path: root/sq-guide.md

---
title: "User guide for Sequoia-PGP `sq`"
subtitle: "Keeping private things private"
author: "The Sequoia-PGP project"
documentclass: report
...

# Document status {.unnumbered}

This document is very much a work in progress. Nothing is finished and
final. Parts haven't been
written yet. That said, feedback on what is written, or the structure
of the document is very much welcome. The source code for this
document is in version control on the `gitlab.com` site at:

```
https://gitlab.com/sequoia-pgp/sq-user-guide/
```

If you find mistakes or missing parts from the outline, please open an
issue or a merge request.

# Prelude: quick start {.unnumbered}

If you already understand the core cryptographic concepts in OpenPGP,
and you're in a hurry to get started, this chapter is for you. This
chapter distils the main content of this guide into examples showing a
small number of common use cases. No explanations.

```{.sh .numberLines}
$ sq key generate --userid="My Name" --userid="<me@example.com>" --export=key.pgp
$ sq key extract-cert --output=cert.pgp key.pgp
$ ls -l
total 8
-rw-rw-r-- 1 liw liw 1772 Oct 15 16:18 cert.pgp
-rw-rw-r-- 1 liw liw 1967 Oct 15 16:18 key.pgp
-rw-rw-r-- 1 liw liw  476 Oct 15 16:18 key.pgp.rev
```

```{.sh .numberLines}
$ sq sign --signer-key=key.pgp --output=foo.pgp foo.md
$ sq sign --signer-key=key.pgp --detached --output=foo-sig.pgp foo.md
$ ls -l foo*
-rw-r--r-- 1 liw liw 1086 Oct 15 16:19 foo.md
-rw-rw-r-- 1 liw liw 1825 Oct 15 16:20 foo.pgp
-rw-rw-r-- 1 liw liw  325 Oct 15 16:20 foo-sig.pgp
```

```{.sh .numberLines}
$ sq verify --signer-cert=cert.pgp --output=checked.md foo.pgp
Good signature from 84B292ABCE27285B
1 good signature.
$ sq verify --signer-cert=cert.pgp --detached foo-sig.pgp foo.md
Good signature from 84B292ABCE27285B
1 good signature.
$ ls -l checked*
-rw-rw-r-- 1 liw liw 1086 Oct 15 16:23 checked.md
```

\newpage

```{.sh .numberLines}
$ sq encrypt --recipient-cert=cert.pgp --signer-key=key.pgp --output=bar.pgp foo.md
$ ls -l bar.pgp
-rw-rw-r-- 1 liw liw 2076 Oct 15 16:26 bar.pgp
```

```{.sh .numberLines}
$ sq decrypt --recipient-key=key.pgp --signer-cert=cert.pgp --output=decrypted.md bar.pgp
Encrypted using AES with 256-bit key
Compressed using ZIP
Good signature from 84B292ABCE27285B
1 good signature.
$ cmp foo.md decrypted.md
$ ls -l decrypted.md
-rw-rw-r-- 1 liw liw 1086 Oct 15 16:27 decrypted.md
```

# Introduction

## What are Sequoia-PGP and `sq`?

[Sequoia-PGP project]: https://sequoia-pgp.org/
[Rust programming language]: https://www.rust-lang.org/
[sequoia-openpgp]: https://gitlab.com/sequoia-pgp/sequoia/-/tree/main/openpgp

The [Sequoia-PGP project][] works to make use of cryptography for privacy
and authentication in communication more commonplace. The project
produces and maintains an implementation of the OpenPGP standard
that's easy and uncomplicated to use.

OpenPGP is used widely in the IT industry and by free and open source
projects to verify the authenticity of software packages, and for
encrypting and authenticating messages.

`sq` is the command line tool provided by Sequoia-PGP. It's easy and
uncomplicated to use. Sequoia-PGP also provides a library for the
[Rust programming language][], called [sequoia-openpgp][]. However,
the library is only of interest to software developers, and this guide
is aimed at users of the `sq` tool.

## Why use OpenPGP?

The cryptography in OpenPGP helps with three main problems:

* keeping private things private
* identifying if data has changed
* authenticating communication (make sure who it's from)

The OpenPGP standard also
specifies protocols and data formats for managing the ecosystem of
OpenPGP users. The standard specifies, for example, what form
cryptographic messages take. This allows different parties in a
communication to use different programs. One correspondent might
use a laptop computer, while another may use a mobile phone. The
software running on those will be radically different, but as long as
they follow the same standard, secure communication using OpenPGP is
possible.


## Who is this guide aimed for?

This guide is aimed at those who want to
communicate privately, and receive and send messages and data safely with other
people. The guide does not require any background in cryptography,
mathematics, or programming, but does require using a computer via the
command line.

However, note that this guide aims to get the reader to a point
where they can use cryptography at all. It does not aim to teach the
reader how to protect against motivated, targeted attacks from
organized crime, or intelligence organizations. Such protection will
probably build on top of cryptography, but it is outside the scope of
this guide.

You can think of this guide as giving you the first turn of the
_security rachet_. Every turn of the ratchet improves security a
little bit, but achieving protection against targeted attacks will
require a great many turns.


## Scope of this guide

This guide covers the important concepts in using cryptography as
specified by the OpenPGP standard:

* keys, certificates, and their management
* certifying that a name or email address should be associated with a key
* signing files and verifying signatures
* encrypting and decrypting data

The guide shows how to use the `sq` command line tool from
Sequoia-PGP. It does not cover integrating Sequoia-PGP with mail
software, version control, file transfer software, or other
applications. (That will be covered by other documentation.)

## Structure of the guide

This guide has the following structure:

* The prelude chapter is a guide for getting started quickly. It's
  aimed at readers who are already familiar with the relevant
  cryptographic concepts, having perhaps used other software, and just
  want to see how to do a few basic things. That chapter provides no
  explanation, just shows the commands. You can safely skip it if you
  want to.
* The appendices show how to perform tasks to achieve specific goals.
  These are expanded versions of the prelude
  examples, but with detailed, step-by-step explanations of
  everything. These "how-to" guides are useful for getting things
  done without having to wade through long discussions about
  underlying concepts.
* There is also an appendix with a glossary, which can be helpful for
  looking up unknown terminology, and another appendix with links to
  additional material relevant to Sequoia-PGP and cryptography.
* The rest of this guide is discussions of the concepts needed
  to understand how cryptography works, and how to use it well.

Notably, this guide is not meant to be a reference guide. It does not
try to cover every aspect of the `sq` tool in detail. The built-in
help, which you can get by running `sq help` or `sq encrypt --help`,
is always up to date and a good way to look up details.


# Installing `sq`

This chapter explains how to install `sq` in various ways. It is by
necessity always going to be incomplete, but the authors would
gratefully accept changes for additional target systems.

## On various platforms

### Debian

On a Debian system (version 11 or later):

```
apt install sq
```

Note that on Debian 11 (bullseye), the version of `sq` is rather old.
You may want to install a back-ported version from the usual Debian
location for them.

### Fedora
FIXME.

### Arch

FIXME.

### FreeBSD

FIXME.

### OpenBSD

FIXME.

### NetBSD

FIXME.

### macOS

FIXME.

### Windows

FIXME.

## From source code on all platforms

To build and install `sq` from source, you need to have the [Rust][]
toolchain installed, in particular `cargo` and `rustc`. You also need
a number of non-Rust build dependencies installed; see the
[README.md][] for an up-to-date list.

To build and install the latest released version of `sq`, run the
following command:

```{.sh .numberLines}
$ cargo install sequoia-sq
```

To build `sq` from the current development version, get its source
code from [GitLab][]:

```{.sh .numberLines}
$ git clone https://gitlab.com/sequoia-pgp/sequoia.git
$ cargo install --path=sequoia/sq
```

[GitLab]: https://gitlab.com/sequoia-pgp/sequoia
[Rust]: https://www.rust-lang.org/
[README.md]: https://gitlab.com/sequoia-pgp/sequoia/-/blob/main/README.md


# On cryptography {#cryptography}

The science of keeping private communication private
(confidentiality), verifying that a message hasn't been modified
(integrity), and determining who created a message (authentication) is
called _cryptography_.

Cryptography is not just for spies.  Cryptography allows everyday
activities such as shopping and banking to happen without rampant
theft. It also allows journalists working on stories about the rich,
powerful, or corrupt to communicate with their sources with less fear
of prematurely revealing what they're doing.

Just as having a seat belt in a car won't help you if you don't use
it, or may even hurt you if you use it wrong, you need to understand a
few concepts to effectively use cryptography.  This chapter presents
the essential ideas that you need to understand to not just be safe,
but to avoid endangering yourself or others.

If you are concerned about a targeted attack on you or people you
communicate with, then this chapter is not enough.  You also need
training in operational security from a digital security trainer.
[Freedom of the Press Foundation][] is one organization that offers
training material, and courses.

  [Freedom of the Press Foundation]: https://freedom.press/training/

## Public key cryptography

OpenPGP uses public key cryptography.  To use public key cryptography,
you need two things: a _public key_ and a _private key_.

A public key and a private key form a pair.  They work together as
follows.  Say Alice wants to send a confidential message to Bob.  She
encrypts the message using Bob's public key, sends him the encrypted
message, and Bob decrypts it using his private key:

```text
       Alice       |        Internet        |      Bob
                   |                        |
  Encrypt Message  |                        |   Decrypt Message
  with Bob's      --->  Encrypted Message  ---> with Bob's
  Public Key       |                        |   Private Key
```

How Alice sends the message to Bob doesn't matter.  Someone who
intercepts the message can't decrypt it unless they have Bob's private
key.  And, even though Alice encrypted the message using Bob's public
key, Bob's public key can't be used to help decrypt the message.  A
public key is a one-way street.

The term public key includes the word public, because for it to be
useful, it needs to be widely published: Alice needs Bob's public key
to encrypt a message to him.

Likewise, the private key includes the word private, because it should
be hidden.  If someone else had Bob's private key, they could decrypt
the message that Alice sent him.

In short: you want people to have access to your public key; it should
be public.  Your private key, however, is private; like a secret, you
shouldn't share your private key with anyone.

Digital signatures work in a similar manner.  Alice creates a digital
signature using her private key (because no one else should be able to
sign a document in her name!).  And, to verify a signature, Bob uses
her public key, because anyone should be able to verify the signature.

In OpenPGP, your public key is just one part of a thing called a
_certificate_.  A certificate is a collection of public keys (you need
a different one for encryption and signing), some information about
you, like your name or alias, and your email address, and information
about what features your software supports.  A certificate doesn't
include your private keys.  You can and should share your certificate
with people you want to communicate with.

In OpenPGP, private keys are stored in a _key_.  A certificate never
includes private keys; a key does include private keys.  You should
share your certificate with other people; you should never share your
key with other people.


```text
  Do Share                 Keep Private

  OpenPGP                  OpenPGP
  Certificate              Key
  +------------+           +-------------+-----------+
  | Public     |           | Public      | Private   |
  | Key        |           | Key         | Key       |
  |            |           |             |           |
  | Public     |           | Public      | Private   |
  | Key        |           | Key         | Key       |
  |            |           |             +-----------+
  | User ID    |           | User ID     |
  |            |           |             |
  | Preference |           | Preferences |
  +------------+           +-------------+
```

## Password-based encryption

There is another type of encryption, which uses passwords.  This is
called symmetric encryption, because you use the same key to encrypt
and decrypt a message.

OpenPGP also supports password-based encryption.  Oftentimes, your key
will be protected with a password so you'll need to enter your
password before you can decrypt or sign a message.  But, you can also
use a password to encrypt a message.  Unlike a certificate, if you
publish a password, then everyone can decrypt your message.  This
means passwords are a lot more inconvenient than public keys.  Unlike
public keys, you can't share them willy-nilly, and you definitely
can't publish them in a directory like a telephone book.  Passwords
have to stay secret to be useful.

## Authentication

Encryption and signing are two of the three essential functions that
you need to communicate privately.  The last is called authentication.
It helps answer the following questions: When Alice sends a message to
Bob, does she have the right certificate?  And, when Bob receives a
message from Alice, can he be sure it really came from her?

Authentication helps prevent two different problems.  The first is
impersonation.  If Alice and Bob communicate regularly, and Bob gets a
message that purports to be from Alice, but is written in a different
style, then he may become worried that it is not really from Alice.
But, if Bob doesn't recognize these social cues, then he might be
tricked.  This is how phishing works.  Today, people are taught to
recognize impersonation.  This requires schooling, and vigilance.
Authentication addresses this problem in a different, more reliable
way: if Bob can authenticate Alice's key, and a message is signed
using Alice's key, then Bob can be confident that the message really
came from Alice.

```
           |                         |
  Mallory ---> Message from "Alice" ---> Bob
           |                         |
```

The second problem, interception, is more subtle and can't be solved
using social cues.  If Mallory wants to read what Alice and Bob send
to each other, then he can try to eavesdrop on their communication
channel.

Encryption is a prerequisite, but it is not sufficient to prevent
Mallory from intercepting the messages.  Imagine that Alice and Bob
send each other their certificates via email.  If Mallory is able to
intercept these initial, unencrypted messages, then he can replace the
certificates with his own.  Now, Alice and Bob will have the wrong
certificates, and when Alice sends Bob a message, she'll encrypt it
using Mallory's certificate.  When Mallory intercepts the message, he
can decrypt it, since actually Alice encrypted it to him.  And, he can
even fool Bob by reencrypting it using Bob's real certificate, and
forwarding that version to Bob.  Bob will be able to decrypt the
message as usual and won't suspect a thing!

```
          |                                     |
  Alice ---> Message ---> Mallory ---> Message ---> Bob
          |                                     |
```

The only practical way to prevent this type of attack is to
authenticate certificates.

Authentication can be done directly.  For instance, when Alice and Bob
meet in person, Alice and Bob can exchange business cards with their
certificates' ID numbers (in OpenPGP, ID numbers are called
_fingerprints_).  When Bob gets home, he can add what the correct
certificate for is Alice to his address book.  And, Alice can do the
same for Bob.  Alice and Bob will now use the right certificate, and
will detect an interception attack.  This is effective, because it's
much harder for Mallory to switch the fingerprints at a physical
meeting than to intercept and modify an email.

Another approach is for Alice and Bob to use a trusted third party,
which is sometimes called a _certification authority_ (_CA_).  For
instance, if Alice and Bob work at the same company, their IT
administrator could record everybody's fingerprint, and publish
appropriate certifications in a publicly available directory.  Now,
Alice just needs to authenticate the IT administrator; she doesn't
have to worry about authenticating her coworkers' certificates.  A
convenient way to run a CA like this is to use [OpenPGP CA][].

  [OpenPGP CA]: https://openpgp-ca.org

Interception attacks are a real concern.  The Government
Communications Headquarters (GCHQ), Britain's intelligence and
security organization, has proposed [Ghost], an authentication-layer
backdoor that they want secure messengers to implement.  Their
argument is that subverting authentication allows secure messengers to
help governments without actually violating their claim that
communication is end-to-end encrypted.  While technically true, this
is the moral equivalent of building a backdoor into the encryption,
and is, in effect, a new attempt at the failed [Crypto Wars] of the
1990s.

  [Ghost]: https://www.lawfareblog.com/principles-more-informed-exceptional-access-debate
  [Crypto Wars]: https://en.wikipedia.org/wiki/Crypto_Wars

## Advantages of public key cryptography

Using public key cryptography allows some very interesting things:

* I can publish my certificate, and anyone can send me a confidential
  message by encrypting it using my certificate, and be assured that
  only I can read the message.

  Examples: Activists in oppressive regimes using this can stop their
  governments from eavesdropping on them. Journalists can protect
  communications with their sources. Corporations don't need to worry
  their trade secrets leak to competitors.

* I can send them a reply, sign it with my private key, and the recipient
  can be sure it's from me, because they can check the signature using my
  public certificate.

  Examples: The CFO can't be fooled by forged emails from the CEO
  telling them to pay a fraudulent invoice. Activists can't be fooled
  by messages from the secret police to go to the town square at noon
  wearing a pink rose. Journalists can be sure the message is from
  their source and not someone who's trying to interfere with their reporting.

* These can be combined: If I have their certificate, I can be sure
  that the message someone send me is really from them, and that only
  they can read my response.

* I can publish a software release, and sign the file using my
  private key. Anyone downloading the release can be sure they get
  what I published by checking the signature with my certificate.

The mathematical and cryptographic details of how this works are
outside the scope of this guide, but see the [references](#references)
for links to explanations.

## Limitations of cryptography

When thinking about cryptography it's important to remember that
it has limitations. For example, no cryptography can prevent the
intended recipient from willfully sharing an encrypted message they
receive. If you send a photo of your safe combination to someone
encrypted with their certificate, they can decrypt it, and share the
picture with the highest bidder.

Also, no cryptography provides any protection if keys aren't kept
private. If I accidentally publish my key as a front page
advert on the New York Times, cryptography can't prevent others from
using that to decrypt messages intended for me, or publishing messages
that claim to be from me.

Further, cryptography doesn't protect against violence used to coerce
either party in a secure communication from disclosing secrets.

Finally, cryptography relies on some assumptions of what kind of
attacks are feasible, whether they're based on mathematics or raw
computing power. Over time, attacks on cryptographic protocols,
algorithms, and implementations only get stronger. Every few years,
attackers have a breakthrough, and some classes of cryptography
suddenly become so weak normal people can break them. Then the
purveyors and users of cryptography move to newer, stronger
alternatives.

For most people, these are quite unlikely scenarios. Most people do
not actually have enemies who are a threat specifically to them. If
you do, or you suspect you do, be very careful what you do and what
advice you follow.  You need to seek advice beyond this guide.  In
particular, you need training in operational security.  A digital
security trainer can help you.  [Freedom of the Press Foundation][] is
one organization that offers training material, and courses.

  [Freedom of the Press Foundation]: https://freedom.press/training/

# General principles of the `sq` interface

`sq` is a command line tool using subcommands and options. Global
options come before the subcommand on the command line, and options
specific to the subcommand come after.

Some options have both long and short forms. Thus, for example,
`--output` may be shortened to just `-o`. The
examples in this guide use the long form, as that's clearer and easier
to understand without explanation. In practice, the short form is
often more convenient to type.

Some options are "flags", and the use of the option is enough. For
example, to request binary output, the option` --binary` is enough.
Other options require more information to be provided. For example,
the option `--recipient-key` to specify what key to use for a
recipient when encrypting needs to be provided the name of the file in
which the key is stored. Such option values can be specified as the
command line argument after the option, or appended to the option
itself using `=foo` syntax. Thus, the following are
equivalent:

```
sq encrypt --recipient-cert cert.pgp --output bar.pgp foo.md
sq encrypt --recipient-cert=cert.pgp --output=bar.pgp foo.md
```

This guide uses the latter syntax to make it clearer when an option is
given a value without the reader having to look up each option.

The `sq` command has built-in help text that can be accessed using the
`help` command or the `--help` option:

```
sq help
sq --help
sq help key
sq key --help
sq help key generate
sq key generate --help
```

Use the help feature liberally to find out all the subcommands and
options, and whether an option is a flag or takes a value, and what
other arguments the command accepts.



# Managing one's own key

This chapter concentrates on creating and managing a private key for oneself.
Please see the [glossary](#glossary) for definitions of terms. Some of
the terminology `sq` uses is specific to cryptography in general, or
to OpenPGP, and some is specific to `sq` itself.

## Why use keys and certificates?

Your key is, in the context of public key cryptography, you. It's a
digital artifact that represents you to everyone else. Nobody else has
your key. You and your key are inseparable, and to everyone else, your
key is you, and you are your key.

That is, of course, romantic balderdash. A key is a large random
number. It has no free will, it has no agency, it can't think, it
doesn't feel, it can't act, it can't enjoy a cup of hot tea in the
morning while writing a book, it's not alive. In no real way is it
you. Except when it comes to secure communication, your key stands for
you. When someone wants to send a confidential message to you, they
encrypt it using your certificate, which is mathematically,
inalienably linked to your key. When you want to prove a message comes
from you, you encrypt it with your key.

You can have as many keys as you want. You can have one for work,
another for school, a third for your family and friends, and a fourth one
for publishing poetry online. These keys may be linked or kept
separate, as you prefer.

## Types of keys and algorithms

Over time, as cryptographic attacks have weakened the protections of
cryptographic defences, different types of keys and algorithms have
been developed and enhanced. While a through discussion of these is
beyond the scope of this guide, the list below gives a summary.

* Encryption algorithm **RSA**: this is the oldest known public key
  encryption algorithm. It has stayed strong, except as attackers are
  gaining faster computers and better attack software, the keys used
  for RSA have needed to become much longer. Where a 384 bit key was
  OK in the early 1990s, a 2048 bit or even a 4096 bit key is
  preferred thirty years later. The longer an RSA key is, the more
  computing power it takes to use it, and this makes strong
  cryptography with RSA slower.

* **Elliptic curve cryptography**: where RSA relies on the
  computational difficulty of factoring large numbers, elliptic curve
  relies on certain operations in geometry involving elliptical curves
  in two-dimensional space being hard to undo. There are several
  curves in popular use, and some of the older ones are now considered
  to provide only weak protection against attackers. New curves get
  developed from time to time, to provide more strength. With elliptic
  curves, the choice of curve also specifies the size of the key.

* **Hash** algorithms are one-way mathematical functions, where if
  you have a file you can compute a result from its contents easily,
  but if you have the result, it's really hard to get back the file. A
  _cryptographic hash_ is one where it's really hard to find any
  file with the given hash value. As with encryption algorithms,
  hashes need to be made stronger as time goes by. OpenPGP
  originally mandated the use of the MD5 algorithm, but that is no longer
  suitable. Currently SHA256 is the preferred hash algorithm.

Because key types and algorithms need to be improved over time, the
OpenPGP standard allows replacing them in newer versions of the
standard without fundamentally changing the structure of the OpenPGP
protocol. Each version of OpenPGP supports multiple key types and
algorithms, which allows for a managed migration towards stronger
security, and without losing access to older files and messages.


## Why use subkeys?

Even someone having only one cryptographic key may benefit from having
other keys for specific purposes. For example, they might have a very
strong primary key as their primary key, and additional, auxiliary
keys for encryption or digital signatures. Such auxiliary keys can be
tied to the primary key using _certifications_, which we'll cover in
more detail later. For now, a certification uses the primary key to
declare that the auxiliary key can be used instead of the primary key
for a specific purpose. The auxiliary key then becomes a _subkey_,
and other users of OpenPGP will use it automatically, if they have
your certificate. This setup has several benefits:

* you can have separate subkeys for encryption, signing, or
  authentication
* it's harder to leak or misuse the primary key, as it's only used
  rarely
* you can use a smaller key when less security is OK in exchange for
  faster use
* you can have a separate subkey for each device you have, or put
  them on a hardware security token
* you can replace the subkeys easily: others trust your certificate,
  and will happily use any subkey they can verify using your
  certificate

All of this is managed pretty much automatically using OpenPGP
software.


## Why would keys expire automatically?

A key, whether a primary key or a subkey, can be set to expire at a given
time. This is a precaution against you losing access to the primary
key: if the key expires, others won't use it anymore. You can extend
the expiration as often as you wish, although that requires getting
your updated certificate to everyone who needs to use it.

Another, more subtle benefit of expiring keys is that a short
expiration time (of, say, one year) forces everyone else to refresh
their copy of your certificate. This routine means they will also get
a revocation update for the key, if there's ever a need for that.

You can also set subkeys to expire. This has the same benefits as
expiring the primary key.

Changing expiration times can be a chore. There's a security benefit
to it, but if it's inconvenient for you, you may want to consider not
expiring keys, or only expire subkeys. Despite the benefits, it's
better to have a non-expiring key than not have a key at all.


## Generating a key

To generate a key with `sq`:

```sh
sq key generate --userid="My Name" --userid="<me@example.com>" --export=key.pgp
```

A key can have any number of _user identifiers_ (or _user ids_). The
Sequoia project suggests that it's best to have separate user ids for
name and email address to allow them to be certified separately (we'll
discuss what that means later). Traditionally they have been combined
into one id, and that still works.

When a email program looks up a certificate for a recipient, it uses
the email address to do so. At least one user id should contain the
email address for the lookup to work.

You can set an expiration time at the time of creating a key, if you
want. See the `--expires` and `--expires-in` options.

Generating a key with `sq` results in two files. The key is put in the
file you name as the argument to the `--export` option. A _key
revocation certificate_ is written to a file with that name and `.rev`
appended to it (or you can specify the name yourself, with the
`--rev-cert` option). The revocation certificate tells others that
your key is longer usable. If, for example, you lose the file with the
key, you can share the revocation certificate with others, and they
(or their OpenPGP software) will know to not use that key
anymore. We'll cover key revocation in more detail later.

You can choose the cryptographic algorithm, and whether the key should
have subkeys for signing or encrypting messages. See the `--help`
output for a list of options.


## Extracting a certificate from a key

Given a key, you can extract the certificate linked to it:

```
sq key extract-cert --output=cert.pgp key.pgp
```

The `cert.pgp` file is the certificate (choose whatever name you want
for it). You need to re-extract the certificate every time you make a
change to the key that would shared with others: user ids, expiration
times, subkeys.

Note that while you can extract a certificate from a key, the other
direction is not possible.


## Sharing your certificate with others

A certificate contains no secrets, and you can safely share it with
anyone: include it as an attachment in every email you send; put it on
your web home page; put it on your profile on social media sites such
as Facebook, Twitter, Mastodon, or GitHub; publish a photo of it on
a photo sharing site; print it on business cards. We'll cover more
options later in the chapter on managing keys in a community.

A caveat: a certificate does contain all the user ids on your key, so
if any of those is not public information you may want to remove them
from your key before extracting the certificate. You may want to have
an entirely separate key for that. User ids are tied to the primary
key, subkeys inherit them from their primary.

# Using digital signatures

## Why use signatures?

Digital signatures are used to show who sent a message and that it
hasn't been changed. See the chapter on [cryptography](#cryptography)
for a longer discussion.

It's important to note that signatures are good not just for messages,
but for any kind of data, including files and cryptographic keys.

* You can sign a file to prove that you've seen it. Others can then
  verify that the signature and file match, and trust that the file
  they have is the same one you signed.

  This does not prove that you created the file, only that you had the
  file at the time you signed it.

* When a software distributor, such as Debian, Fedora, Red Hat, or the
  Apple app store, prepares a software package for distribution, they
  sign it using the their key. When a system downloads a
  package from the distributor, it verifies the package signature
  using the distributor's public key, which it already has, as it
  comes pre-installed in Linux distributions and on Apple devices. If
  the package fails to match its signature, the package won't be
  installed.

  This is done to protect the system from installing software that has been
  modified maliciously. It can also be used as a mechanism
  for preventing software from being installed that isn't approved by the
  distributor. Linux distributions don't do that, but big corporations
  seeking a dominant market position tend to like it.

* When you add a User ID to a key, any user ids attached to it are signed by
  the key. This prevents other people from adding their email address
  to your key. If they could do that, they would get your email. While
  they can't read it, if it's encrypted, they can at least make sure
  you don't get it.

* When you add subkeys, they are signed by the primary key to prove
  that you, the key holder, wants the subkey to be used.

## Making a signature

To sign a file with `sq`, you need your key (not the certificate). The
command to sign is:

```sh
sq sign --signer-key=key.pgp --output=foo.pgp foo.md
```

This signs the file `foo.md`, and writes the signed file to `foo.pgp`.
That file contains both the contents of `foo.md` and a signature.

Having the signature be part of the file can be convenient, but it can
also be inconvenient. Sometimes it's easier to have the signature
separate from the data. That's called a _detached signature_. If
nothing else it means that you don't have two copies of the data,
which can be costly in terms of disk space. A detached signature is
also handy if someone else already has a copy of the data and you just
want to prove your copy is identical.

To make a detached signature:

```
sq sign --detached --signer-key=key.pgp --output=foo.sig foo.md
```

Note the `--detached` option. The signature, but none of the original
data, is written to `foo.sig`. The detached signature is small,
and its size does not depend on the size of the signed data. That's
because the detached signature only contains the _cryptographic hash_
of the original data.

## Verifying a signature

Verifying the signature of a signed file is done like this:

```
sq verify --signer-cert=cert.pgp --output=checked.md foo.pgp
```

The output will say something like this:

```
Good signature from 84B292ABCE27285B
1 good signature.
```

The mysterious number `84B292ABCE27285B` is the key identifier of the
key that made the signature. It should match the certificate you've
provided.

If the signature doesn't verify correctly, the error message is clear
(see [#768](https://gitlab.com/sequoia-pgp/sequoia/-/issues/768)):

```
thread 'main' panicked at 'It is an error to consume more than data returns: Custom { kind: InvalidInput, error: "Bad CRC sum." }', openpgp/src/parse.rs:4988:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

(That output is a bug. See issue
[#768](https://gitlab.com/sequoia-pgp/sequoia/-/issues/768). This
part of the guide will be updated once the bug is fixed.)

A detached signature is verified in a similar fashion, but you need to
be a little more verbose and give the names of both the data file and
the signature file:

```
$ sq verify --detached --signer-cert=cert.pgp foo.sig foo.md
```

## Authenticating a certificate

If you have a suspicious mind set, you may have spotted a glaring
error in the above discussion of verifying digital signatures: you
must have the certificate of the sender, and you need to be sure it's
actually their certificate.

Sometimes that's easy. If you're downloading software to install using
a "package manager" (Linux distributions), or the app installation
program on a mobile phone, the certificate was pre-installed.

If you're downloading software directly from its publisher's website,
they probably provide the certificate on their website as well. If
you're downloading things over HTTPS (instead of the unencrypted,
unprotected, un-lamented plain HTTP), you can _probably_ trust the
certificate. It's possible to spoof that, but it's not easy.

You may gain more trust in the certificate by verifying that the one
you have is one that a lot of other people have, and have had a long
time. You can do this by asking a lot of people. We'll return to this
topic later in the chapter on managing keys at a community level.


# Using encryption

## Encrypting a file

Encrypting things is how you keep your private data private. There's
a large, global debate about privacy, and who deserves to keep what
private, and how to still catch bad guys who harm others. We shan't get
into that here.

To encrypt a file using `sq`:

```
sq encrypt --recipient-cert=cert.pgp --output=bar.pgp foo.md
```

This encrypts the file `foo.md`, using the certificate in `cert.pgp`,
and writes the result into `bar.pgp`.

Note that the encryption is done only for the explicitly specified
recipients. If you want to read the encrypted output later,
you need to add yourself as a recipient.

The output file has a name different from the input file so that the
filename, which is not encrypted, does not reveal anything about the
contents to someone who happens to see it.

You can optionally also sign the data by adding the
`--signer-key=key.pgp` option to the encryption command.

## Decrypting a file

To decrypt an encrypted file:

```
sq decrypt --recipient-key=key.pgp --output=decrypted.md bar.pgp
```

The output is written to `decrypted.md`. If the encrypted data was
also signed, and you add the `--signer-cert=cert.pgp` option, the
decryption will check the signature. If the signature fails to match,
the data is not written into the output file to avoid anyone trusting
an unauthenticated message.



# Managing digital keys and certificates on a community level

FIXME: This chapter will discuss how to manage keys and certificates
among large groups of people. It will discuss how to build strong
trust that a key belongs to a specific person or organization. It will
discuss various ways of distributing certificates.


* how do I find someone's key?
* how do I check it's their key?
* how do I get updates to their key?
  - user ids
  - subkeys
  - expiration changes
  - revocation of primary key or subkey



# Appendix: How to...? {.unnumbered}

This appendix has task-oriented guides for achieving specific goals.
Each how-to guide will explain every step, giving command line
examples, but will not go into detail about what is happening or why.
These how-to guides are aimed at people who need to achieve a specific
goal, have some understanding of OpenPGP concepts, but don't currently
care to understand deeply. As such, the how-to guides will repeat
specifics that have been covered in the rest of the book.

## How to verify that a downloaded file is the one its author made
## How to sign a file to share with others
## How to decrypt a message from someone else
## How to encrypt a message for someone else
## How to generate a key, with subkeys, and a certificate
## How to distribute certificate to others
## How to certify someone else's user id



# Appendix: Switching from GnuPG to Sequoia-PGP {.unnumbered}

This appendix is aimed at people who already know how to use `gpg`,
the command line tool from GnuPG that roughly corresponds to `sq`. It
shows how to do specific tasks using either `gpg` or `sq`. It will

GnuPG stores keys and certificates in the `~/.gnupg` directory, or
the directory named in the `GNUPGHOME` environment variable. They're not
easily accessed directly as files, and are referred to via the user id
or using a hexadecimal key identifier or key fingerprint. The set of
keys and certificates in that directory is called a _keyring_. possibly
be a comparison table, for easy review.

GnuPG typically outputs binary files. The `--armor` option tells it to
write a textual representation. That representation is still not
human-readable, but can be easier to transmit over various channels
that expect text instead of binary data.

## Generate a key and certificate

```
gpg --quick-gen-key "Tomjon <tomjon@example.com>"
```

## Export certificate into a file

```
gpg --export --armor tomjon > tomjon.asc
```

## Import a certificate into your keyring

```
gpg --import certificate.asc
```

## List all certificates in your keyring

Either all keys, or keys with a user id that contains a string:

```
gpg --list-keys
gpg --list-keys tomjon
```

Output for one key looks something like:

```
pub   rsa4096 2015-03-01 [SC] [expires: 2025-01-10]
      DBE5439D97D8262664A1B01844E17740B8611E9C
uid           [ unknown] Lars Wirzenius <liw@liw.fi>
uid           [ unknown] Lars Wirzenius <liw@iki.fi>
uid           [ unknown] Lars Wirzenius <lwirzenius@wikimedia.org>
sub   rsa4096 2015-03-01 [S]
sub   rsa4096 2015-03-01 [E]
```

The first word of the line tells you what the line contains:

* `pub`---a public key (i.e., certificate)
* `sec`---a secret key (i.e., key, in `--list-secret-keys` output)
* `sec#`---a secret key is known to exist, but isn't actually in the keyring
* `uid`---a userid attached to the key
* `sub`---a subkey

The `unknown` tells you how much you've told GnuPG you trust that user
id. There's also information about type and length of a key, what it's
used for (signing, certifying, encrypting), and key fingerprint
(`DBE5439D97D8262664A1B01844E17740B8611E9C` above). The fingerprint is
the strongest way to refer to a key.

## List all private keys in your keyring

```
gpg --list-secret-keys
```

## Sign a file

Drop `--armor` for binary output. Output goes to `hello.txt.asc` with
`--armor`, or `hello.txt.pgp` without.

```
gpg --sign --armor hello.txt
```

## Check a file's signature

```
gpg --verify hello.txt.gpg
```


## Sign a file---detached signature

Drop `--armor` for binary output. Output goes to `hello.txt.asc`
with `--armor`, or `hello.txt.sig` without.

```
gpg --detach-sign --armor hello.txt
```

## Check a file's detached signature

```
gpg --verify hello.txt.sig hello.txt
```

## Encrypt a file

Output goes to `hello.txt.pgp` (with `--armor` to `hello.txt.asc`).
The `--recipient` option can be shortened to `-r`. By default, this
encrypts _only_ for the explicitly named recipients, so if one wants
to decrypt the file later oneself, one needs to remember to encrypt it
for oneself.

```
gpg --encrypt --recipient liw -r tomjon hello.txt
gpg --encrypt --armor --recipient liw -r tomjon hello.txt
```

## Decrypt a file

Output goes to the standard output unless `--output` is used. Note
that GnuPG may output the cleartext, even if the signature fails.

```
gpg --decrypt hello.txt.gpg
```

# Appendix: Glossary {#glossary .unnumbered}

This appendix explains all the specialist terminology related to
OpenPGP and Sequoia-PGP. It includes both the terms Sequoia prefers
(e.g., "certificate") and the older terminology for the same thing
("public key").

authenticate

: to verify the origin of a message, using a digital signature

certificate

: the public key in public key cryptography; meant to be distributed
  widely; _see_ public key

certification

: assuring that the user id and key belong to a specific person, using
  a signature on the user id

cipher

: an encryption algorithm

cleartext

: data that has not been encrypted, even if it's not text

decrypt

: convert encrypted data into cleartext

encrypt

: convert cleartext data into a form nobody can read unless it was
  encrypted for them

identity

: a deep philosophial problem; for OpenPGP, the user id attached to
  one's key

key

: a key for encrypting and decrypting data; _see_ private key, public key

key pair

: in public key cryptography, the two parts of a key; _see_ private
  key, public key

private key

: the private part of a key in public key cryptography; this is meant
  to not be shared with anyone

public key

: the public part of a key in public key cryptography; _see_ certificate

sign

: to encrypt data with one's private key, to be decrypted using one's
  certificate

signature

: the result of signing data; _see_ sign

user id

: a name and/or email address attatched to a key; there can be many
  such user ids attached to one key

verify

: to check that signed data matches its signature



# Appendix: References {#references .unnumbered}

* [OpenPGP on Wikipedia](https://en.wikipedia.org/wiki/Pretty_Good_Privacy#OpenPGP)
* [PGP on Wikipedia](https://en.wikipedia.org/wiki/Pretty_Good_Privacy)
* [Public key cryptography on Wikipedia](https://en.wikipedia.org/wiki/Public-key_cryptography)
* [RFC 4880](https://datatracker.ietf.org/doc/html/rfc4880), the
  Internet standard specification for OpenPGP
* [Sequoia-PGP website](https://sequoia-pgp.org/)


# Appendix: Copyright license {.unnumbered}

Copyright 2021  The pep foundation

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          it is cured within 30 days of Your discovery of the
          violation; or

       2. upon express reinstatement by the Licensor.

     For the avoidance of doubt, this Section 6(b) does not affect any
     right the Licensor may have to seek remedies for Your violations
     of this Public License.

  c. For the avoidance of doubt, the Licensor may also offer the
     Licensed Material under separate terms or conditions or stop
     distributing the Licensed Material at any time; however, doing so
     will not terminate this Public License.

  d. Sections 1, 5, 6, 7, and 8 survive termination of this Public
     License.


Section 7 -- Other Terms and Conditions.

  a. The Licensor shall not be bound by any additional or different
     terms or conditions communicated by You unless expressly agreed.

  b. Any arrangements, understandings, or agreements regarding the
     Licensed Material not stated herein are separate from and
     independent of the terms and conditions of this Public License.


Section 8 -- Interpretation.

  a. For the avoidance of doubt, this Public License does not, and
     shall not be interpreted to, reduce, limit, restrict, or impose
     conditions on any use of the Licensed Material that could lawfully
     be made without permission under this Public License.

  b. To the extent possible, if any provision of this Public License is
     deemed unenforceable, it shall be automatically reformed to the
     minimum extent necessary to make it enforceable. If the provision
     cannot be reformed, it shall be severed from this Public License
     without affecting the enforceability of the remaining terms and
     conditions.

  c. No term or condition of this Public License will be waived and no
     failure to comply consented to unless expressly agreed to by the
     Licensor.

  d. Nothing in this Public License constitutes or may be interpreted
     as a limitation upon, or waiver of, any privileges and immunities
     that apply to the Licensor or You, including from the legal
     processes of any jurisdiction or authority.


=======================================================================

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will be considered the “Licensor.” The text of the Creative Commons
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of Creative Commons without its prior written consent including,
without limitation, in connection with any unauthorized modifications
to any of its public licenses or any other arrangements,
understandings, or agreements concerning use of licensed material. For
the avoidance of doubt, this paragraph does not form part of the
public licenses.

Creative Commons may be contacted at creativecommons.org.

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