51026930ef
Using -o makes sure that the file is created with 0600 permissions instead of whatever the default umask setting is. Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
922 lines
36 KiB
Markdown
922 lines
36 KiB
Markdown
# Kernel developer PGP guide
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Updated: 2018-01-24
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*Status: CURRENT, BETA*
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### Target audience
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This document is aimed at Linux kernel developers, and especially subsystem
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maintainers. It contains a subset of information discussed in the more
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general "Protecting Code Integrity" guide found in the same repository. If you
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are not a Linux kernel developer, you should read the more general guide
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instead.
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This document covers the following topics:
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1. How to improve your PGP key security
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2. When and how to use PGP with git
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3. How to verify fellow kernel developer identities
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### Structure
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Each section is split into two areas:
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- A checklist of actionable items
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- Free-form list of considerations that explain what dictated these decisions,
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together with configuration instructions
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#### Checklist priority levels
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The items in each checklist include the priority level, which we hope will
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help guide your decision:
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- _(ESSENTIAL)_ items should definitely be high on the consideration list.
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- _(NICE)_ to have items will improve the overall security, but will affect how
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you interact with your work environment, and probably require learning new
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habits or unlearning old ones.
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Remember, these are only guidelines. If you feel these priority levels do not
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reflect your commitment to security, you should adjust them as you see fit.
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## The role of PGP in Linux Kernel development
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PGP helps ensure the integrity of the code that is produced by the Linux
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Kernel development community and, to a lesser degree, establish trusted
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communication channels between developers via PGP-signed email exchange.
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The Linux Kernel source code is available in two main formats:
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- Distributed source repositories (git)
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- Periodic release snapshots (tarballs)
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Both git repositories and tarballs carry PGP signatures of the kernel
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developers who create official kernel releases. These signatures offer a
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cryptographic guarantee that downloadable versions made available via
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kernel.org or any other mirrors are identical to what these developers have on
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their workstations. To this end:
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- git repositories provide PGP signatures on all tags
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- tarballs provide detached PGP signatures with all downloads
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### Trusting the developers, not infrastructure
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Ever since the 2011 compromise of core kernel.org systems, the main operating
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principle of the Kernel Archives project has been to assume that any part of
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the infrastructure can be compromised at any time. For this reason, the
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administrators have taken deliberate steps to emphasize that trust must always
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be placed with developers and never with the code hosting infrastructure,
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regardless of how good the security practices for the latter may be.
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The above guiding principle is the reason why this guide is needed. We want to
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make sure that by placing trust into developers we do not simply shift the
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blame for potential future security incidents to someone else. The goal is to
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provide a set of guidelines developers can use to create a secure working
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environment and safeguard the PGP keys used to establish the integrity of the
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Linux Kernel itself.
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## PGP tools
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### Checklist
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- [ ] Make sure you use GnuPG version 2 _(ESSENTIAL)_
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- [ ] Configure gpg-agent _(ESSENTIAL)_
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- [ ] Set up a refresh cronjob _(ESSENTIAL)_
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### Considerations
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### Installing GnuPG
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Your distro should already have GnuPG installed by default, you just need to
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verify that you are using version 2.x and not the legacy 1.4 release --
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many distributions still package both, with the default `gpg` command invoking
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GnuPG v.1. To check, run:
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$ gpg --version | head -n1
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If you see `gpg (GnuPG) 1.4.x`, then you are using GnuPG v.1. Try the `gpg2`
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command (if you don't have it, you may need to install the gnupg2 package):
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$ gpg2 --version | head -n1
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If you see `gpg (GnuPG) 2.x.x`, then you are good to go. This guide will
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assume you have the version 2.2 of GnuPG (or later). If you are using version
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2.0 of GnuPG, then some of the commands in this guide will not work, and you
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should consider installing the latest 2.2 version of GnuPG. Versions of
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gnupg-2.1.11 and later should be compatible for the purposes of this guide as
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well.
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##### Making sure you always use GnuPG v.2
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If you have both `gpg` and `gpg2` commands, you should make sure you are
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always using GnuPG v2, not the legacy version. You can enforce this by setting
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the appropriate alias:
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$ alias gpg=gpg2
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You can put that in your `.bashrc` to make sure it's always the case.
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#### Configure gpg-agent options
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The GnuPG agent is a helper tool that will start automatically whenever you
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use the `gpg` command and run in the background with the purpose of caching
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the private key passphrase. It is no longer necessary to start it manually
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at the beginning of your shell session.
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There are two options you should know in order to tweak when the passphrase
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should be expired from cache:
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- `default-cache-ttl` (seconds): If you use the same key again before the
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time-to-live expires, the countdown will reset for another period.
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The default is 600 (10 minutes).
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- `max-cache-ttl` (seconds): Regardless of how recently you've used the key
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since initial passphrase entry, if the maximum time-to-live countdown
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expires, you'll have to enter the passphrase again. The default is 30
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minutes.
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If you find either of these defaults too short (or too long), you can edit
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your `~/.gnupg/gpg-agent.conf` file to set your own values:
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# set to 30 minutes for regular ttl, and 2 hours for max ttl
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default-cache-ttl 1800
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max-cache-ttl 7200
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#### Set up a refresh cronjob
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You will need to regularly refresh your keyring in order to get the latest
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changes on other people's public keys, which is best done with a daily
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cronjob:
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$ crontab -e
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Add the following on a new line:
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@daily /usr/bin/gpg2 --refresh >/dev/null 2>&1
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**NOTE**: check the full path to your `gpg` or `gpg2` command and use the `gpg2`
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command if regular `gpg` for you is the legacy GnuPG v.1.
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## Protecting your master PGP key
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### Checklist
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- [ ] Understand the "master" key vs. subkeys _(ESSENTIAL)_
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- [ ] Ensure your private key passphrase is strong _(ESSENTIAL)_
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- [ ] Create a separate **[S]** subkey _(ESSENTIAL)_
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- [ ] Back up the master key using paperkey _(ESSENTIAL)_
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- [ ] Back up your whole `.gnupg` directory to encrypted media _(ESSENTIAL)_
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- [ ] Remove the master key from your homedir _(ESSENTIAL)_
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### Considerations
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This guide assumes that you already have a PGP key that you use for Linux
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Kernel development purposes. If you do not yet have one, please see the
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"Protecting Code Integrity" document in this repository for guidance on how to
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create a new one.
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You should also make a new key if your current one is weaker than 2048 bits
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(RSA).
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#### Understanding the "Master" (Certify) key
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In this and next section we'll talk about the "master key" and "subkeys". It
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is important to understand the following:
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1. There are no technical differences between the "master key" and "subkeys."
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2. At creation time, we assign functional limitations to each key by
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giving it specific capabilities.
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3. A PGP key can have 4 capabilities.
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- **[S]** key can be used for signing
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- **[E]** key can be used for encryption
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- **[A]** key can be used for authentication
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- **[C]** key can be used for certifying other keys
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4. A single key may have multiple capabilities.
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The key carrying the **[C]** (certify) capability is considered the "master"
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key because it is the only key that can be used to indicate relationship with
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other keys. Only the **[C]** key can be used to:
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- add or revoke other keys (subkeys) with S/E/A capabilities
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- add, change or revoke identities (uids) associated with the key
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- add or change the expiration date on itself or any subkey
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- sign other people's keys for the web of trust purposes
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By default, GnuPG creates the following when generating new keys:
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- A master key carrying both Certify and Sign capabilities (**[SC]**)
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- A separate subkey with the Encryption capability (**[E]**)
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If you used the default parameters when generating your key, then that is what
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you will have. You can verify by running `gpg --list-secret-keys`, for
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example:
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sec rsa2048 2018-01-23 [SC] [expires: 2020-01-23]
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000000000000000000000000AAAABBBBCCCCDDDD
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uid [ultimate] Alice Dev <adev@kernel.org>
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ssb rsa2048 2018-01-23 [E] [expires: 2020-01-23]
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Any key carrying the **[C]** capability is your master key, regardless of any
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other capabilities it may have assigned to it.
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The long line under the `sec` entry is your key fingerprint -- whenever you
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see `[fpr]` in the examples below, that 40-character string is what it refers
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to.
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#### Ensure your passphrase is strong
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GnuPG uses passphrases to encrypt your private keys before storing them on
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disk. This way, even if your `.gnupg` directory is leaked or stolen in its
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entirety, the attackers cannot use your private keys without first obtaining
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the passphrase to decrypt them.
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It is absolutely essential that your private keys are protected by a
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strong passphrase. To set it or change it, use:
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$ gpg --change-passphrase [fpr]
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#### Create a separate Signing subkey
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Our goal is to protect your master key by moving it to offline media, so
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if you only have a combined **[SC]** key, then you should create a separate
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signing subkey.
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##### RSA vs. ECC subkeys
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GnuPG 2.1 and later has full support for Elliptic Curve Cryptography, with
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ability to combine ECC subkeys with traditional RSA master keys. The main
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upside of ECC cryptography is that it is much faster computationally and
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creates much smaller signatures when compared byte for byte with 2048+ bit RSA
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keys.
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Unless you plan on using a smartcard device that does not support ECC
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operations, we recommend that you create an ECC signing subkey for your kernel
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work:
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$ gpg --quick-add-key [fpr] ed25519 sign
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If for some reason you prefer to stay with RSA subkeys, just replace "ed25519"
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with "rsa2048" in the above command.
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Remember to tell the keyservers about this change, so others can pull down
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your new subkey:
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$ gpg --send-key [fpr]
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#### Back up your master key for disaster recovery
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The more signatures you have on your PGP key from other developers, the more
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reasons you have to create a backup version that lives on something other than
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digital media, for disaster recovery reasons.
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The best way to create a printable hardcopy of your private key is by using
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the `paperkey` software written for this very purpose. See `man paperkey` for
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more details on the output format and its benefits over other solutions.
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Paperkey should already be packaged for most distributions.
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Run the following command to create a hardcopy backup of your private key:
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$ gpg --export-secret-key [fpr] | paperkey -o /tmp/key-backup.txt
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Print out that file (or pipe the output straight to lpr), then take a pen and
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write your passphrase on the margin of the paper. **This is strongly
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recommended** because the key printout is still encrypted with that
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passphrase, and if you ever change it you will not remember what it used to be
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when you had created the backup -- *guaranteed*.
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Put the resulting printout and the hand-written passphrase into an envelope
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and store in a secure and well-protected place, preferably away from your
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home, such as your bank vault.
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**NOTE ON PRINTERS**: Your printer is probably no longer a simple dumb device
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connected to your parallel port, but since the output is still encrypted with
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your passphrase, printing out even to "cloud-integrated" modern printers
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should remain a relatively safe operation. One option is to change the
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passphrase on your master key immediately after you are done with paperkey.
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#### Back up your whole GnuPG directory
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**!!!Do not skip this step!!!**
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It is important to have a readily available backup of your PGP keys should you
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need to recover them (this is different from the disaster-level preparedness
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we did with `paperkey`). You will also rely on these external copies whenever
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you need to use your Certify key -- such as when making changes to your own
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key or signing other people's keys after conferences and meetups.
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##### Prepare detachable encrypted storage
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Start by getting a small USB "thumb" drive (preferably two!) that you will use
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for backup purposes. You will need to encrypt them before using them for our
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purposes:
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- [Ubuntu instructions](https://help.ubuntu.com/community/EncryptedFilesystemsOnRemovableStorage)
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For the encryption passphrase, you can use the same one as on your master key.
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##### Back up your GnuPG directory
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Once the encryption process is over, re-insert the USB drive and make sure it
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gets properly mounted. Find out the full mount point of the device, for
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example by running the `mount` command (under Linux, external media usually
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gets mounted under `/media/disk`).
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Once you know the full mount path, copy your entire GnuPG directory there:
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$ cp -a ~/.gnupg [/media/disk/name]/gnupg-backup
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(Note: If you get any `Operation not supported on socket` errors, those are
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benign and you can ignore them.)
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You should now test to make sure everything still works:
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$ gpg --homedir=[/media/disk/name]/gnupg-backup --list-key [fpr]
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If you don't get any errors, then you should be good to go. Unmount the USB
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drive, distinctly label it so you don't blow it away next time you need to use
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a random USB drive, and put in a safe place -- but not too far away, because
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you'll need to use it every now and again for things like editing identities,
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adding or revoking subkeys, or signing other people's keys.
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#### Remove the master key from your homedir
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The files in our home directory are not as well protected as we like to think.
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They can be leaked or stolen via many different means:
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- by accident when making quick homedir copies to set up a new workstation
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- by systems administrator negligence or malice
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- via poorly secured backups
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- via malware in desktop apps (browsers, pdf viewers, etc)
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- via coercion when crossing international borders
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Protecting your key with a good passphrase greatly helps reduce the risk of
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any of the above, but passphrases can be discovered via keyloggers,
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shoulder-surfing, or any number of other means. For this reason, the
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recommended setup is to remove your master key from your home directory and
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store it on offline storage.
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##### Removing your master key
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Please see the previous section and make sure you have backed up your GnuPG
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directory in its entirety. What we are about to do will render your key
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useless if you do not have a usable backup!
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First, identify the keygrip of your master key:
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$ gpg --with-keygrip --list-key [fpr]
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The output will be something like this:
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pub rsa2048 2018-01-24 [SC] [expires: 2020-01-24]
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000000000000000000000000AAAABBBBCCCCDDDD
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Keygrip = 1111000000000000000000000000000000000000
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uid [ultimate] Alice Dev <adev@kernel.org>
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sub rsa2048 2018-01-24 [E] [expires: 2020-01-24]
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Keygrip = 2222000000000000000000000000000000000000
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sub ed25519 2018-01-24 [S]
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Keygrip = 3333000000000000000000000000000000000000
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Find the keygrip entry that is beneath the `pub` line (right under the master
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key fingerprint). This will correspond directly to a file in your `~/.gnupg`
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directory:
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$ cd ~/.gnupg/private-keys-v1.d
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$ ls
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1111000000000000000000000000000000000000.key
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2222000000000000000000000000000000000000.key
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3333000000000000000000000000000000000000.key
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All you have to do is simply remove the `.key` file that corresponds to the
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master keygrip:
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$ cd ~/.gnupg/private-keys-v1.d
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$ rm 1111000000000000000000000000000000000000.key
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Now, if you issue the `--list-secret-keys` command, it will show that the
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master key is missing (the `#` indicates it is not available):
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$ gpg --list-secret-keys
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sec# rsa2048 2018-01-24 [SC] [expires: 2020-01-24]
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000000000000000000000000AAAABBBBCCCCDDDD
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uid [ultimate] Alice Dev <adev@kernel.org>
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ssb rsa2048 2018-01-24 [E] [expires: 2020-01-24]
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ssb ed25519 2018-01-24 [S]
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You should also remove any `secring.gpg` files in the `~/.gnupg` directory,
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which are left over from earlier versions of GnuPG.
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##### If you don't have the "private-keys-v1.d" directory
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If you do not have a `~/.gnupg/private-keys-v1.d` directory, then your
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secret keys are still stored in the legacy `secring.gpg` file used by GnuPG
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v1. Making any changes to your key, such as changing the passphrase or adding
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a subkey, should automatically convert the old `secring.gpg` format to use
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`private-keys-v1.d` instead.
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Once you get that done, make sure to delete the obsolete `secring.gpg` file,
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which still contains your private keys.
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## Move the subkeys to a dedicated crypto device
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### Checklist
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- [ ] Get a GnuPG-compatible hardware device _(NICE)_
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- [ ] Configure the device to work with GnuPG _(NICE)_
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- [ ] Set the user and admin PINs _(NICE)_
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- [ ] Move your subkeys to the device _(NICE)_
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### Considerations
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Even though the master key is now safe from being leaked or stolen, the
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subkeys are still in your home directory. Anyone who manages to get their
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hands on those will be able to decrypt your communication or fake your
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signatures (if they know the passphrase). Furthermore, each time a GnuPG
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operation is performed, the keys are loaded into system memory and can be
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stolen from there by sufficiently advanced malware (think Meltdown and
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Spectre).
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The best way to completely protect your keys is to move them to a specialized
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hardware device that is capable of smartcard operations.
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#### The benefits of smartcards
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A smartcard contains a cryptographic chip that is capable of storing private
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keys and performing crypto operations directly on the card itself. Because the
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key contents never leave the smartcard, the operating system of the computer
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into which you plug in the hardware device is not able to retrieve the
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private keys themselves. This is very different from the encrypted USB storage
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device we used earlier for backup purposes -- while that USB device is plugged
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in and mounted, the operating system is able to access the private key
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contents.
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Using external encrypted USB media is not a substitute to having a
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smartcard-capable device.
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#### Available smartcard devices
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Unless all your laptops and workstations have smartcard readers, the easiest
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is to get a specialized USB device that implements smartcard functionality.
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There are several options available:
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- [Nitrokey Start](https://shop.nitrokey.com/shop/product/nitrokey-start-6):
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Open hardware and Free Software, based on FSI Japan's
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[Gnuk](http://www.fsij.org/doc-gnuk/). Offers support for ECC keys,
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but fewest security features (such as resistance to tampering or some
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side-channel attacks).
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- [Nitrokey Pro](https://shop.nitrokey.com/shop/product/nitrokey-pro-3):
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Similar to the Nitrokey Start, but more tamper-resistant and offers more
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security features, but no ECC support.
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- [Yubikey 4](https://www.yubico.com/product/yubikey-4-series/): Proprietary
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hardware and software, but cheaper than Nitrokey Pro and comes available in
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the USB-C form that is more useful with newer laptops. Offers
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additional security features such as FIDO U2F, but no ECC.
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[LWN has a good review](https://lwn.net/Articles/736231/) of some of the above
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models, as well as several others. If you want to use ECC keys, your best bet
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among commercially available devices is the Nitrokey Start.
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|
#### Configuring your smartcard device
|
|
|
|
Your smartcard device should Just Work (TM) the moment you plug it into any
|
|
modern Linux workstation. You can verify it by running:
|
|
|
|
$ gpg --card-status
|
|
|
|
If you see full smartcard 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 help via usual support channels.
|
|
|
|
##### 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), 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.
|
|
|
|
##### 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.
|
|
|
|
#### 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.
|
|
|
|
$ gpg --edit-key [fpr]
|
|
|
|
Secret subkeys are available.
|
|
|
|
pub rsa2048/AAAABBBBCCCCDDDD
|
|
created: 2018-01-23 expires: 2020-01-23 usage: SC
|
|
trust: ultimate validity: ultimate
|
|
ssb rsa2048/1111222233334444
|
|
created: 2018-01-23 expires: never usage: E
|
|
ssb ed25519/5555666677778888
|
|
created: 2017-12-07 expires: never usage: S
|
|
[ultimate] (1). Alice Dev <adev@kernel.org>
|
|
|
|
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, the **[E]** subkey):
|
|
|
|
gpg> key 1
|
|
|
|
In the output, you should now see `ssb*` on the **[E]** key. The `*` indicates
|
|
which 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.
|
|
|
|
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# rsa2048 2018-01-24 [SC] [expires: 2020-01-24]
|
|
000000000000000000000000AAAABBBBCCCCDDDD
|
|
uid [ultimate] Alice Dev <adev@kernel.org>
|
|
ssb> rsa2048 2018-01-24 [E] [expires: 2020-01-24]
|
|
ssb> ed25519 2018-01-24 [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 | grep 'private-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.
|
|
|
|
#### Mounting your master key offline storage
|
|
|
|
You will need your master key for any of the operations below, so you will
|
|
first need to mount your backup offline storage and tell GnuPG to use it.
|
|
|
|
$ 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 master key 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):
|
|
|
|
$ gpg --quick-set-expire [fpr] 2020-07-01
|
|
|
|
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 their copy of
|
|
linux.git has not been tampered with by a malicious third party?
|
|
|
|
Or what happens if a backdoor is discovered in the code 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
|
|
|
|
- [ ] Configure git to use your key _(ESSENTIAL)_
|
|
- [ ] Configure git to always sign annotated tags _(NICE)_
|
|
- [ ] Decide if you're going to use commit signing _(NICE)_
|
|
|
|
### Considerations
|
|
|
|
#### 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]
|
|
|
|
**IMPORTANT**: 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 pulling from has not been
|
|
maliciously altered.
|
|
|
|
##### 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 "how to verify kernel developer identities"
|
|
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
|
|
use them in Linux Kernel development, since it relies on patches sent to
|
|
the mailing list, and this workflow does not preserve PGP commit signatures.
|
|
|
|
If you have your working git tree publicly available at some git hosting
|
|
service (kernel.org, infradead.org, ozlabs.org, or others), then the
|
|
recommendation is that you sign all your git commits even if upstream
|
|
developers do not directly benefit from this practice. Should there ever be a
|
|
need to perform code forensics or track code provenance, even externally
|
|
maintained trees carrying PGP commit signatures will be extremely valuable for
|
|
such purposes.
|
|
|
|
##### Creating signed commits
|
|
|
|
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
|
|
|
|
#### 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`).
|
|
|
|
## How to verify kernel developer identities
|
|
|
|
Signing tags and commits is easy, but how does one go about verifying that the
|
|
key used to sign something belongs to the actual kernel developer and not to
|
|
a malicious imposter?
|
|
|
|
### Checklist
|
|
|
|
- [ ] Configure auto-key-retrieval using WKD and DANE _(ESSENTIAL)_
|
|
- [ ] Configure trust-model to `tofu+pgp` _(ESSENTIAL)_
|
|
- [ ] Learn how to use keyservers (more) safely _(ESSENTIAL)_
|
|
|
|
### Considerations
|
|
|
|
#### Configure auto-key-retrieval using WKD and DANE
|
|
|
|
If you are not already someone with an extensive collection of other
|
|
developers' public keys, then you can jumpstart your keyring by relying
|
|
on key auto-discovery and auto-retrieval. GnuPG can piggyback on other
|
|
delegated trust technologies, namely DNSSEC and TLS, to get you going if the
|
|
prospect of starting your own Web of Trust from scratch is too daunting.
|
|
|
|
Add the following to your `~/.gnupg/gpg.conf`:
|
|
|
|
auto-key-locate wkd,dane,local
|
|
auto-key-retrieve
|
|
|
|
DNS-Based Authentication of Named Entities ("DANE") is a method for publishing
|
|
public keys in DNS and securing them using DNSSEC signed zones. Web Key
|
|
Directory ("WKD") is the alternative method that uses https lookups for the
|
|
same purpose. When using either DANE or WKD for looking up public keys, GnuPG
|
|
will validate DNSSEC or TLS certificates, respectively, before adding
|
|
auto-retrieved public keys to your local keyring.
|
|
|
|
Kernel.org publishes the WKD for all developers who have kernel.org accounts.
|
|
Once you have the above changes in your `gpg.conf`, you can auto-retrieve the
|
|
keys for Linus Torvalds and Greg Kroah-Hartman (if you don't already have
|
|
them):
|
|
|
|
$ gpg --locate-keys torvalds@kernel.org gregkh@kernel.org
|
|
|
|
If you have a kernel.org account, then you should make sure that you have
|
|
[added the kernel.org UID to your key](https://korg.wiki.kernel.org/userdoc/mail#adding_a_kernelorg_uid_to_your_pgp_key)
|
|
to make WKD more useful to other kernel developers.
|
|
|
|
#### 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. While
|
|
it remains an important aspect of the OpenPGP specification, 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
|
|
valid. 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.
|
|
|
|
We recommend that you use the combined TOFU+PGP trust model (which is the new
|
|
default in GnuPG v2). To set it, add (or modify) the `trust-model` setting in
|
|
`~/.gnupg/gpg.conf`:
|
|
|
|
trust-model tofu+pgp
|
|
|
|
#### Learn to use keyservers (more) safely
|
|
|
|
If, despite setting `auto-key-retrieve`, you still get a "No public key" error
|
|
when trying to validate someone's tag, then you should attempt to lookup that
|
|
key using a keyserver. It is important to keep in mind that there is
|
|
absolutely no guarantee that the key you retrieve from a keyserver belongs to
|
|
the actual person -- that much is by design. You are supposed to use the Web
|
|
of Trust to establish key validity.
|
|
|
|
How to properly maintain the Web of Trust is beyond the scope of this
|
|
document, simply because doing it properly requires both effort and dedication
|
|
that tends to be beyond the caring threshold of most human beings. Here are
|
|
some shortcuts that will help you reduce the risk of importing a malicious
|
|
key.
|
|
|
|
First, let's say you've tried to run `git verify-tag` but it returned an error
|
|
saying the key is not found:
|
|
|
|
$ git verify-tag sunxi-fixes-for-4.15-2
|
|
gpg: Signature made Sun 07 Jan 2018 10:51:55 PM EST
|
|
gpg: using RSA key DA73759BF8619E484E5A3B47389A54219C0F2430
|
|
gpg: issuer "wens@...org"
|
|
gpg: Can't check signature: No public key
|
|
|
|
Let's query the keyserver for more info about that key fingerprint (the
|
|
fingerprint probably belongs to a subkey, so we can't use it directly without
|
|
finding out the ID of the master key it is associated with):
|
|
|
|
$ gpg --search DA73759BF8619E484E5A3B47389A54219C0F2430
|
|
gpg: data source: hkp://keys.gnupg.net
|
|
(1) Chen-Yu Tsai <wens@...org>
|
|
4096 bit RSA key C94035C21B4F2AEB, created: 2017-03-14, expires: 2019-03-15
|
|
Keys 1-1 of 1 for "DA73759BF8619E484E5A3B47389A54219C0F2430". Enter number(s), N)ext, or Q)uit > q
|
|
|
|
Locate the ID of the master key in the output, in our example
|
|
`C94035C21B4F2AEB`. Now say `q` and display the key of Linus Torvalds that you
|
|
have on your keyring:
|
|
|
|
$ git --list-key torvalds@kernel.org
|
|
pub rsa2048 2011-09-20 [SC]
|
|
ABAF11C65A2970B130ABE3C479BE3E4300411886
|
|
uid [ unknown] Linus Torvalds <torvalds@kernel.org>
|
|
sub rsa2048 2011-09-20 [E]
|
|
|
|
Next, open the [PGP pathfinder](https://pgp.cs.uu.nl/). In the "From" field,
|
|
paste the key fingerprint of Linus Torvalds from the output above. In the "To"
|
|
field, paste they key-id you found via `gpg --search` of the unknown key, and
|
|
check the results:
|
|
|
|
- [From Linus to Chen-Yu](https://pgp.cs.uu.nl/paths/79BE3E4300411886/to/C94035C21B4F2AEB.html)
|
|
|
|
If you get a few decent trust paths, then it's a pretty good indication that
|
|
it is a valid key. You can add it to your keyring from the keyserver now:
|
|
|
|
$ gpg --recv-key C94035C21B4F2AEB
|
|
|
|
This process is not perfect, and you are obviously trusting the administrators
|
|
of the PGP Pathfinder service to not be malicious. However, if you do not
|
|
carefully maintain your own web of trust, then it is an improvement over
|
|
blindly trusting keyservers.
|