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+---
+title: PGP basics
+date: Version 1.0.1 for 2019-05-08
+...
+
+PGP and GnuPG—some basics
+=============================================================================
+
+Use of unnecessary swearing while using crypto software...
+has been approved
+
+
+---
+
+Everyday uses of PGP
+=============================================================================
+
+* email: sign, encrypt (all personal emails I send are signed by default)
+
+* git: signed tags, signed commits (for releases)
+
+* software releases: sign your tarballs, downloadables
+
+* SSH: you can use your PGP key as an SSH key
+
+    * I use a PGP subkey on my Yubikey for SSH
+
+* amaze people at parties, be an international celebrity
+
+---
+
+Threat models
+=============================================================================
+
+* Why would anyone attack **you**?
+
+    * you, your data, or your computing resources might be valuable
+    * you might be a stepping stone to another target, such as
+      your employer
+    * you might be an easy target
+
+* An attacker may want to...
+
+    * **impersonate** you
+    * **falsify** a file you need or provide, such as a software
+      download
+    * **eavesdrop** your communications to steal sensitive information
+
+---
+
+* An attacker may be...
+
+    * a government: US, UK, Russia, China, etc, possibly illegally
+    * law enforcement, possibly illegally
+    * a large private organisation, possibly your employer
+    * a criminal organisation
+    * a minor criminal
+    * someone with too much time and a twisted sense of humour
+    * someone you know who doesn't like you
+
+---
+
+Defences
+=============================================================================
+
+* cryptography
+
+* widespread use of cryptography ("herd immunity")
+
+* general infosec and opsec: keep your devices secure, and don't
+  do stuff that leaves you open for an attack
+
+* this can be really difficult, depending who you are and who's
+  attacking you; if you're a desirable target, you need to learn to
+  be secure, but if you're not, a few basic precautions is enough
+
+* compare with driving
+
+    * if you're just running errands, you'll be OK after normal
+      driving lessons
+    * if you're in a Formula 1 race, you need to learn a lot more and
+      work hard to even be allowed in the car
+
+* if the attacker is sufficiently motivated or sufficiently
+  well-funded, and has sufficient time, you will probably eventually
+  lose, sorry
+
+---
+
+The kernel of wisdom is moderation in all things
+-----------------------------------------------------------------------------
+
+* It's easy to become despondent, but don't. While security is never
+  perfect, even a little effort will improve things a lot. You might
+  run faster than others, so the bear won't attack you.
+
+* It's easy to try too hard, but don't become a cryptography or
+  security absolutist. While it's good if everyone tries their best,
+  and improves their cryptographic defences over time, don't despair
+  if what they or you have isn't perfect.
+
+* **Cryptography absolutism** is when you say it's not worth using
+  cryptography at all, unless you're protected against every
+  conceivable scenario.
+
+---
+
+Basics of public key cryptography
+=============================================================================
+
+* Everyone has a key pair: a **public** and a **secret** key, which
+  are linked together using advanced maths, such that data encrypted
+  with one can only be decrypted with the other
+
+    * you keep the **secret key secure** so that only you have a copy,
+      and only you can use it
+    * you **publish the public key** so that everyone can have a copy
+
+* Prove data or message is from you by **signing**: encrypt with your
+  secret key, anyone can check by decrypting with your public key
+
+    * actually, a cryptographic **hash** is signed, not the whole
+      message
+
+* Keep communications **secret**: everyone can encrypt with your
+  public key, only you can decrypt using your secret key
+
+---
+
+* Public key cryptography solves the key distribution problem.
+  Traditional (symmetric) cryptography requires a shared secret or
+  code between sender and receiver, and this can be complicated to
+  arrange.
+
+* RSA, ElGamal, Diffie-Hellman, elliptic curve, etc: interesting maths
+  if that's your thing, not required to be understood for using crypto
+  (this might be a little controversial)
+
+---
+
+Storing keys securely
+=============================================================================
+
+* Consider your threat models: what do you need to protect against?
+
+* You should probably at least keep your main key pair off-line,
+  on a USB stick, and only use it in a device whose security you
+  trust
+
+* Subkeys: the main key is kept safe and forms the identity, and
+  subkeys are additional keys, linked to the main key. New subkeys can be
+  generated at will for specific purposes, such as for keeping on a
+  laptop for email
+
+---
+
+* Subkeys can be stored on your normal devices, since they're easy
+  to replace with new ones if they're compromised
+
+* A secret key is data "at rest" (as opposed to "in transit"), and is
+  basically only protected by the passphrase you set.
+
+    * Choose a long passphrase that you won't forget. GnuPG can handle
+      any length you can remember and type.
+
+    * You can change the passphrase later if you want to make it
+      stronger.
+
+* Subkeys can also be stored on special devices for more secure key
+  storage, such as Yubikeys
+
+    * Highly recommended for many use cases
+
+    * Hardware restrictions prevent access of key
+
+
+---
+
+Creating a key, with subkeys, with GnuPG
+=============================================================================
+
+* Main key is used for keysigning, changing key uids, subkeys for
+  everything else
+
+* Install GnuPG aka GPG aka gpg, version 2.1 or later (Linux, Mac, Windows)
+
+* **`gpg --gen-key`**, use defaults unless you know better
+
+* Also create subkeys, at least one for encryption and one for signing
+
+* Follow one of these:
+
+    * <https://alexcabal.com/creating-the-perfect-gpg-keypair>
+
+    * <https://blog.liw.fi/posts/2017/05/29/using_a_yubikey_4_for_ensafening_one_s_encryption/>
+
+* Remember to create a **key revocation certificate** and store that
+  somewhere safe that you can access without your PGP key
+
+---
+
+Publishing a public key
+=============================================================================
+
+* A network of key servers exist for this, to make it easy to find
+  keys
+
+* Always use full fingerprints or 64-bit ("long") key ids to refer
+  to keys
+
+    * 32-bit (8 hex digit) short ids are no longer secure, and there
+      are hoax keys with real names (see <https://evil32.com/>)
+
+---
+
+* **In principle** it doesn't matter which keyserver you use, the
+  built-in default should be good enough, except sometimes it isn't
+
+    * New keys and updates to keys propagate across the network
+      automatically, but sometimes it takes time, even weeks
+
+    * This pool of keyservers seems to be better than most:<br/>
+      <http://sks-keyservers.net>
+
+    * Add this line to **~/.gnupg/gpg.conf** and **dirmngr.conf**:
+      <br/> `keyserver hkps://hkps.pool.sks-keyservers.net`
+
+    * Technically, only dirmngr.conf should be needed, but in my
+      experience things work better if it's in both.
+
+---
+
+Signing a key
+=============================================================================
+
+* You can add any names and email addresses you like to a key
+  generate; **anyone can create a new key and put your name on it;**
+  how can anyone trust any given key?
+
+* "Web of trust" vs "Trust on first use"
+
+* Keys can have signatures, which mean **"the person who signed this
+  key is confident that this key belongs to the person named in
+  the key"**
+
+    * Does *not* mean "is a trustworthy person, you should loan them
+      money and your car"
+    * "Is confident" is up to the signer, there are no rules
+
+---
+
+* Key signatures are also published on the keyservers
+
+    * However, this is currently not possible due to attack
+
+* You can tell GnuPG which keys' signatures you trust, and how
+  much, and GnuPG will tell you if you can trust a key, even if
+  you haven't signed it yourself
+
+    * compare with introducing people in real life
+
+    * "I trust Alice to introduce me to new people and not lie who
+      they are"
+
+---
+
+Signing a key: HOWTO, part 1
+=============================================================================
+
+* You need the fingerprint of the key you want to sign (KEYID below)
+  and to get the actual key from the keyservers; or you can get in a
+  file via email or similar
+
+    * **`gpg --recv-key KEYID`**
+    * **`gpg --import FILENAME`**
+
+* Sign key key, which is done by "editing" the key
+
+    * **`gpg --edit-key KEYID`**
+    * this will result in a **`gpg>`** prompt
+    * enter the **`sign`** command, *check the fingerprint*, answer yes
+      if you want to sign; this will ask for your key passphrase
+    * you may also have a GPG agent running, in which case you may
+      not need to provide a passphrase; the agent keeps the private
+      key and remembers it without you having to enter a passphrase
+      every time
+    * enter the **`save`** command at the prompt
+
+
+---
+
+Signing a key: HOWTO, part 2
+=============================================================================
+
+* Export and encrypt the signed key:
+
+    * **`gpg --export --armor KEYID > key.pub`**
+    * **`gpg --encrypt -r KEYID key.pub`**
+    * GnupPG may tell you that the key isn't ultimately trusted; tell
+      it to use the key anyway
+
+* Email the encrypted file **`key.pub.gpg`** (note suffix!) to the email
+  address listed on the key
+
+* This ensures the person who controls the secret key actually
+  receives email to the address given on the key
+
+
+---
+
+Receiving a signature for your key
+=============================================================================
+
+* Decrypt the email
+
+* Save the signed copy of your key to a file
+
+* Import the key: **`gpg --import FILENAME`**
+
+* Send the update key to the keyserver: **`gpg --send-key YOURKEYID`**
+
+
+
+---
+
+Signing a key: the easy way
+=============================================================================
+
+* Works on Debian and derived Linux distributions, maybe others
+
+* **`apt install signing-party`**
+
+* **`caff FINGERPRINT`**
+
+* may require for a mail transport agent to be configured on the
+  laptop
+
+---
+
+Extending key expiration date
+=============================================================================
+
+* Every key can have an expiration date
+
+* The secret key holder can extend the expiration date
+
+* It's optional, and not terribly useful
+
+    * makes sure you don't forget how to use GnuPG
+
+    * if you lose your secret key (careless!) it warns people to not
+      use your key after it's expired
+
+* Make sure you have a revocation certificate and can get that even if
+  you lose your secret key
+
+---
+
+Using a USB stick: why?
+=============================================================================
+
+* Store main key only on USB stick
+
+* Keep USB stick physically safe
+
+    * full-disk encryption with stong passphrase
+    * maybe stored in a safe
+    * possibly guarded by armed guards with dogs
+    * castles, moats, and sharks are probably overkill, but do
+      consider crocodiles
+    * some of this advice may be in jest
+
+* Also, only use USB stick in a computer you trust
+
+* You may want to mark all copies of the stick clearly
+
+* Have subkeys on laptop for everyday use
+
+---
+
+Using a USB stick: overall approach
+=============================================================================
+
+* GnuPG keeps everything in **`~/.gnupg`** by default, override by
+  setting the **`GNUPGHOME`** environment variable
+
+* Use this to have two GnuPG instances: **main key on a USB stick**,
+  and **subkeys on your laptop**
+
+    * format a dedicated USB stick with a suitable filesystem (e.g.,
+      **ext4**), give it a filesystem **label**, and mount it (e.g.,
+      **`/media/liw/usb-stick`**)
+    * make backup copies of the stick; probably best give each backup
+      copy a dedicated filesystem label so you know which one is which
+
+---
+
+* To use the main key, set **`export GNUPGHOME=/media/liw/usb-stick`**
+  for key signing, importing signatures, creating new subkeys
+
+* For everything else, don't set the environment variable
+
+* GnuPG sometimes starts **background services**; you should **kill
+  them** when switching between keys, or you'll be confused
+
+    * **`gpgconf --kill gpg-agent`**
+    * **`gpgconf --kill dirmngr`**
+
+---
+
+Using a USB stick: moving main key
+=============================================================================
+
+* Create your PGP and subkeys normally on your laptop, mount your USB stick
+
+    * Mount point it something like **`/media/liw/usb-stick`**
+
+* Export secret subkeys to a file (on laptop)
+
+    * **`gpg --export-secret-subkeys YOURKEYID > secret.key`**
+
+* Move the laptop's GnuPG stuff to USB stick.
+
+    * **`mv ~/.gnupg /media/liw/usbstick/gnupg`**
+
+---
+
+* Import the exported keys to laptop
+
+    * **`gpg --import secret.key`**
+    * this creates a new **`~/.gnupg`** directory; you may want to
+      configure it
+
+* Check results: **`gpg -K`**
+
+    * should show **`sec#`** to indicate missing main secret key
+    * should show **`ssb`** to indicate secret subkey being present
+
+---
+
+Legalese
+=============================================================================
+
+Copyright 2019 Lars Wirzenius
+
+This content is licensed under the Creative Commons
+Attribution-ShareAlike 4.0 International ([CC BY-SA 4.0][]) licence.
+
+[CC BY-SA 4.0]: https://creativecommons.org/licenses/by-sa/4.0/