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 .. _pgpguide:
 
 ===========================
 Kernel Maintainer PGP guide
 ===========================
 
 :Author: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
 
 This document is aimed at Linux kernel developers, and especially at
 subsystem maintainers. It contains a subset of information discussed in
 the more general "`Protecting Code Integrity`_" guide published by the
 Linux Foundation. Please read that document for more in-depth discussion
 on some of the topics mentioned in this guide.
 
 .. _`Protecting Code Integrity`: https://github.com/lfit/itpol/blob/master/protecting-code-integrity.md
 
 The role of PGP in Linux Kernel development
 ===========================================
 
 PGP helps ensure the integrity of the code that is produced by the Linux
 kernel development community and, to a lesser degree, establish trusted
 communication channels between developers via PGP-signed email exchange.
 
 The Linux kernel source code is available in two main formats:
 
 - Distributed source repositories (git)
 - Periodic release snapshots (tarballs)
 
 Both git repositories and tarballs carry PGP signatures of the kernel
 developers who create official kernel releases. These signatures offer a
 cryptographic guarantee that downloadable versions made available via
 kernel.org or any other mirrors are identical to what these developers
 have on their workstations. To this end:
 
 - git repositories provide PGP signatures on all tags
 - tarballs provide detached PGP signatures with all downloads
 
 .. _devs_not_infra:
 
 Trusting the developers, not infrastructure
 -------------------------------------------
 
 Ever since the 2011 compromise of core kernel.org systems, the main
 operating principle of the Kernel Archives project has been to assume
 that any part of the infrastructure can be compromised at any time. For
 this reason, the administrators have taken deliberate steps to emphasize
 that trust must always be placed with developers and never with the code
 hosting infrastructure, regardless of how good the security practices
 for the latter may be.
 
 The above guiding principle is the reason why this guide is needed. We
 want to make sure that by placing trust into developers we do not simply
 shift the blame for potential future security incidents to someone else.
 The goal is to provide a set of guidelines developers can use to create
 a secure working environment and safeguard the PGP keys used to
 establish the integrity of the Linux kernel itself.
 
 .. _pgp_tools:
 
 PGP tools
 =========
 
 Use GnuPG v2
 ------------
 
 Your distro should already have GnuPG installed by default, you just
 need to verify that you are using version 2.x and not the legacy 1.4
 release -- many distributions still package both, with the default
 ``gpg`` command invoking GnuPG v.1. To check, run::
 
     $ gpg --version | head -n1
 
 If you see ``gpg (GnuPG) 1.4.x``, then you are using GnuPG v.1. Try the
 ``gpg2`` command (if you don't have it, you may need to install the
 gnupg2 package)::
 
     $ gpg2 --version | head -n1
 
 If you see ``gpg (GnuPG) 2.x.x``, then you are good to go. This guide
 will assume you have the version 2.2 of GnuPG (or later). If you are
 using version 2.0 of GnuPG, then some of the commands in this guide will
 not work, and you should consider installing the latest 2.2 version of
 GnuPG. Versions of gnupg-2.1.11 and later should be compatible for the
 purposes of this guide as well.
 
 If you have both ``gpg`` and ``gpg2`` commands, you should make sure you
 are always using GnuPG v2, not the legacy version. You can enforce this
 by setting the appropriate alias::
 
     $ alias gpg=gpg2
 
 You can put that in your ``.bashrc`` to make sure it's always the case.
 
 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. 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
 
 .. note::
 
     It is no longer necessary to start gpg-agent manually at the
     beginning of your shell session. You may want to check your rc files
     to remove anything you had in place for older versions of GnuPG, as
     it may not be doing the right thing any more.
 
 Set up a refresh cronjob
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 You will need to regularly refresh your keyring in order to get the
 latest changes on other people's public keys, which is best done with a
 daily cronjob::
 
     @daily /usr/bin/gpg2 --refresh >/dev/null 2>&1
 
 Check the full path to your ``gpg`` or ``gpg2`` command and use the
 ``gpg2`` command if regular ``gpg`` for you is the legacy GnuPG v.1.
 
 .. _master_key:
 
 Protect your master PGP key
 ===========================
 
 This guide assumes that you already have a PGP key that you use for Linux
 kernel development purposes. If you do not yet have one, please see the
 "`Protecting Code Integrity`_" document mentioned earlier for guidance
 on how to create a new one.
 
 You should also make a new key if your current one is weaker than 2048 bits
 (RSA).
 
 Master key vs. Subkeys
 ----------------------
 
 Subkeys are fully independent PGP keypairs that are tied to the "master"
 key using certifying key signatures (certificates). It is important to
 understand the following:
 
 1. There are no technical differences between the "master key" and "subkeys."
 2. At creation time, we assign functional limitations to each key by
    giving it specific capabilities.
 3. A PGP key can have 4 capabilities:
 
    - **[S]** key can be used for signing
    - **[E]** key can be used for encryption
    - **[A]** key can be used for authentication
    - **[C]** key can be used for certifying other keys
 
 4. A single key may have multiple capabilities.
 5. A subkey is fully independent from the master key. A message
    encrypted to a subkey cannot be decrypted with the master key. If you
    lose your private subkey, it cannot be recreated from the master key
    in any way.
 
 The key carrying the **[C]** (certify) capability is considered the
 "master" key because it is the only key that can be used to indicate
 relationship with other keys. Only the **[C]** key can be used to:
 
 - add or revoke other keys (subkeys) with S/E/A capabilities
 - add, change or revoke identities (uids) associated with the key
 - add or change the expiration date on itself or any subkey
 - sign other people's keys for web of trust purposes
 
 By default, GnuPG creates the following when generating new keys:
 
 - A master key carrying both Certify and Sign capabilities (**[SC]**)
 - A separate subkey with the Encryption capability (**[E]**)
 
 If you used the default parameters when generating your key, then that
 is what you will have. You can verify by running ``gpg --list-secret-keys``,
 for example::
 
     sec   rsa2048 2018-01-23 [SC] [expires: 2020-01-23]
           000000000000000000000000AAAABBBBCCCCDDDD
     uid           [ultimate] Alice Dev <adev@kernel.org>
     ssb   rsa2048 2018-01-23 [E] [expires: 2020-01-23]
 
 Any key carrying the **[C]** capability is your master key, regardless
 of any other capabilities it may have assigned to it.
 
 The long line under the ``sec`` entry is your key fingerprint --
 whenever you see ``[fpr]`` in the examples below, that 40-character
 string is what it refers to.
 
 Ensure your passphrase is strong
 --------------------------------
 
 GnuPG uses passphrases to encrypt your private keys before storing them on
 disk. This way, even if your ``.gnupg`` directory is leaked or stolen in
 its entirety, the attackers cannot use your private keys without first
 obtaining the passphrase to decrypt them.
 
 It is absolutely essential that your private keys are protected by a
 strong passphrase. To set it or change it, use::
 
     $ gpg --change-passphrase [fpr]
 
 Create a separate Signing subkey
 --------------------------------
 
 Our goal is to protect your master key by moving it to offline media, so
 if you only have a combined **[SC]** key, then you should create a separate
 signing subkey::
 
     $ gpg --quick-addkey [fpr] ed25519 sign
 
 Remember to tell the keyservers about this change, so others can pull down
 your new subkey::
 
     $ gpg --send-key [fpr]
 
 .. note:: ECC support in GnuPG
 
     GnuPG 2.1 and later has full support for Elliptic Curve
     Cryptography, with ability to combine ECC subkeys with traditional
     RSA master keys. The main upside of ECC cryptography is that it is
     much faster computationally and creates much smaller signatures when
     compared byte for byte with 2048+ bit RSA keys. Unless you plan on
     using a smartcard device that does not support ECC operations, we
     recommend that you create an ECC signing subkey for your kernel
     work.
 
     If for some reason you prefer to stay with RSA subkeys, just replace
     "ed25519" with "rsa2048" in the above command. Additionally, if you
     plan to use a hardware device that does not support ED25519 ECC
     keys, like Nitrokey Pro or a Yubikey, then you should use
     "nistp256" instead or "ed25519."
 
 
 Back up your master key for disaster recovery
 ---------------------------------------------
 
 The more signatures you have on your PGP key from other developers, the
 more reasons you have to create a backup version that lives on something
 other than digital media, for disaster recovery reasons.
 
 The best way to create a printable hardcopy of your private key is by
 using the ``paperkey`` software written for this very purpose. See ``man
 paperkey`` for more details on the output format and its benefits over
 other solutions. Paperkey should already be packaged for most
 distributions.
 
 Run the following command to create a hardcopy backup of your private
 key::
 
     $ gpg --export-secret-key [fpr] | paperkey -o /tmp/key-backup.txt
 
 Print out that file (or pipe the output straight to lpr), then take a
 pen and write your passphrase on the margin of the paper. **This is
 strongly recommended** because the key printout is still encrypted with
 that passphrase, and if you ever change it you will not remember what it
 used to be when you had created the backup -- *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::
 
     Your printer is probably no longer a simple dumb device connected to
     your parallel port, but since the output is still encrypted with
     your passphrase, printing out even to "cloud-integrated" modern
     printers should remain a relatively safe operation. One option is to
     change the passphrase on your master key immediately after you are
     done with paperkey.
 
 Back up your whole GnuPG directory
 ----------------------------------
 
 .. warning::
 
     **!!!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``. You will also rely
 on these external copies whenever you need to use your Certify key --
 such as when making changes to your own key or signing other people's
 keys after conferences and summits.
 
 Start by getting a small USB "thumb" drive (preferably two!) that you
 will use for backup purposes. You will need to encrypt them using LUKS
 -- refer to your distro's documentation on how to accomplish this.
 
 For the encryption passphrase, you can use the same one as on your
 master key.
 
 Once the encryption process is over, re-insert the USB drive and make
 sure it gets properly mounted. Copy your entire ``.gnupg`` directory
 over to the encrypted storage::
 
     $ cp -a ~/.gnupg /media/disk/foo/gnupg-backup
 
 You should now test to make sure everything still works::
 
     $ gpg --homedir=/media/disk/foo/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 master key from  your homedir
 ----------------------------------------
 
 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 master key from your home directory
 and store it on offline storage.
 
 .. warning::
 
     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 master key::
 
     $ gpg --with-keygrip --list-key [fpr]
 
 The output will be something like this::
 
     pub   rsa2048 2018-01-24 [SC] [expires: 2020-01-24]
           000000000000000000000000AAAABBBBCCCCDDDD
           Keygrip = 1111000000000000000000000000000000000000
     uid           [ultimate] Alice Dev <adev@kernel.org>
     sub   rsa2048 2018-01-24 [E] [expires: 2020-01-24]
           Keygrip = 2222000000000000000000000000000000000000
     sub   ed25519 2018-01-24 [S]
           Keygrip = 3333000000000000000000000000000000000000
 
 Find the keygrip entry that is beneath the ``pub`` line (right under the
 master key fingerprint). This will correspond directly to a file in your
 ``~/.gnupg`` directory::
 
     $ cd ~/.gnupg/private-keys-v1.d
     $ ls
     1111000000000000000000000000000000000000.key
     2222000000000000000000000000000000000000.key
     3333000000000000000000000000000000000000.key
 
 All you have to do is simply remove the .key file that corresponds to
 the master keygrip::
 
     $ cd ~/.gnupg/private-keys-v1.d
     $ rm 1111000000000000000000000000000000000000.key
 
 Now, if you issue the ``--list-secret-keys`` command, it will show that
 the master key is missing (the ``#`` indicates it is not available)::
 
     $ gpg --list-secret-keys
     sec#  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]
 
 You should also remove any ``secring.gpg`` files in the ``~/.gnupg``
 directory, which are left over from earlier versions of GnuPG.
 
 If you don't have the "private-keys-v1.d" directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 If you do not have a ``~/.gnupg/private-keys-v1.d`` directory, then your
 secret keys are still stored in the legacy ``secring.gpg`` file used by
 GnuPG v1. Making any changes to your key, such as changing the
 passphrase or adding a subkey, should automatically convert the old
 ``secring.gpg`` format to use ``private-keys-v1.d`` instead.
 
 Once you get that done, make sure to delete the obsolete ``secring.gpg``
 file, which still contains your private keys.
 
 .. _smartcards:
 
 Move the subkeys to a dedicated crypto device
 =============================================
 
 Even though the master 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 mounted, the
 operating system is able to access the private key contents.
 
 Using external encrypted USB media is not a substitute to having a
 smartcard-capable device.
 
 Available smartcard devices
 ---------------------------
 
 Unless all your laptops and workstations have smartcard readers, the
 easiest is to get a specialized USB device that implements smartcard
 functionality. There are several options available:
 
 - `Nitrokey Start`_: Open hardware and Free Software, based on FSI
   Japan's `Gnuk`_. One of the few available commercial devices that
   support ED25519 ECC keys, but offer fewest security features (such as
   resistance to tampering or some side-channel attacks).
 - `Nitrokey Pro 2`_: Similar to the Nitrokey Start, but more
   tamper-resistant and offers more security features. Pro 2 supports ECC
   cryptography (NISTP).
 - `Yubikey 5`_: proprietary hardware and software, but cheaper than
   Nitrokey Pro and comes available in the USB-C form that is more useful
   with newer laptops. Offers additional security features such as FIDO
   U2F, among others, and now finally supports ECC keys (NISTP).
 
 `LWN has a good review`_ of some of the above models, as well as several
 others. Your choice will depend on cost, shipping availability in your
 geographical region, and open/proprietary hardware considerations.
 
 .. note::
 
     If you are listed in MAINTAINERS or have an account at kernel.org,
     you `qualify for a free Nitrokey Start`_ courtesy of The Linux
     Foundation.
 
 .. _`Nitrokey Start`: https://shop.nitrokey.com/shop/product/nitrokey-start-6
 .. _`Nitrokey Pro 2`: https://shop.nitrokey.com/shop/product/nitrokey-pro-2-3
 .. _`Yubikey 5`: https://www.yubico.com/products/yubikey-5-overview/
 .. _Gnuk: https://www.fsij.org/doc-gnuk/
 .. _`LWN has a good review`: https://lwn.net/Articles/736231/
 .. _`qualify for a free Nitrokey Start`: https://www.kernel.org/nitrokey-digital-tokens-for-kernel-developers.html
 
 Configure 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.
 
 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.
 
 .. note::
 
     Despite having the name "PIN", neither the user PIN nor the admin
     PIN on the card need to be numbers.
 
 .. warning::
 
     Some devices may require that you move the subkeys onto the device
     before you can change the passphrase. Please check the documentation
     provided by the device manufacturer.
 
 Move 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/foo/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).
 
 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]
 
 Updating your work directory after any changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 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
 
 Using gpg-agent over ssh
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 You can forward your gpg-agent over ssh if you need to sign tags or
 commits on a remote system. Please refer to the instructions provided
 on the GnuPG wiki:
 
 - `Agent Forwarding over SSH`_
 
 It works more smoothly if you can modify the sshd server settings on the
 remote end.
 
 .. _`Agent Forwarding over SSH`: https://wiki.gnupg.org/AgentForwarding
 
 
 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`_?
 
 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.
 
 .. _`nothing to do with it`: https://github.com/jayphelps/git-blame-someone-else
 
 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
     $ git config --global gpgv.program gpgv2
 
 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 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 [...]
 
 If you are verifying someone else's git tag, then you will need to
 import their PGP key. Please refer to the
 ":ref:`verify_identities`" section below.
 
 .. note::
 
     If you get "``gpg: Can't check signature: unknown pubkey
     algorithm``" error, you need to tell git to use gpgv2 for
     verification, so it properly processes signatures made by ECC keys.
     See instructions at the start of this section.
 
 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
 
 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. Furthermore, when rebasing your repository to match
 upstream, even your own PGP commit signatures will end up discarded. For
 this reason, most kernel developers don't bother signing their commits
 and will ignore signed commits in any external repositories that they
 rely upon in their work.
 
 However, 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.
 
 We recommend this for the following reasons:
 
 1. Should there ever be a need to perform code forensics or track code
    provenance, even externally maintained trees carrying PGP commit
    signatures will be valuable for such purposes.
 2. If you ever need to re-clone your local repository (for example,
    after a disk failure), this lets you easily verify the repository
    integrity before resuming your work.
 3. If someone needs to cherry-pick your commits, this allows them to
    quickly verify their integrity before applying them.
 
 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
 
 .. note::
 
     Make sure you configure ``gpg-agent`` before you turn this on.
 
 .. _verify_identities:
 
 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?
 
 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 `add the kernel.org
 UID to your key`_ to make WKD more useful to other kernel developers.
 
 .. _`add the kernel.org UID to your key`: https://korg.wiki.kernel.org/userdoc/mail#adding_a_kernelorg_uid_to_your_pgp_key
 
 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
 
 How to use keyservers (more) safely
 -----------------------------------
 
 If you 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 PGP keyservers 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 display the key of Linus Torvalds that you
 have on your keyring::
 
     $ gpg --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, find a trust path from Linus Torvalds to the key-id you found via ``gpg
 --search`` of the unknown key.  For this, you can use several tools including
 https://github.com/mricon/wotmate,
 https://git.kernel.org/pub/scm/docs/kernel/pgpkeys.git/tree/graphs, and
 https://the.earth.li/~noodles/pathfind.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 (in
 fact, this goes against :ref:`devs_not_infra`). However, if you
 do not carefully maintain your own web of trust, then it is a marked
 improvement over blindly trusting keyservers.

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