevent it from
falling into malicious hands.
#### Before you create the certify key
Before you create your certify key you need to pick your primary identity and
your passphrase.
##### Primary identity
Identities are strings using 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
GnuPG supports many algorithms, but we will only consider the below two:
- RSA for the certification key
- ECC (Elliptic Curve) for all other subkeys
We use RSA for the certification key mainly for compatibility reasons -- there
is probably still some tooling that does not properly handle ECC keys, so
sticking with RSA for the certification key makes sense. We will use it only
very occasionally, so the slowness/size considerations are unimportant.
All of the day-to-day work will be done using subkeys, so picking ECC makes
perfect sense there -- it will be faster and the resulting signatures will be
dramatically smaller.
#### Generate the certification key
To generate your new certification 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 2021-05-01 [C] [expires: 2023-05-01]
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 certification 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 -o /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 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 UIDs 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 the Encryption subkey _(ESSENTIAL)_
- [ ] Generate the Signing subkey _(ESSENTIAL)_
- [ ] Generate the Authentication subkey _(NICE)_
- [ ] Upload your public keys to a PGP keyserver _(NICE)_
- [ ] Set up a refresh cronjob _(NICE)_
### Considerations
Now that we've created the certification key, let's create the keys you'll
actually be using for day-to-day work. Before we do this, we need to pick the
ECC algorithm flavor.
#### ED25519 or NIST?
We won't go into the reasons behind why ECC is split into two camps. All you
need to know is that many people consider ed25519 "more pure" due to the way
its underlying curve primitives were selected and NIST "less pure" because
that selection process was not as public or thorough.
Do you plan to use a hardware device like a Yubikey for storing your subkeys?
Then pick NIST (use "nistp256" instead of "cv25519" and "ed25519" below). If
you just plan to store your subkeys on your computer, then pick ED25519 (the
GnuPG default).
Since you can revoke subkeys and create new ones at any time, this is not a
life or death kind of decision. If in doubt, pick ed25519.
#### Create the subkeys
To create the subkeys, run:
$ gpg --quick-add-key [fpr] cv25519 encr
$ gpg --quick-add-key [fpr] ed25519 sign
You can also create the Authentication key, which will allow you to use your
PGP key for ssh purposes:
$ gpg --quick-add-key [fpr] ed25519 auth
You can review your key information using `gpg --list-key [fpr]`:
pub rsa4096 2021-05-01 [C] [expires: 2023-05-01]
111122223333444455556666AAAABBBBCCCCDDDD
uid [ultimate] Alice Engineer <alice@example.org>
uid [ultimate] Alice Engineer <allie@example.net>
sub cv25519 2021-05-01 [E]
sub ed25519 2021-05-01 [S]
#### Upload your public key to GitHub
If you use GitHub in your development, you should upload your key following
the instructions they have provided:
- [Adding a PGP key to your GitHub account](https://help.github.com/articles/adding-a-new-gpg-key-to-your-github-account/)
To generate the public key output suitable to paste in, just run:
$ gpg --export --armor [fpr]
#### Upload your public key to keys.openpgp.org
To make it easier for others to find your public key, you can upload it to the
keys.openpgp.org keyserver. Please follow the instructions provided here:
- [Upload to keys.openpgp.org](https://keys.openpgp.org/about/usage#gnupg-upload)
## Moving your certification key to offline storage
### Checklist
- [ ] Prepare encrypted detachable storage _(ESSENTIAL)_
- [ ] Back up your GnuPG directory _(ESSENTIAL)_
- [ ] Remove the certification key from your home directory _(NICE)_
- [ ] Remove the revocation certificate from your home directory _(NICE)_
### Considerations
Why would you want to remove your certification (**[C]**) key from your home
directory? This is generally done to prevent it 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 for any developer to have their PGP keys stolen --
in the Free Software world this is basically equal 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:
- [Apple instructions](https://support.apple.com/kb/PH25745)
- [Linux instructions](https://help.ubuntu.com/community/EncryptedFilesystemsOnRemovableStorage)
For the encryption passphrase, you can use the same one as on your private
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 ~/.gnupg [/media/disk/name]/gnupg-backup
(Note: If you get any `Operation not supported on socket` errors, those are
benign and you can ignore them.)
You should now test to make sure everything 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 editing identities,
adding or revoking subkeys, or signing other people's keys.
#### Remove the certification key
The files in our home directory are not as well protected as we like to think.
They can be leaked or stolen via many different means:
- by accident when making quick homedir copies to set up a new workstation
- by systems administrator negligence or malice
- via poorly secured backups
- via malware in desktop apps (browsers, pdf viewers, etc)
- via coercion when crossing international borders
Protecting your key with a good passphrase greatly helps reduce the risk of
any of the above, but passphrases can be discovered via keyloggers,
shoulder-surfing, or any number of other means. For this reason, the
recommended setup is to remove your certification key from your home directory
and store it on offline storage.
##### Removing your certification 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 render your key
useless if you do not have a usable backup!
First, identify the "keygrip" of your certification key:
$ gpg --with-keygrip --list-key [fpr]
The output will be something like this:
pub rsa4096 2021-05-01 [C] [expires: 2023-05-01]
111122223333444455556666AAAABBBBCCCCDDDD
Keygrip = AAAA999988887777666655554444333322221111
uid [ultimate] Alice Engineer <alice@example.org>
uid [ultimate] Alice Engineer <allie@example.net>
sub cv25519 2021-05-01 [E]
Keygrip = BBBB999988887777666655554444333322221111
sub ed25519 2021-05-01 [S]
Keygrip = CCCC999988887777666655554444333322221111
Find the keygrip entry that is beneath the `pub` line (right under the public
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
certification keygrip:
$ cd ~/.gnupg/private-keys-v1.d
$ rm AAAA999988887777666655554444333322221111.key
Now, if you issue the `--list-secret-keys` command, it will show that the
**[C]** key is not present (indicated by the `#` character):
$ gpg --list-secret-keys
sec# rsa4096 2021-05-01 [C] [expires: 2023-05-01]
111122223333444455556666AAAABBBBCCCCDDDD
uid [ultimate] Alice Engineer <alice@example.org>
uid [ultimate] Alice Engineer <allie@example.net>
ssb cv25519 2021-05-01 [E]
ssb ed25519 2021-05-01 [S]
#### Remove the revocation certificate
Another file you should remove (but keep in backups) is the revocation
certificate that was automatically created with your certification 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 key passphrase.
Just as with the certification 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 certification key is now safe from being leaked or stolen, the
subkeys are still in your 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). Furthermore, each time a GnuPG
operation is performed, the keys are loaded into system memory and can be
stolen from there by sufficiently advanced malware (think Meltdown and
Spectre).
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](https://shop.nitrokey.com/shop/product/nitrokey-start-6):
Open hardware and Free Software: one of the cheapest options for GnuPG use,
but with fewest extra security features.
- [Nitrokey Pro](https://shop.nitrokey.com/shop/product/nitrokey-pro-3):
Similar to the Nitrokey Start, but is tamper-resistant and offers more
security features (but not U2F, see the Fido U2F section of the guide); only
supports NIST ECC cryptography.
- [Yubikey](https://www.yubico.com/): 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; only supports NIST ECC cryptography.
If you want to use ED25519 subkeys, then your only choice is a Nitrokey Start,
though once Nitrokey 3 Pro is available, it should also be considered.
#### 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
[...omitted...]
gpg/card> admin
Admin commands are allowed
gpg/card> passwd
You should set the user PIN (1) and Admin PIN (3). Please make sure to record
and store these in a safe place -- especially the Admin PIN. 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 PGP 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: 2021-05-01 expires: 2023-05-01 usage: C
trust: ultimate validity: ultimate
ssb cv25519/1111222233334444
created: 2021-05-01 expires: never usage: E
ssb ed25519/5555666677778888
created: 2021-05-01 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: 2021-05-01 expires: 2023-05-01 usage: C
trust: ultimate validity: ultimate
ssb* cv25519/1111222233334444
created: 2021-05-01 expires: never usage: E
ssb ed25519/5555666677778888
created: 2021-05-01 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 PGP 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 delete 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 2021-05-01 [C] [expires: 2023-05-01]
111122223333444455556666AAAABBBBCCCCDDDD
uid [ultimate] Alice Engineer <alice@example.org>
uid [ultimate] Alice Engineer <allie@example.net>
ssb> cv25519 2021-05-01 [E]
ssb> ed25519 2021-05-01 [S]
The `>` in the `ssb>` output indicates that the subkey is only available on
the 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!
### Other common GnuPG operations
Here is a quick reference for some common operations you'll need to do with
your PGP key.
In all of the below commands, the `[fpr]` is your key fingerprint.
#### Mounting your offline storage
You will need your certification key for any of the operations below, so you
will first need to mount your backup offline storage and tell GnuPG to use it.
First, find out where the media got mounted, e.g. by looking at the output of
the `mount` command. Then, locate the directory with the backup of your GnuPG
directory and tell GnuPG to use that as its home:
$ export GNUPGHOME=/media/disk/name/gnupg-backup
$ gpg --list-secret-keys
You want to make sure that you see `sec` and not `sec#` in the output (the `#`
means the key is not available and you're still using your regular home
directory location).
##### Updating your regular GnuPG working directory
After you make any changes to your key using the offline storage, you will
want to import these changes back into your regular working directory:
$ gpg --export | gpg --homedir ~/.gnupg --import
$ unset GNUPGHOME
#### Extending key expiration date
The certification key we created has the default expiration date of 2 years
from the date of creation. This is done both for security reasons and to make
obsolete keys eventually disappear from keyservers.
To extend the expiration on your key by a year from current date, just run:
$ gpg --quick-set-expire [fpr] 1y
You can also use a specific date if that is easier to remember (e.g. your
birthday, January 1st, or Canada Day, 2030):
$ gpg --quick-set-expire [fpr] 2030-07-01
Remember to send the updated key back to keyservers:
$ gpg --send-key [fpr]
#### Revoking identities
If you need to revoke an identity (e.g. you changed employers and your old
email address is no longer valid), you can use a one-liner:
$ gpg --quick-revoke-uid [fpr] 'Alice Engineer <aengineer@example.net>'
You can also do the same with the menu mode using `gpg --edit-key [fpr]`.
Once you are done, remember to send the updated key back to keyservers:
$ gpg --send-key [fpr]
## Using PGP with Git
One of the core features of Git is its decentralized nature -- once a
repository is cloned to your system, you have full history of the project,
including all of its tags, commits and branches. However, with hundreds of
cloned repositories floating around, how does anyone verify that the
repository you downloaded has not been tampered with by a malicious third
party? You may have cloned it from GitHub or some other official-looking
location, but what if someone had managed to trick you?
Or what happens if a backdoor is discovered in one of the projects you've
worked on, and the "Author" line in the commit says it was done by you, while
you're pretty sure you had [nothing to do with it](https://github.com/jayphelps/git-blame-someone-else)?
To address both of these issues, Git introduced PGP integration. Signed tags
prove the repository integrity by assuring that its contents are exactly the
same as on the workstation of the developer who created the tag, while signed
commits make it nearly impossible for someone to impersonate you without
having access to your PGP keys.
### Checklist
- [ ] Understand signed tags, commits _(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)_
- [ ] Configure gpg-agent options _(ESSENTIAL)_
### Considerations
Git implements multiple levels of integration with PGP, first starting with
signed tags, and then introducing signed commits.
#### 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 records 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 with each commit, so as not to
re-checksum the entire tree unnecessarily if only a small part of it was
touched.
Then it calculates and stores the checksum of the toplevel tree, 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 include 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 still 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" -- 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
- information about the tagger (name, email, time of tagging)
- the tag message
A PGP-signed tag is simply an annotated tag with all these entries 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, contents, 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, that gives you
cryptographic assurance that _all contents in the repository, including all of
its history, 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 -- 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. Tag signatures and commit
PGP signatures provide exact same security assurances about the repository and
its entire history.
#### 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 secret keys, you can tell git which
key should be used (`[fpr]` is the fingerprint of your key):
$ git config --global user.signingKey [fpr]
#### 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. in order to introduce backdoors).
##### How to verify signed tags
To verify a signed tag, simply use the `verify-tag` command:
$ git verify-tag [tagname]
If you are verifying someone else's git tag, then you will need to import
their PGP key. Please refer to the "Trusted Team communication" document in
the same repository for guidance on this topic.
##### 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. Many projects use signed commits as a
sort of "Committed-by:" line equivalent that records code provenance -- the
signatures are rarely verified by others except when tracking down project
history. In a sense, signed commits are used for "tamper evidence," and not to
"tamper-proof" the git workflow.
To create a signed commit, you just need to pass the `-S` flag to the `git
commit` command (it's capital `-S` due to collision with another flag):
$ 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 (that can
always come at a later time).
##### How to verify signed commits
To verify a single commit you can use `verify-commit`:
$ git verify-commit [hash]
You can also look at repository logs and request that all commit signatures
are verified and shown:
$ git log --pretty=short --show-signature
##### 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 the merge will fail 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 -- the human side of the equation is what makes adopting strict commit
signing for your project 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 project receiving your patches will not be able to
verify them directly with git.
You can still sign the emails containing the patches, though.
#### Configure git to always sign commits
You can tell git to always sign commits:
git config --global commit.gpgSign true
Or you can train your muscle memory to always pass the `-S` flag to all `git
commit` operations (this includes `--amend`).
#### Configure gpg-agent options
The GnuPG agent is a helper tool that will start automatically whenever you
use the `gpg` command and run in the background with the purpose of caching
the private key passphrase. This way you only have to unlock your key once to
use it repeatedly (very handy if you need to sign a bunch of git operations in
an automated script without having to continuously retype your passphrase).
There are two options you should know in order to tweak when the passphrase
should be expired from cache:
- `default-cache-ttl` (seconds): If you use the same key again before the
time-to-live expires, the countdown will reset for another period.
The default is 600 (10 minutes).
- `max-cache-ttl` (seconds): Regardless of how recently you've used the key
since initial passphrase entry, if the maximum time-to-live countdown
expires, you'll have to enter the passphrase again. The default is 30
minutes.
If you find either of these defaults too short (or too long), you can edit
your `~/.gnupg/gpg-agent.conf` file to set your own values:
# set to 30 minutes for regular ttl, and 2 hours for max ttl
default-cache-ttl 1800
max-cache-ttl 7200
##### Bonus: Using gpg-agent with ssh
If you've created an **[A]** (Authentication) key and moved it to the
smartcard, you can use it with ssh for adding 2-factor authentication for your
ssh sessions. You just need to tell your environment to use the correct socket
file for talking to the agent.
All you need is add this to your `.bashrc`:
export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
Then start a new login session for the changes to take effect:
$ bash
$ ssh-add -L
The last command should list the SSH representation of your PGP Auth key (the
comment should say `cardno:XXXXXXXX` at the end to indicate it's coming from
the smartcard).
To enable key-based logins with ssh, just add the `ssh-add -L` output to
`~/.ssh/authorized_keys` on remote systems you log in to. Congratulations,
you've just made your ssh credentials extremely difficult to steal.
As a bonus, you can get other people's PGP-based ssh keys from public
keyservers, should you need to grant them ssh access to anything:
$ gpg --export-ssh-key [keyid]
This can come in super handy if you need to allow developers access to git
repositories over ssh.
## Protecting online accounts
### Checklist
- [ ] Get a U2F-capable device _(ESSENTIAL)_
- [ ] Enable 2-factor authentication for your online accounts _(ESSENTIAL)_
- [ ] GitHub/GitLab
- [ ] Google
- [ ] Social Media
- [ ] Use U2F as primary mechanism, with TOTP as fallback _(ESSENTIAL)_
### Considerations
You may have noticed how a lot of your online developer identity is tied to
your email address. If someone can gain access to your mailbox, they would be
able to do a lot of damage to you personally, and to your reputation as a free
software developer. Protecting your email accounts is just as important as
protecting your PGP keys.
#### Two-factor authentication with Fido U2F
[Two-factor authentication](https://en.wikipedia.org/wiki/Multi-factor_authentication)
is a mechanism to improve account security by requiring a physical token in
addition to a username and password. The goal is to make sure that even if
someone steals your password (via keylogging, shoulder surfing, or other
means), they still wouldn't be able to gain access to your account without
having in their possession a specific physical device ("something you have"
factor).
The most widely known mechanisms for 2-factor authentication are:
- SMS-based verification
- Time-based One-Time Passwords (TOTP) via a smartphone app, such as
the "Google Authenticator" or similar solutions
- Hardware tokens supporting Fido U2F
SMS-based verification is easiest to configure, but has the following
important downsides: it is useless in areas without signal (e.g. most building
basements), and can be defeated if the attacker is able to intercept or divert
SMS messages, for example by cloning your SIM card.
TOTP-based multi-factor authentication offers more protection than SMS, but
has important scaling downsides (there are only so many tokens you can add to
your smartphone app before finding the correct one becomes unwieldy). Plus,
there's no avoiding the fact that your secret key ends up stored on the
smartphone itself -- which is a complex, globally connected device that may or
may not have been receiving timely security patches from the manufacturer.
Most importantly, neither TOTP nor SMS methods protect you from phishing
attacks -- if the phisher is able to steal both your account password and the
2-factor token, they can replay them on the legitimate site and gain access to
your account.
[Fido U2F](https://en.wikipedia.org/wiki/Universal_2nd_Factor) is a standard
developed specifically to provide a mechanism for 2-factor authentication
*and* to combat credential phishing. The U2F protocol will store each site's
unique key on the USB token and will prevent you from accidentally giving the
attacker both your password and your one-time token if you try to use it on
anything other than the legitimate website.
Both Chrome and Firefox support U2F 2-factor authentication, and hopefully
other browsers will soon follow.
#### Get a token capable of Fido U2F
There are [many options available](http://www.dongleauth.info/dongles/) for
hardware tokens with Fido U2F support, but if you're already ordering a
smartcard-capable physical device, then your best option is a Yubikey 4, which
supports both.
#### Enable 2-factor authentication on your online accounts
You definitely want to enable this option on the email provider you are using
(especially if it is Google, which has excellent support for U2F). Other sites
where this functionality should be enabled are:
- **GitHub**: it probably occurred to you when you uploaded your PGP public key
that if anyone else is able to gain access to your account, they can replace
your key with their own. If you publish code on GitHub, you should take care
of your account security by protecting it with U2F-backed authentication.
- **GitLab**: for the same reasons as above.
- **Google**: if you have a google account, you will be surprised how many
sites allow logging in with Google authentication instead of site-specific
credentials.
- **Facebook**: same as above, a lot of online sites offer the option to
authenticate using a Facebook account. You should 2-factor protect your
Facebook account even if you do not use it.
- Other sites, as you deem necessary. See
[dongleauth.info](http://www.dongleauth.info) for inspiration.
#### Configure TOTP failover, if possible
Many sites will allow you to configure multiple 2-factor mechanisms, and the
recommended setup is:
- U2F token as the primary mechanism
- TOTP phone app as the secondary mechanism
This way, even if you lose your U2F token, you should be able to re-gain
access to your account. Alternatively, you can enroll multiple U2F tokens
(e.g. you can get another cheap token that only does U2F and use it for
backup reasons).
## Further reading
By this point you have accomplished the following important tasks:
1. Created your developer identity and protected it using PGP cryptography.
2. Configured your environment so your identity is not easily stolen by moving
your certification key offline and your subkeys to an external hardware
device.
3. Configured your git environment to ensure that anyone using your project is
able to verify the integrity of the repository and its entire history.
4. Secured your online accounts using 2-factor authentication.
You are already in a good place, but you should also read up on the following
topics:
- How to secure your team communication (see the document in this repository).
Decisions regarding your project development and governance require just as
much careful protection as any committed code, if not so. Make sure that
your team communication is trusted and the integrity of all decisions is
verified.
- How to secure your workstation (see the document in this repository). Your
goal is to minimize risky behaviour that would cause your project code to be
contaminated, or your developer identity to be stolen.
- How to write secure code (see various documentation related to the
programming languages and libraries used by your project). Bad, insecure
code is still bad, insecure code even if there is a PGP signature on the
commit that introduced it.