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__________________________________
GNUPG FREQUENTLY ASKED QUESTIONS
Robert J. Hansen et al.
__________________________________
1 Foreword
==========
Welcome to the GnuPG Frequently Asked Questions (FAQ)! Before we
begin, there’s just a few things that need to be addressed. It’s
regrettable these things have to be included, but society has become
very litigious.
1.1 Trademark notice
~~~~~~~~~~~~~~~~~~~~
- PGP is a trademark of Symantec Corporation.
- Windows is a trademark of the Microsoft Corporation.
- Macintosh, OS X and Mac OS X are all trademarks of the Apple
Corporation.
- Linux is a trademark of Linus Torvalds.
- Solaris is a trademark of Oracle Corporation.
- Some cryptographic algorithms mentioned in this FAQ may be
trademarked.
The use of these, or any other, marks is solely for identification
purposes.
1.2 Licensing
~~~~~~~~~~~~~
This document is © 2012, Robert J. Hansen <[rjh@sixdemonbag.org]> and
A.M. Kuchling <[amk@amk.ca]>. You are free to make use of this
document in accordance with the [Creative Commons
Attribution-ShareAlike 3.0 license], with the exception of Randall
Munroe’s XKCD comic, which is released under terms of the [Creative
Commons Attribution-NonCommercial 2.5] license; alternately, you may
make use of it under terms of the GNU General Public License (version
3 or, at your discretion, any later version), again excepting
Mr. Munroe’s works.
[rjh@sixdemonbag.org]
mailto:rjh@sixdemonbag.org?subject=The%20GnuPG%20FAQ
[amk@amk.ca] mailto:amk@amk.ca?subject=The%20GnuPG%20FAQ
[Creative Commons Attribution-ShareAlike 3.0 license]
http://creativecommons.org/licenses/by-sa/3.0/
[Creative Commons Attribution-NonCommercial 2.5]
http://creativecommons.org/licenses/by-nc/2.5/
1.3 Disclaimer of liability
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Although the contents of this document are believed to be correct, the
author(s) cannot promise it is error-free and cannot assume liability
for any errors.
Table of Contents
_________________
1 Foreword
.. 1.1 Trademark notice
.. 1.2 Licensing
.. 1.3 Disclaimer of liability
2 Welcome
.. 2.1 What conventions are used in this FAQ?
.. 2.2 Who maintains this FAQ?
.. 2.3 Is this the official GnuPG FAQ?
.. 2.4 When was this FAQ last checked for accuracy?
3 General questions
.. 3.1 What’s GnuPG?
.. 3.2 How do I pronounce GnuPG?
.. 3.3 Is it compatible with Symantec’s PGP?
.. 3.4 Which operating systems does it run on?
.. 3.5 How much does it cost?
.. 3.6 From where can I download it…
.. 3.7 Is there source code available for it?
.. 3.8 What’s Free Software, and why does it matter?
.. 3.9 How can I donate money to the GnuPG project?
.. 3.10 How can I help with GnuPG development?
4 Where can I get more information?
.. 4.1 How can I spot the charlatans?
.. 4.2 What are some useful mailing lists?
.. 4.3 What are some useful webpages?
5 What email clients support GnuPG on…
.. 5.1 … Microsoft Windows?
.. 5.2 … Mac OS X?
.. 5.3 … Linux or FreeBSD?
6 Is GnuPG available as a ‘portable app’?
7 What do all these strange words mean?
.. 7.1 What’s ‘public-key cryptography’?
.. 7.2 What’s ‘symmetric cryptography’?
.. 7.3 What’s a ‘key’?
.. 7.4 What’s a ‘certificate’?
.. 7.5 What’s a ‘keyserver’?
.. 7.6 What’s RSA?
.. 7.7 What’s DSA?
.. 7.8 What’s Elgamal?
.. 7.9 What’s AES?
.. 7.10 What are Twofish and Blowfish?
.. 7.11 What’s 3DES?
.. 7.12 What’s Camellia?
.. 7.13 What are SHA-1, SHA-224, SHA-256, SHA-384, SHA-512 and SHA-3?
.. 7.14 What’s MD5?
.. 7.15 What are CAST, CAST5, and CAST5-128?
.. 7.16 What are ZLIB, ZIP and BZIP?
.. 7.17 What’s a ‘revocation certificate’?
.. 7.18 What’s a ‘designated revoker’?
.. 7.19 What does ‘validity’ mean?
.. 7.20 What does ‘trust’ mean?
.. 7.21 What does ‘ownertrust’ mean?
8 How do I start using GnuPG?
.. 8.1 Does GnuPG need to be ‘tuned’ before use?
.. 8.2 How large should my key be?
.. 8.3 What algorithm should I use?
.. 8.4 Why does it take so long to generate a certificate?
.. 8.5 What should I do after making my certificate?
.. 8.6 Where does GnuPG look for configuration options?
.. 8.7 What options should I put in my configuration file?
.. 8.8 Is there any particular keyserver I should use?
.. 8.9 What’s the difference between an ‘option’ and a ‘command’?
.. 8.10 What are the most commonly used options?
.. 8.11 What are the most commonly used commands?
.. 8.12 How do I use another person’s certificate?
.. 8.13 Why can’t I read emails I’ve sent, and how do I fix it?
.. 8.14 How do I encrypt a file for multiple recipients?
.. 8.15 How do I sign a file with multiple certificates?
.. 8.16 How do I combine encryption with signing?
.. 8.17 How do I force GnuPG to make printable-text output?
.. 8.18 How do I create an ‘inline signature’?
.. 8.19 How can I use GnuPG in an automated environment?
.. 8.20 I’m a programmer and I need a GnuPG library. Is there one?
.. 8.21 I’m a programmer and I need a way to call GnuPG internals directly. Is there a library for this?
9 What common problems come up?
.. 9.1 Why is GnuPG warning me this certificate might not belong to whom I think it does?
.. 9.2 Why is GnuPG warning me about using insecure memory?
.. 9.3 Why is GnuPG changing my message?
10 What are some common best practices?
.. 10.1 How can I choose a strong passphrase?
.. 10.2 How can I keep my revocation certificate safe?
.. 10.3 How can I keep my computer safe from malware?
.. 10.4 Should I use encrypted disk software like TrueCrypt, BitLocker or FileVault?
11 Advanced topics
.. 11.1 Why does GnuPG use RSA-2048 by default?
.. 11.2 Do other high-security applications use RSA-2048?
.. 11.3 Why doesn’t GnuPG default to using RSA-4096?
.. 11.4 Why do people advise against using RSA-4096?
.. 11.5 Why does GnuPG support RSA-4096 if it’s such a bad idea?
.. 11.6 Can any of the ciphers in GnuPG be brute-forced?
.. 11.7 Has GnuPG ever been successfully attacked?
.. 11.8 Should I use PGP/MIME for my emails?
.. 11.9 What are the best algorithms in GnuPG?
.. 11.10 Why is my DSA key limited to 3072 bits?
.. 11.11 Why does my DSA-1024 key use a different digest algorithm than my DSA-2048 or DSA-3072 key?
2 Welcome
=========
Welcome to the *unofficial* GnuPG FAQ.
*As of this writing, this FAQ is both inaccurate and incomplete. It is
a work in progress. Do not rely on the answers found herein.*
2.1 What conventions are used in this FAQ?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As is par for the course with everything involving computers, there
are an awful lot of acronyms in this FAQ. For most of them, holding
the mouse pointer over the acronym will reveal a tooltip containing
the full expansion of the acronym. Hovering over the letters FAQ, for
instance, will reveal the words “Frequently Asked Questions.” These
acronyms are presented in a small-caps font in order to make them
easier to recognize when reading.
2.2 Who maintains this FAQ?
~~~~~~~~~~~~~~~~~~~~~~~~~~~
[Robert J. Hansen]. Please feel free to contact me should there be an
error in this FAQ, whether typographical, grammatical, or factual.
When writing, the editorial “we” refers to the general consensus of
the GnuPG community. This consensus is hammered out on the GnuPG-Users
mailing list. All members of the GnuPG community are invited to
participate. Individual people within the community may give their
own editorial comments: these will be set off by square brackets,
italicized, and initialed by their author. The different editors are:
- wk: Werner Koch <[wk@gnupg.org]>
- rjh: Robert J. Hansen <[rjh@sixdemonbag.org]>
/[Do we have any other editors we need to add? — rjh]/
[Robert J. Hansen]
mailto:rjh@sixdemonbag.org?subject%3DThe%20GnuPG%20FAQ
[wk@gnupg.org] mailto:wk@gnupg.org?subject%3DThe%20GnuPG%20FAQ
[rjh@sixdemonbag.org]
mailto:rjh@sixdemonbag.org?subject=The%20GnuPG%20FAQ
2.3 Is this the official GnuPG FAQ?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Not yet.
2.4 When was this FAQ last checked for accuracy?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
October 2012.
3 General questions
===================
Since no company controls GnuPG, there’s really no single vendor for
GnuPG. Instead, there’s a robust community surrounding GnuPG which
has produced versions of it for several different operating systems.
But first, let’s cover the basics.
3.1 What’s GnuPG?
~~~~~~~~~~~~~~~~~
GnuPG is cryptographic software that helps people ensure the
confidentiality, integrity and assurance of their data. Let’s try
that again: GnuPG is…
- /Cryptographic./ The word “cryptography” is derived from two Greek
words, κρυπτός (pronounced “kryptos,” meaning “hidden”) and γράφω
(pronounced “graphein,” meaning “writing”). Cryptography is the
mathematical study of codes and ciphers.
- /Software./ This one should already be obvious.
- /Confidentiality./ No one except authorized parties should be able
to read your data.
- /Integrity./ It shouldn’t be possible to tamper with a message
unnoticeably.
- /Assurance./ An assurance is not a guarantee. There are no
guarantees in life, and software is no different. An assurance just
means there is good reason to be confident of something — here, it
means that when GnuPG is correctly used, people may be confident the
data is confidential and/or possesses integrity.
GnuPG may be used by itself as a command-line application (i.e., to be
run at a Terminal prompt or a Windows command prompt), or integrated
into popular email clients. It’s also used by some instant messaging
clients, such as Psi.
3.2 How do I pronounce GnuPG?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
“guh-NEW-pee-gee.”
3.3 Is it compatible with Symantec’s PGP?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Largely, yes. It can be made to interoperate with anything from PGP
5.0 and onwards, and has excellent interoperability with the most
recent releases.
3.4 Which operating systems does it run on?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Too many to list! It’s known to run on Microsoft Windows, Mac OS X,
the various free Unixes, AIX, Solaris, HPUX, OpenVMS, and more. People
are even working on porting it to smartphones such as Android.
3.5 How much does it cost?
~~~~~~~~~~~~~~~~~~~~~~~~~~
There is no fixed price. Many sites on the internet offer legal
downloads of it for free.
3.6 From where can I download it…
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Lots of different places, but no one site hosts binaries for all
operating systems.
… for Microsoft Windows?
------------------------
A convenient Windows installer is available from [GPG4WIN].
[GPG4WIN] http://www.gpg4win.org
… for Mac OS X?
---------------
The [GPGtools project] has everything needed to get started.
[GPGtools project] http://www.gpgtools.org
… for Linux?
------------
The bad news is there is no single, consistent way to install GnuPG on
Linux systems. The good news is that it’s usually installed by
default, so nothing needs to be downloaded!
* … for Debian GNU/Linux?
GnuPG is installed by default with Debian GNU/Linux. If for some
reason it is not, use the APT package manager to install the package
“gnupg2”.
* … for OpenSUSE?
A recent version of GnuPG is part of the default OpenSUSE
installation.
* … for Fedora?
See the [instructions for CentOS]. Typically, though, Fedora ships
with a much more recent version of GnuPG, and as such you shouldn’t
need to install anything.
[instructions for CentOS] See section 3.6.3.4
* … for CentOS or RHEL?
An older (but still secure!) version of GnuPG is part of every CentOS
installation. To get the latest version, use the YUM package manager
to install the package “gnupg2”.
* … for Ubuntu?
See the instructions for [Debian GNU/Linux].
[Debian GNU/Linux] See section 3.6.3.1
* … for Slackware?
Install the `gnupg' package for GnuPG 1.4, or the `gnupg2' package for
GnuPG 2.0.
* … for Gentoo?
To install GnuPG on Gentoo, run the following command as root:
`emerge gnupg'
The Gentoo documentation includes a [GnuPG User Guide].
[GnuPG User Guide] http://www.gentoo.org/doc/en/gnupg-user.xml
… for FreeBSD?
--------------
GnuPG is included in the ports collection. To install it, run the
following commands as root:
,----
| cd /usr/ports/security/gnupg
| make install clean
`----
Alternatively, you can install GnuPG using a package manager:
,----
| sudo pkg_add -r gnupg
`----
3.7 Is there source code available for it?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Yes! The person, business or group that provided you with the GnuPG
binary is required to give you the source code upon your request.
3.8 What’s Free Software, and why does it matter?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The word “free” should evoke ideas of liberty, not price. An awful
lot of the software industry does not respect your freedoms: your
freedom to use the software for any purpose, your freedom to study and
learn from how it works, your freedom to share it with others who
might benefit from it, and more. Free Software is the antithesis of
this: Free Software is meant to respect your rights. You may use the
software for any purpose: you may inspect and modify the source code:
you may share the software and/or your modifications with others.
3.9 How can I donate money to the GnuPG project?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The best way is to visit the [g10 Code donation page].
[g10 Code donation page] http://g10code.com/gnupg-donation.html
3.10 How can I help with GnuPG development?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Development discussion takes place on the gnupg-devel mailing list.
Go to the [GnuPG mailing list page] for links to subscribe and to the
list's archives.
The [GnuPG project's bug tracker] is also publicly available.
[GnuPG mailing list page]
http://www.gnupg.org/documentation/mailing-lists.en.html
[GnuPG project's bug tracker] https://bugs.gnupg.org/gnupg/
4 Where can I get more information?
===================================
The good news is the internet is a treasure trove of information. The
bad news is that the internet is a festering sewer of misinformation,
conspiracy theories, and half-informed speculations all masquerading
as informed commentary.
The following mailing lists and web pages are generally known for
having a strong signal-to-noise ratio. Despite this, we strongly urge
skepticism.
4.1 How can I spot the charlatans?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
First, beware of all absolutes. Almost every question in either the
fields of computer security or cryptography can honestly be answered
with, “it depends.” Real experts will avoid giving blanket yes-or-no
questions except to the simplest and most routine of questions. They
will instead hem and haw and explain the several different factors
that must be weighed. Hucksters will promise you absolute truth.
Second, the experts really don’t care whether you take their advice.
Hucksters often want to be seen as authorities, and if you fail to
take their advice they may harangue you about how you’re taking
chances with your data, how you’re acting irresponsibly, and so on.
Third, experts genuinely don’t want you to trust them. An expert will
instead point to the published literature (usually in a dead-tree
edition with the imprimatur of a reputable publishing house) and tell
you what the reference books say. They want you to trust the
reference books, not them. Hucksters will go on about their extensive
personal experience or refer to papers that have only ever been
self-published on websites.
Fourth, experts try not to scare people. The world is a scary enough
place without it being made moreso. Hucksters will try to scare you,
in order to keep you listening to them and dependent on them for
information on how to be ‘safe.’
Fifth, experts will quickly admit when they are wrong and give credit
to the person bringing the error to their attention. Hucksters tend
to take challenges as personal affronts.
4.2 What are some useful mailing lists?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The good news is, there are many!
The GnuPG-Users mailing list
----------------------------
Subscribing: visit the [GnuPG-Users webpage]
Unsubscribing: see above
List moderator: <[gnupg-users-owner@gnupg.org]>
Supports PGP/MIME?: No
Languages supported: English
GnuPG-Users is home to the largest community of GnuPG users on the
net. The list is very lightly moderated and somewhat freewheeling, but
overall it has an excellent signal-to-noise ratio. The level of
technical discussion is sometimes a little daunting for the newcomer,
but on the whole it’s a wonderful resource.
[GnuPG-Users webpage]
http://lists.gnupg.org/mailman/listinfo/gnupg-users
[gnupg-users-owner@gnupg.org]
mailto:gnupg-users-owner@gnupg.org?subject%3DThe%20GnuPG-Users%20list
The Enigmail mailing list
-------------------------
Subscribing: Visit the [Enigmail mailing list page]
Unsubscribing: See above
List moderator(s):
- John Clizbe <[john@enigmail.net]>
- Olav Seyfarth <[olav@enigmail.net]>
- Patrick Brunschwig <[patrick@enigmail.net]>
- Ludwig Hügelschäfer <[ludwig@enigmail.net]>
- Daniele Raffo <[daniele@enigmail.net]>
- Robert J. Hansen <[rob@enigmail.net]>
Supports PGP/MIME: Yes
Languages supported: English, Deutsch, Schwyzerdütsch, Español
Enigmail integrates GnuPG with [Mozilla Thunderbird] and/or [Mozilla
Seamonkey]. It’s one of the most popular ways to use GnuPG, and the
mailing list provides a friendly place to learn how it works and get
started using it.
The list is lightly moderated.
[Enigmail mailing list page]
https://admin.hostpoint.ch/mailman/listinfo/enigmail-users_enigmail.net
[john@enigmail.net]
mailto:john@enigmail.net?subject=The%20Enigmail%20list
[olav@enigmail.net]
mailto:olav@enigmail.net?subject=The%20Enigmail%20list
[patrick@enigmail.net]
mailto:patrick@enigmail.net?subject=The%20Enigmail%20list
[ludwig@enigmail.net]
mailto:ludwig@enigmail.net?subject=The%20Enigmail%20list
[daniele@enigmail.net]
mailto:daniele@enigmail.net?subject=The%20Enigmail%20list
[rob@enigmail.net]
mailto:rob@enigmail.net?subject=The%20Enigmail%20list
[Mozilla Thunderbird] http://www.getthunderbird.com
[Mozilla Seamonkey] http://www.seamonkey-project.org/
PGP-Basics
----------
Subscribing: visit the [PGP-Basics webpage]
Unsubscribing: see above
List moderator: Mike Daigle <[mdaigle@gswot.org]>
Supports PGP/MIME: No
Languages supported: English
PGP-Basics was established over a decade ago specifically to provide a
place where newcomers to GnuPG and PGP could learn about
communications security. The list is low-volume, lightly-moderated,
and remarkably friendly to new users.
[PGP-Basics webpage] http://tech.groups.yahoo.com/group/PGP-Basics
[mdaigle@gswot.org]
mailto:mdaigle@gswot.org?subject=The%20PGP-Basics%20list
PGPNET
------
Subscribing: visit the [PGPNET page]
Unsubscribing: see above
List moderator(s): Unknown
Supports PGP/MIME?: No
Languages supported: Unknown
PGPNET exists to provide people with the opportunity to practice
sending and receiving encrypted, signed, and encrypted-and-signed
traffic in a group environment.
[PGPNET page] http://tech.groups.yahoo.com/group/PGPNET
4.3 What are some useful webpages?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As a general rule, the huckster quotient of webpages at-large is
fairly high. That said, there are some web resources we recommend.
They can be broken up into homepages for specific GnuPG-related
projects, and sites of general interest.
Where can I find the homepage for…
----------------------------------
Many of the projects associated with GnuPG maintain their own
websites. If you have problems with an associated project, please
check their website first: they might be able to give you faster and
better help than the GnuPG community can.
* … GnuPG?
GnuPG’s homepage can be found at [[http://www.gnupg.org]].
[[http://www.gnupg.org]] http://www.gnupg.org
* … Enigmail?
Enigmail, a plugin for Mozilla Thunderbird that adds strong GnuPG
support, can be found at [[http://enigmail.net]].
[[http://enigmail.net]] http://enigmail.net
* … GPGTools?
Mac OS X users may wish to visit the GPGTools project at
[[http://www.gpgtools.org]].
[[http://www.gpgtools.org]] http://www.gpgtools.org
* … GPG4WIN?
GPG4WIN, the Windows port of GnuPG, maintains a homepage at
[[http://www.gpg4win.org]].
[[http://www.gpg4win.org]] http://www.gpg4win.org
Where can I find webpages covering…
-----------------------------------
Although the GnuPG community generally finds these websites to be
useful, your mileage may significantly vary. There are wide
differences of opinion about some of them. They’re worth visiting and
worth reading, but make sure to read skeptically.
* … an easy introduction to cryptography?
There is no such thing as an easy introduction to cryptography.
However, PGP Corporation has a well-regarded [/Introduction to
Cryptography/].
[/Introduction to Cryptography/]
http://www.cs.unibo.it/babaoglu/courses/security/resources/documents/intro-to-crypto.pdf
* … the deeper mathematics of cryptography?
The maintainer of this list also keeps a gentle(-ish) [introduction to
the mathematics and computer science of cryptography].
[introduction to the mathematics and computer science of cryptography]
http://keyservers.org/~rjh/cryptofaq.xhtml
* … best practices for using GnuPG?
At present, there are no reputable web pages detailing GnuPG best
practices.
* … the politics of cryptography?
The inclusion of a site on this list is not an endorsement of that
site’s political leanings.
Probably the best-known organization is the [Electronic Frontier
Foundation], which has been at the vanguard of electronic civil
liberties for over twenty years.
The [Free Software Foundation] is also deeply involved in these
matters, although in a different way than the EFF.
[Electronic Frontier Foundation] http://www.eff.org
[Free Software Foundation] http://www.fsf.org
5 What email clients support GnuPG on…
======================================
Many email clients offer strong GnuPG integration.
5.1 … Microsoft Windows?
~~~~~~~~~~~~~~~~~~~~~~~~
- Thunderbird
Plugin?: Yes, via [Enigmail]
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
With the Enigmail plugin, Thunderbird becomes one of the most
popular GnuPG-aware email clients. it’s under active development
and is compatible with the latest Thunderbird releases, with a
friendly and welcoming user community.
- Kontact
Plugin?: No (natively supported)
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
Kontact is KDE’s integrated personal information manager of KDE. It
runs anywhere that KDE does, and even on some mobile devices as
Kontact Touch.
[Enigmail] http://enigmail.net
5.2 … Mac OS X?
~~~~~~~~~~~~~~~
- Thunderbird
Plugin?: Yes, via [Enigmail]
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
With the Enigmail plugin, Thunderbird becomes one of the most
popular GnuPG-aware email clients. it’s under active development
and is compatible with the latest Thunderbird releases, with a
friendly and welcoming user community.
- Gnus
Plugin?: Yes, via [EasyPG]
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
- Mutt
Plugin?: No (natively supported)
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
Make sure to put `set crypt_use_gpgme' in your `~/.muttrc' file.
- Apple Mail
Plugin?: Yes, via [GPGtools]
Supports GnuPG versions: 2.0
Supports PGP/MIME: No
Actively developed: Yes
PGP/MIME support in Apple Mail+GPGtools is not absent, just
temporarily broken.
[Enigmail] http://enigmail.net
[EasyPG] http://www.emacswiki.org/emacs/EasyPG
[GPGtools] http://www.gpgtools.org
5.3 … Linux or FreeBSD?
~~~~~~~~~~~~~~~~~~~~~~~
- Thunderbird
Plugin?: Yes, via [Enigmail]
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
With the Enigmail plugin, Thunderbird becomes one of the most
popular GnuPG-aware email clients. it’s under active development
and is compatible with the latest Thunderbird releases, with a
friendly and welcoming user community.
- Gnus
Plugin?: Yes, via [EasyPG]
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
- Mutt
Plugin?: No (natively supported)
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
Make sure to put `set crypt_use_gpgme' in your `~/.muttrc' file.
- Kontact
Plugin?: No (natively supported)
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
Kontact is KDE’s integrated personal information manager of KDE. It
runs anywhere that KDE does, and even on some mobile devices as
Kontact Touch.
- Evolution
Plugin?: No (natively supported)
Supports GnuPG versions: 1.4, 2.0
Supports PGP/MIME: Yes
Actively developed: Yes
[Enigmail] http://enigmail.net
[EasyPG] http://www.emacswiki.org/emacs/EasyPG
6 Is GnuPG available as a ‘portable app’?
=========================================
For Windows users, check [PortableApps.com].
[PortableApps.com]
http://portableapps.com/apps/internet/thunderbird_portable
7 What do all these strange words mean?
=======================================
Cryptography tends to use a whole lot of specialized language and
jargon. In this section some of it will be deciphered.
7.1 What’s ‘public-key cryptography’?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the 1970s new ideas came to the forefront of the cryptanalytic
world. One of the most important was the development of asymmetric
cryptography (also often called “public-key cryptography”).
Asymmetric cryptography is built around problems that are very hard in
one direction, and very easy in another. Consider the number 2,701.
If you were to be asked for its prime factors, you would find it a
daunting challenge. If you were to be given the numbers 37 and 73,
though, it wouldn’t take but a minute to discover the answer was
2,701. Multiplying two numbers to yield a third number is easy:
finding those two numbers, given the third, is hard.
Asymmetric cryptography uses these asymmetric problems as the
building-blocks of cryptography. It’s easy to create an encrypted
message which neither you nor anyone else save the intended recipient
can decrypt. To continue the metaphor, you and everyone else get to
wrestle with the hard problem (“factor 2,701”). The intended
recipient knows a secret piece of information which makes the problem
easy (“factor 2,701, given that one of the factors is 73”).
This manages to overcome the major flaw with symmetric cryptography.
Your public key can be shared with the entire world, even your
enemies, and your communications will still be secure. Compare this
to symmetric cryptography, where as soon as the key became public
knowledge the entire system was broken.
7.2 What’s ‘symmetric cryptography’?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
One of the earliest ciphers was the shift cipher, which was allegedly
used by Julius Caesar in his campaign against the Gauls. He took his
plaintext and shifted each letter three positions up in the alphabet,
wrapping around once he reached the end (so that ‘Z’ would become
‘C’). His correspondents would reverse the process: by moving each
letter in the encrypted text down three letters the original message
would be recovered. Knowing how to encrypt the text also gave the
knowledge of how to decrypt the text: the process wasn’t identical
(one shifted up, the other shifted down), but knowing one process the
other one could trivially be discovered.
This trait, that of encryption and decryption being two sides of the
same coin, is the defining trait of symmetric cryptography.
Modern-day symmetric ciphers are much more complex than Caesar’s
scheme, but they still work in fundamentally the same way. Knowledge
of how to encrypt reveals knowledge of how to decrypt, and vice-versa.
The symmetry between those two operations leads to the name “symmetric
cryptography”.
Symmetric cryptography is fast, well-studied, and safe. It has one
critical drawback, though: you have to have a secure communications
channel by which you can share the key with someone. If you already
have a secure communications channel, though, do you really need
cryptography?
7.3 What’s a ‘key’?
~~~~~~~~~~~~~~~~~~~
The word ‘key’ is unfortunately ambiguous. It can either refer to the
mathematical structures that allow encryption, decryption, signing and
verification to occur, or to the rather large blobs of data that
contain those mathematical structures as well as information about the
person associated with it, additional subkeys, and so forth.
With respect to the large blobs of data, it is preferable to call them
‘certificates’, so that the word ‘key’ may be unambiguously recognized
as meaning just the mathematical structures. Unfortunately, this is a
custom that seems to be honored mostly in the breach.
7.4 What’s a ‘certificate’?
~~~~~~~~~~~~~~~~~~~~~~~~~~~
A certificate is a large data structure that contains one or more
[keys], and optionally information that identifies the user,
designated revokers, who has vouched for this certificate, and so on.
[keys] Unknown reference
7.5 What’s a ‘keyserver’?
~~~~~~~~~~~~~~~~~~~~~~~~~
A keyserver is a service that publishes public-key certificates and
makes them searchable. You can upload your certificate to a keyserver
so that other users can find it. There are distributed networks of
keyservers that share keys, so you only need to upload your key once
to that network.
One widely-used keyserver network is [sks-keyservers.net]. SKS stands
for “Synchronising Key Server”. You can use this network by supplying
the `--keyserver pool.sks-keyservers.net' option.
[sks-keyservers.net] http://www.sks-keyservers.net/
7.6 What’s RSA?
~~~~~~~~~~~~~~~
RSA is the world’s premier [asymmetric cryptographic algorithm], and
is built on the difficulty of factoring extremely large composites.
GnuPG supports RSA with [key] sizes of between 1024 and 4096 bits.
[asymmetric cryptographic algorithm] See section 7.1
[key] See section 7.3
7.7 What’s DSA?
~~~~~~~~~~~~~~~
The United States’ National Institute for Standards and Technology
([NIST]) established the Digital Signature Algorithm (DSA) as a
government standard for digital signatures. Originally, it supported
key lengths between 512 and 1024 bits. Recently, NIST has declared
512-bit keys obsolete: now, DSA is available in 1024, 2048 and
3072-bit lengths.
DSA belongs to the Elgamal family of algorithms, and is very
well-regarded.
[NIST] http://www.nist.gov
7.8 What’s Elgamal?
~~~~~~~~~~~~~~~~~~~
Elgamal may refer to either a family of cryptographic algorithms built
around the difficulty of computing discrete logarithms in a finite
field, or one particular [asymmetric encryption algorithm] based on
that problem. The former is normally referred to as “the Elgamal
family,” and the latter is normally referred to as simply “Elgamal.”
GnuPG supports the Elgamal asymmetric encryption algorithm in [key]
lengths ranging from 1024 to 4096 bits.
There is also an Elgamal signature algorithm, which GnuPG no longer
supports.
[asymmetric encryption algorithm] See section 7.1
[key] See section 7.3
7.9 What’s AES?
~~~~~~~~~~~~~~~
Leading up to the year 2000, it was obvious that the old Data
Encryption Standard (DES) was on its last legs and needed to be
replaced. 3DES was available as a stopgap measure, but there was a
lot of pressure to make a new encryption standard that made use of the
last few decades of cryptologic research.
The United States National Institute of Standards and Technology
([NIST]) held an open competition to select the new encryption
standard. In the summer of 2000, a cipher named Rijndael (pronounced
“RAIN-doll”) was selected as the new Advanced Encryption Standard, or
AES.
AES is a thoroughly modern cipher design and may be used with
confidence.
[NIST] http://www.nist.gov
7.10 What are Twofish and Blowfish?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Blowfish and Twofish are well-regarded symmetric ciphers. Blowfish
should not be used to encrypt files larger than 4Gb in size, but
Twofish has no such restrictions. These algorithms are modern, and
may be used with confidence.
7.11 What’s 3DES?
~~~~~~~~~~~~~~~~~
In the 1970s, IBM developed a new symmetric cipher called the Data
Encryption Standard (DES). They overdesigned it horribly: even after
three decades, the only way to break DES is by brute force.
Unfortunately, standard DES has a small enough keyspace to be
susceptible to brute-forcing.
A new variant of DES was needed. 3DES, which is made of three DES
algorithms running together with three independent keys, was the
result. 3DES is ungainly, ugly, slow, and has all the aesthetics of a
Soviet workers’ housing bloc. It has also withstood three decades of
cryptanalysis and is still going strong.
Due to its 1970s-era 64-bit block size, it should not be used to
encrypt more than about 4Gb of data. Beyond that, though, it is solid
as a rock, and very few GnuPG users will ever notice a problem with
it. Provided you’re not encrypting more than 4Gb of data you may use
3DES with confidence.
7.12 What’s Camellia?
~~~~~~~~~~~~~~~~~~~~~
During roughly the same time period that [NIST] was running the
Advanced Encryption Standard trials, Japan’s [CRYPTREC] and the
European Union's [NESSIE] were running their own similar trials.
Camellia is the cipher that won the NESSIE and CRYPTREC trials, much
in the same way that Rijndael won the United States’ AES trials.
Camellia is a thoroughly modern cipher design and may be used with
confidence.
[NIST] http://www.nist.gov
[CRYPTREC] http://www.cryptrec.jp/english/
[NESSIE] http://www.cryptonessie.org/
7.13 What are SHA-1, SHA-224, SHA-256, SHA-384, SHA-512 and SHA-3?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Secure Hash Algorithms are cryptographic hash functions originally
devised by the United States’ National Security Agency. The
algorithms have been made publicly available and have been subjected
to an astonishing amount of peer review.
- *SHA* and/or *SHA-0*: the original Secure Hash Algorithm, generating
160-bit outputs. Flaws were discovered in it almost immediately.
SHA-0 never gained much traction in the cryptologic community, and
it is not present in GnuPG.
- *SHA-1*: This is SHA-0 with the flaws fixed, and not much else in
the way of changes. It still generates 160-bit outputs. SHA-1 has
not aged well. Although it is still believed to be safe, it would
be advisable to use another, different hash function if possible.
- *SHA-224*: This is a massively-overhauled SHA-1 which generates
224-bit outputs. It is believed to be safe, with no warnings about
its usage.
- *SHA-256*: This is a massively-overhauled SHA-1 which generates
256-bit outputs. It is believed to be safe, with no warnings about
its usage.
- *SHA-384*: This is a massively-overhauled SHA-1 which generates
384-bit outputs. It is believed to be safe, with no warnings about
its usage.
- *SHA-512*: This is a massively-overhauled SHA-1 which generates
512-bit outputs. It is believed to be safe, with no warnings about
its usage.
- *SHA-3*: SHA-3 is a completely new hash algorithm that makes a clean
break with the previous SHAs. It is believed to be safe, with no
warnings about its usage. At present, GnuPG does not support SHA-3.
Support for SHA-3 is forthcoming: expect it soon.
7.14 What’s MD5?
~~~~~~~~~~~~~~~~
MD5 is a 128-bit cryptographic hash function invented by Ron Rivest
(the ‘R’ of ‘RSA’) in the early 1990s. For many years it was one of
the standard algorithms of the field, but it has not aged well and is
widely considered to be completely obsolete.
You don’t need to worry about MD5, though: although GnuPG can read
MD5-based signatures, it will only generate MD5-based signatures if
you jump through a lot of hoops. This is for your own safety, to
prevent people from accidentally generating MD5-based signatures.
7.15 What are CAST, CAST5, and CAST5-128?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Carlisle Adams and Stafford Tavares (the “CA” and the “ST” in “CAST”)
developed the CAST algorithm in 1996. It was later approved for
Canadian government use.
CAST has many names: CAST, CAST5, CAST5-128 and CAST-128 all refer to
the same algorithm.
Internally, CAST is distinctly similar to Blowfish, another
well-respected algorithm. Like 3DES, its 64-bit block size means it
should not be used to encrypt files larger than 4Gb in size. With
that said, though, CAST is a modern cipher and may be used with
confidence.
7.16 What are ZLIB, ZIP and BZIP?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ZLIB, ZIP and BZIP refer to different kinds of compression algorithms.
GnuPG will use one of these three algorithms to compress your data
before encrypting it, unless GnuPG can see the data is already
compressed.
7.17 What’s a ‘revocation certificate’?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A revocation certificate is a [certificate] that possesses the
information necessary to mark another certificate as unusable. This
is called ‘revoking’ the certificate.
Recommended procedure is to generate a revocation certificate
immediately after generating a new GnuPG key. If the key is lost
(say, if you have no backups of the key and you lose the smart card or
laptop containing the key), you'll no longer be able to generate a
revocation certificate. Consult [the FAQ instructions] on how to do
this.
[certificate] See section 7.3
[the FAQ instructions] See section 8.5.2
7.18 What’s a ‘designated revoker’?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A designated revoker is a person, identified by a certificate, that
has the authority to revoke another certificate held by a different
person. For instance, if you were using GnuPG in a corporate
environment the IT staff might be listed as a designated revoker for
your certificate, so that when you left the company the IT staff could
revoke your certificate.
7.19 What does ‘validity’ mean?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Although a certificate makes certain assertions about identity, these
assertions cannot be blindly trusted. (Consider, for instance,
whether you should trust a certificate that claims to belong to
`obama@whitehouse.gov'.)
If you trust the certificate’s assertions, you are said to have
‘validated’ the certificate. Validation can be done by fiat or as the
result of a process. For instance, you validate your own certificate
by fiat: “this certificate says it belongs to me, and I trust it.”
Validating other certificates, though, should probably have a little
more rigor involved. How much rigor will depend entirely on your own
particular needs and the threats you face.
7.20 What does ‘trust’ mean?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
‘Trust’ refers to how thoroughly a certificate has been [validated].
The terms are used somewhat interchangeably.
[validated] See section 7.19
7.21 What does ‘ownertrust’ mean?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If a certificate has been [validated], and if you trust the person
owning that certificate to do proper validation of certificates, you
can tell GnuPG “I am willing to trust this person’s validations as if
they were my own.”
/[For instance, I have fully validated John Hawley’s certificate. I
further believe, based on my knowing John, that he will be as careful
about validating a certificate as I would be. So if John validates
your certificate, then if I pull your certificate down from the
keyserver it will show up as a fully validated certificate. — rjh]/
[validated] See section 7.19
8 How do I start using GnuPG?
=============================
The very first thing is to join the [GnuPG-Users mailing list].
You’ll find it to be a welcoming community that’s friendly to
newcomers and is eager to help out.
[GnuPG-Users mailing list] See section 4.2.1
8.1 Does GnuPG need to be ‘tuned’ before use?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No. GnuPG has sensible defaults right out of the box. You don’t need
to tune GnuPG before you can use it.
8.2 How large should my key be?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The overwhelming majority of users will be well-served by generating
2048-bit RSA keys. This is the default behavior for GnuPG.
8.3 What algorithm should I use?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The overwhelming majority of users will be well-served by generating
2048-bit RSA keys. This is the default behavior for GnuPG.
8.4 Why does it take so long to generate a certificate?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The short answer is, “your computer is doing a lot of work.” But
don’t worry: although generating new certificates can take a while,
actually using them once they’re made is quite fast.
8.5 What should I do after making my certificate?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Generate a revocation certificate, and store it in a safe place.
Alternately, you may wish to appoint [a designated revoker].
[a designated revoker] See section 7.18
How do I appoint a designated revoker?
--------------------------------------
A designated revoker is someone whom you trust to revoke your
certificates on your behalf. This person may revoke your certificates
without needing a revocation certificate. For instance, you may wish
to appoint your lawyer as your designated revoker so that, in the
event of your untimely death, your lawyer may revoke your
certificates.
To add a revoker, use the following command line:
`gpg --edit-key' /[your key ID here]/ `addrevoker'
When prompted, enter the key ID of the person whom you wish to appoint
as a revoker. The revoker’s key must be fully validated.
How do I generate a revocation certificate?
-------------------------------------------
A [revocation certificate] marks another certificate as unusable.
To generate a revocation certificate for your key, do:
`gpg --armor --output revoke.asc --gen-revoke' /[your key ID]/
Copy `revoke.asc' to a safe place.
[revocation certificate] See section 7.17
How do I send my certificate to the keyserver network?
------------------------------------------------------
`gpg --keyserver pool.sks-keyservers.net --send-key' /[your
certificate ID]/
You should only upload your own certificates to the keyservers, or
obtain the certificate holder's permission before doing so. In some
circles it's considered rude to upload someone else's certificate; not
everyone wants to publish their key publicly.
8.6 Where does GnuPG look for configuration options?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GnuPG looks at a file called `gpg.conf' to determine various runtime
parameters. On UNIX systems this file can be found in `~/.gnupg'. On
Windows systems it’s a bit more difficult to predict, but try:
Windows XP: FIXME
Windows Vista: FIXME
Windows 7: FIXME
Windows 8: FIXME
8.7 What options should I put in my configuration file?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The good news is, you really shouldn’t need to. That said, the
following is Rob Hansen’s `gpg.conf' file. The italicized text
describes what each piece does: the monospaced text is the actual
content of the file.
/Ensure that all parameters are set for strict OpenPGP conformance.
Later entries will override this, but setting ‘openpgp’ provides a
really good baseline to start from./
`openpgp'
/Make GnuPG a little quieter: don’t warn about insecure memory, don’t
print a greeting message, don’t put comments in GnuPG’s output./
`no-greeting'
`no-secmem-warning'
`no-emit-version'
`no-comments'
/Since keyservers.org sits in my closet, I want GnuPG to always check
it instead of going out on the network to ask another keyserver
halfway around the globe. Most users don’t have a keyserver in their
closet, and will want to substitute pool.sks-keyservers.net here./
`keyserver keyservers.org'
/Whenever I sign a document, use certificate 0xD6B98E10/
`local-user 0xD6B98E10'
/Whenever I encrypt a document, also include certificate 0xD6B98E10 as
a recipient. This allows me to decrypt the messages I send./
`encrypt-to 0xD6B98E10'
/In email, a line beginning with the word ‘From’ can be misinterpreted
by the computer as the start of a new email message. Thus, whenever
GnuPG sees a line starting with ‘From’, it will slightly mangle the
line to prevent this bug from occurring./
`escape-from-lines'
/Use SHA256 instead of SHA-1 for certificate signatures./
`cert-digest-algo SHA256'
/Prefer these digest algorithms, in this order/
`personal-digest-preferences SHA256 SHA512 SHA384 SHA224 RIPEMD160'
/Prefer these ciphers, in this order/
`personal-cipher-preferences TWOFISH CAMELLIA256 AES 3DES'
8.8 Is there any particular keyserver I should use?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Many people have had excellent luck with `pool.sks-keyservers.net'.
8.9 What’s the difference between an ‘option’ and a ‘command’?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Commands tell GnuPG what to do: options tell GnuPG how to do it. For
instance, `encrypt' is a command, and `armor' is an option that tells
GnuPG to ensure the output contains only printable characters.
8.10 What are the most commonly used options?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Produce more output explaining what GnuPG is doing:
`-v', `--verbose'
Some of the most commonly used options are:
Make no changes; this is useful for testing a command line that will
modify keys or generate output:
`-n', `--dry-run'
Send output to the named file:
`-o' /FILE/, `--output' /FILE/
Create ASCII-armored output that can be safely e-mailed, instead of
binary output:
`-a', `--armor'
When encrypting a message, you will usually supply at least one
recipient ID with the recipient option. This option can be supplied
multiple times to encrypt a message to multiple recipients:
`-r' /KEYID/, `--recipient' /KEYID=/ /specify a recipient ID/
8.11 What are the most commonly used commands?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GnuPG's primary functions are to encrypt and decrypt messages, and to
sign and verify them. It's possible to sign without encrypting or
encrypt without signing.
Signing a file's content is done with the `-s' or `--sign' commands.
A variation is `-b' or `--detach-sign', which produces a separate
signature without including the file's content; this is useful for
signing a software archive or other large file. The key to use for
the signature can be specified with the `local-user' setting in your
gpg.conf file, or with the `-u', `--local-user' options.
Encrypting a file's content is done with the `-e' or `--encrypt'
commands. Recipients are specified with the `-r' or `--recipient'
options.
GnuPG's default action is to decrypt and verify its input file,
writing the contents to standard output or to the filename specified
by the `-o' or `--output' options. The `--verify' command will only
verify the signature without writing the file's contents anywhere.
These commands are the most commonly used. GnuPG has many more
commands, largely for managing your keyring containing your private
keys and the certificates of others.
8.12 How do I use another person’s certificate?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to send an encrypted message or verify a signature, you must
obtain the certificate for the sender/signer's public key.
Occasionally you might obtain the certificate physically, by meeting
the certificate holder face-to-face and exchanging the certificate on
some storage medium such as a USB stick, memory card, or portable
disk. Or you might download a copy of the certificate from the
holder's web site.
Once obtained in one of these ways, you can add the certificate to
your collection of public keys by doing:
`gpg --import certificate.txt'
More commonly, you'll download a correspondent's certificate from a
keyserver.
How do I search the keyserver for someone’s certificate?
--------------------------------------------------------
There is also a network of public keyservers, accessible under the
collective hostname `pool.sks-keyservers.net'. GnuPG users can upload
their certificates to the keyservers, and other users can then search
for and download them.
`gpg --keyserver pool.sks-keyservers.net --search' /[email address,
name, key ID, etc.]/
GnuPG will list matching certificates and prompt you to select which
ones you wish to download and add to your keyring.
People will obtain new signatures for their certificates from time to
time. `gpg --refresh-keys' will recheck all of the certificates on
your public key and download any new signatures for those keys.
How do I retrieve a certificate if I already know its fingerprint?
------------------------------------------------------------------
`gpg --keyserver pool.sks-keyservers.net --recv-key' /[fingerprint]/
Why do I need to validate certificates?
---------------------------------------
If you were to receive a letter in the mail that claimed to be from
the President of the United States, would you believe it? Probably
not, because anyone can put together official-looking letterhead:
you’d insist on doing some kind of checking to make sure that no one
was fooling with you.
The same applies to email. A certificate can claim to be from anyone.
You have to make sure that the certificate really belongs to whom it
claims it belongs to. That process of making sure is called
‘validation’.
How do I validate certificates?
-------------------------------
*This advice is controversial.*
It’s controversial for a simple reason: every Tom, Dick and Harry has
their own idea about the “right way” to validate certificates. Some
of these people are well-informed and some of them are just plain
unhinged. In the end, you are responsible for making your own
decisions. That said, the following is generally agreed upon as being
a reasonable procedure:
1. Meet the certificate holder face-to-face.
2. Ask to see two forms of government-issued identification.
3. Upon verifying the person really is who they claim to be, ask this
person to provide their certificate’s fingerprint, their email
address, and where you can obtain a copy of their certificate.
(Example: “My fingerprint is `4541 BB01 8EA4 8F99 19CA 3701 2380
6BE5 D6B9 8E10', and you can find it on
`pool.sks-keyservers.net'.”)
4. On your own computer, retrieve the person’s certificate from the
specified location. Check to make sure the email address they gave
you is one that’s also listed on the certificate. Check to make
sure the fingerprint of the certificate you’ve downloaded matches
the fingerprint the person gave you.
5. `gpg --edit-key' /[their certificate ID]/ `sign'
6. Once signed, `gpg --armor --output signed_cert.asc --export'
/[their certificate ID]/
7. Send the file `signed_cert.asc' to the address they gave you
By following this process you first ensure that you’re speaking to the
right person. By comparing the fingerprints of the certificate you
have against the fingerprint they specified, you’re ensuring that you
have the right certificate. Checking to make sure the email address
they gave you is also listed on the certificate is one more check to
make sure. Once that’s done, presto, Bob’s your uncle: there’s
nothing left to do except sign it and return the newly-signed
certificate to the other person.
8.13 Why can’t I read emails I’ve sent, and how do I fix it?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You encrypted a message to Alice, which means that it requires Alice’s
private key to read it. Only Alice has her private key. That’s why
you can’t read encrypted traffic you generated: only Alice can read
it.
To get around this, add yourself as a recipient (=--recipient = /[your
certificate ID]/).
8.14 How do I encrypt a file for multiple recipients?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Use multiple `--recipient' options. Remember, options come before
commands!
8.15 How do I sign a file with multiple certificates?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Use multiple `--local-user' options. Remember, options come before
commands!
8.16 How do I combine encryption with signing?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`gpg --armor --recipient' /[first recipient’s key ID]/ `--local-user'
/[your key ID]/ `--sign --encrypt' /[filename]/
8.17 How do I force GnuPG to make printable-text output?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Normally, computers use eight-bit binary code. This often presents
trouble for email, which often requires that only printable
(seven-bit) characters may be used. By using the `--armor' flag,
GnuPG will generate output containing only printable characters.
8.18 How do I create an ‘inline signature’?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
An inline signature wraps a textual header and footer around the text
to be signed, leaving the text readable without running GnuPG. This
doesn't conceal the text at all and therefore provides no secrecy, but
if someone edits the text GnuPG will report that the signature is bad.
To generate an inline signature, run
`gpg --armor --output signed_file.asc --local-user' /[your key ID]/
`--clearsign message_file.txt'
To verify the resulting file, simply invoke GnuPG with the filename of
the signed file:
`gpg signed_file.asc'
8.19 How can I use GnuPG in an automated environment?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You should use the `--batch' option. Don't bother to use a passphrase
because there's usually no way to store it more securely than on the
secret keyring itself.
The suggested way to create keys for an automated environment is as
follows. First, on a secure machine:
1. If you want to do automatic signing, create a signing subkey for
your key. Use the interactive key editing menu by issuing the
command:
`gpg --edit-key' /keyID/
Enter "addkey" and select the DSA key type.
2. Make sure that you use a passphrase; this is required by the
current implementation to let you export the secret key.
3. Run:
`gpg --export-secret-subkeys --no-comment' /newsubkeyID/ `>
secring.auto'
4. Copy `secring.auto' and the public keyring to a test directory.
5. Change to the test directory.
6. Run the command:
`gpg --homedir . --edit' /newsubkeyID/
Use the sub-command `passwd' to remove the passphrase from the
subkeys. You may also want to remove all unused subkeys by doing `key
N' and then `delkey' for each subkey.
1. Copy `secring.auto' to the target box somehow.
On the target machine, install `secring.auto' as the secret keyring
and begin writing scripts that invoke GnuPG.
It's a good idea to install an intrusion detection system so that
you will get notice of a successful intrusion. If that happens,
you can revoke all the subkeys installed on that machine and
install new subkeys once the machine is secured again.
8.20 I’m a programmer and I need a GnuPG library. Is there one?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Check out [GPGME (GnuPG Made Easy)].
[GPGME (GnuPG Made Easy)] http://www.gnupg.org/related_software/gpgme/
8.21 I’m a programmer and I need a way to call GnuPG internals directly. Is there a library for this?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No, nor will there be.
9 What common problems come up?
===============================
9.1 Why is GnuPG warning me this certificate might not belong to whom I think it does?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you received an email claiming to be from a Nigerian oil tycoon,
would you believe it? Or would you insist on doing some kind of
verification first, in order to make sure that you’re not being
scammed or swindled?
The same principle applies here. If you’re using a certificate that
claims to belong to Alice, but there’s no evidence it actually belongs
to Alice, GnuPG will warn you that you’re using an untrusted
certificate.
You probably want to validate the certificate; see [this FAQ's
instructions].
[this FAQ's instructions] See section 8.12.4
9.2 Why is GnuPG warning me about using insecure memory?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GnuPG tries to lock memory so that no other process can see it and so
that the memory will not be written to swap. If for some reason it’s
not able to do this (for instance, certain platforms don’t support
this kind of memory locking), GnuPG will warn you that it’s using
insecure memory.
While it’s almost always better to use secure memory, it’s not
necessarily a bad thing to use insecure memory. If you own the
machine and you’re confident it’s not harboring malware, then this
warning can probably be ignored.
9.3 Why is GnuPG changing my message?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GnuPG uses special lines to denote the beginning of a message, the
beginning of a signature, and so forth. These lines start with
“=----- BEGIN=…”. If your text contains a line beginning with a dash,
that line will be slightly mangled in order to prevent GnuPG from
misinterpreting your data as one of its special lines.
10 What are some common best practices?
=======================================
It’s very hard to give advice on this subject, because everyone will
have their own opinion. That said, here are some good guidelines:
- *Join the community.* Join GnuPG-Users and get involved in the
discussions. The conversation is wide-ranging and you’ll encounter
a great variety of thoughts and opinions. Reading GnuPG-Users is
one of the best ways to educate yourself.
- *Practice.* If you don’t practice these skills before they become
necessary, you won’t be able to use these skills effectively.
- *Generate a revocation certificate and keep it safe.*
- *Use a strong passphrase.*
- *Keep your computer free of malware.*
- *Validate certificates correctly.*
10.1 How can I choose a strong passphrase?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If someone manages to obtain your secret key, the only thing
protecting the key will be your passphrase. A passphrase should be 1)
difficult to guess for someone who knows you, and 2) difficult to
brute-force by trying every possible combination of characters.
To meet requirement 1), the passphrase shouldn't be based on
publicly-available information about you: your birthday, your spouse's
name, your school's motto, a line of text from a book, etc. To meet
requirement 2), the passphrase should be long: commercially available
hardware can try 2.8 billion passwords in a day, which is sufficient
to crack a 10-letter all-lowercase password.
One simple approach that produces easy-to-remember passphrases is to
generate four to six random words, as illustrated by the XKCD cartoon
[“Correct, horse! Battery staple!”].
[“Correct, horse! Battery staple!”] http://xkcd.com/936/
10.2 How can I keep my revocation certificate safe?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Good places include safe deposit boxes, kept on file with your lawyer,
placed in a fireproof safe, and so forth. It should be treated as an
important document that needs to be kept safe.
10.3 How can I keep my computer safe from malware?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Although there is no guaranteed way of keeping your system free of
malware, you can reduce your risk quite a lot by following some basic
rules.
1. Keep your system up-to-date. Always apply the latest patches.
2. Stop using old versions of Internet Explorer. If possible, use
[Mozilla Firefox] or [Chromium].
3. Don’t open email attachments unless they are expected and come
from someone you know.
4. Don’t click on email links unless they are expected and come from
someone you know.
5. Be suspicious of requests for personal information, especially if
it’s more detail than is strictly necessary to solve a problem.
[Mozilla Firefox] http://www.getfirefox.com
[Chromium] http://download-chromium.appspot.com/
10.4 Should I use encrypted disk software like TrueCrypt, BitLocker or FileVault?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You can if you want, but it won’t make your private key any more
secure. Your private key is already encrypted: your passphrase is the
key used to decrypt your private key.
11 Advanced topics
==================
These topics are ‘advanced’ in the sense that you really don’t need to
understand them in order to safely and correctly use GnuPG. That
said, if you have a more technical question about GnuPG, you may find
some of the answers in this section.
11.1 Why does GnuPG use RSA-2048 by default?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The United States National Institute of Standards and Technology
([NIST]) believes that 2048-bit [asymmetric cryptography] will be
secure until at least the year 2030. Larger keys are unlikely to
extend this duration very much. Further, large keys come with their
own problems: they cannot be moved to smartcards, mobile devices have
trouble with them, and so on.
GnuPG uses RSA by default instead of DSA not because of any problems
with DSA, but just because RSA has a larger installed user base and is
better supported by other OpenPGP-compatible products.
[NIST] http://www.nist.gov
[asymmetric cryptography] See section 7.1
11.2 Do other high-security applications use RSA-2048?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2048-bit RSA is commonly used to secure SSL root signing certificates.
It’s also used to sign operating system patches, Authenticode
signatures, Java applets and more. RSA-2048 is believed to be safe
against attack until at least the year 2030, so use it with
confidence.
11.3 Why doesn’t GnuPG default to using RSA-4096?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Because it gives us almost nothing, while costing us quite a lot.
Breaking an RSA-10 key requires you to try each prime number between
two and one hundred. There are twenty-five of these, meaning RSA-10
is equivalent to about a 5-bit symmetric cipher. Breaking an RSA-20
key requires you to try each prime number between two and one
thousand: there are 168 of them, meaning RSA-20 is equivalent to about
an 8-bit cipher. Doubling the keylength (from RSA-10 to RSA-20)
didn't give us the benefit that we naively expected. Each additional
bit gives correspondingly less in the way of additional security, and
we quickly reach a point of diminishing returns.
That point of diminishing returns happens around RSA-2048. Once you
move past RSA-2048, you’re really not gaining very much. At the same
time, moving past RSA-2048 means you lose the ability to migrate your
certificate to a smartcard, or to effectively use it on some mobile
devices, or to interoperate with other OpenPGP applications that don’t
handle large keys gracefully.
If you really want a 4096-bit RSA key there’s nothing stopping you:
but we sincerely believe the overwhelming majority of users will be
well-served with RSA-2048.
11.4 Why do people advise against using RSA-4096?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Almost always when people use 4096-bit RSA they’re doing so because
they believe RSA-4096 to be much stronger than it is. The United
States’ National Institute of Standards and Technology ([NIST]) states
that RSA-2048 gives roughly 112 bits of security and RSA-3072 gives
roughly 128. There is no formal recommendation on where RSA-4096
lies, but the general consensus is that it would come in somewhere
around 140 bits — 28 bits of improvement over RSA-2048. This is an
improvement so marginal that it’s really not worth mentioning.
If you need more security than RSA-2048 offers, the way to go would be
to switch to elliptical curve cryptography — not to continue using
RSA.
[NIST] http://www.nist.gov
11.5 Why does GnuPG support RSA-4096 if it’s such a bad idea?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RSA-4096 is not a bad idea: it’s just, generally speaking,
unnecessary. You gain very little in the way of additional resistance
to brute-forcing and cryptanalysis.
11.6 Can any of the ciphers in GnuPG be brute-forced?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No.
The laws of physics require that a certain amount of heat be used in
computation. This is a consequence of the Second Law of
Thermodynamics, and may not be violated under our current
understanding of the laws of physics.
Further, physics requires that a certain amount of time be used in
computation. This is a consequence of the Heisenberg Uncertainty
Principle, and may not be violated under our current understanding of
the laws of physics.
Using these two principles (the [Landauer bound] and the
[Margolus–Levitin limit]), we can determine quite accurately how much
heat would be released by a computer that brute-forced a 128-bit
cipher. The results are profoundly silly: it’s enough to boil the
oceans and leave the planet as a charred, smoking ruin.
This is not to say that GnuPG cannot be successfully attacked. It is
only to say that none of the ciphers in GnuPG are susceptible to
brute-forcing.
[Landauer bound] http://en.wikipedia.org/wiki/Landauer_bound
[Margolus–Levitin limit]
http://en.wikipedia.org/wiki/Margolus%E2%80%93Levitin_theorem
11.7 Has GnuPG ever been successfully attacked?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This depends entirely on what is meant by “successful attack.”
If you mean, “has GnuPG traffic ever been successfully
cryptanalyzed?”, the answer is a flat ‘no’. We are unaware of any
credible reports of any of the ciphers used in GnuPG having ever been
successfully cryptanalyzed.
If you mean, “have people figured out ways to obtain the plaintext
anyway?”, the answer is an emphatic ‘yes.’ In [a 2007 Drug Enforcement
Administration case], a keylogger was installed on a suspect's
computer.
GnuPG protects your traffic against cryptanalysis, but it is not magic
fairy dust that can be sprinkled over your data to make it safe
against all threats.
[a 2007 Drug Enforcement Administration case]
http://news.cnet.com/8301-10784_3-9741357-7.html
11.8 Should I use PGP/MIME for my emails?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Probably not.
PGP/MIME is the official, standardized way of using GnuPG with
electronic mail. PGP/MIME packages the data up as encrypted
attachments. This is the problem with it: attachments often get
mangled, stripped, or otherwise tampered with. For instance, sending
PGP/MIME traffic to the [PGP-Basics mailing list] will result in your
email being completely blank. PGP-Basics is set up to drop all
attachments from messages posted to the list, and that means your
PGP/MIME attachments get dropped.
For many years GNU Mailman would repackage attachments in ways that
would break the PGP/MIME standard and result in unreadable traffic.
These GNU Mailman installations still exist in the wild. For a long
time both [GnuPG-Users] and [Enigmail] ran these buggy versions of GNU
Mailman.
Since PGP/MIME can't reliably be sent to the three largest GnuPG
mailing lists, it’s hard to claim that PGP/MIME is ready for
widespread usage. For now, it’s best to use inline traffic unless you
can be certain that PGP/MIME messages will not be mangled in transit.
[PGP-Basics mailing list] See section 4.2.3
[GnuPG-Users] See section 4.2.1
[Enigmail] See section 4.2.2
11.9 What are the best algorithms in GnuPG?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
MD5 and SHA-1 should be avoided if possible. Beyond that, there is no
“best algorithm” or “best algorithms” in GnuPG. It’s sort of like
asking whether Godzilla or King Kong is better at terrorizing urban
cities: there is no clear-cut winner.
This is not to say you shouldn’t have preferences, though. It is only
to say that GnuPG’s algorithms are so well-designed for what they do
that there is no single “best”. There’s just a lot of personal,
subjective choice. For instance:
- /[I studied Twofish pretty intensively in graduate school, so I tend
to prefer it. — rjh]/
11.10 Why is my DSA key limited to 3072 bits?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The United States’ National Institute of Standards and Technology
([NIST]) is responsible for the DSA specification. NIST has not
published a 4096-bit DSA variant, and thus GnuPG doesn’t offer it.
[NIST] http://www.nist.gov
11.11 Why does my DSA-1024 key use a different digest algorithm than my DSA-2048 or DSA-3072 key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The DSA algorithm has gone through several revisions.
GnuPG’s original implementation of DSA supported 1024-bit keys that
used either SHA-1 or RIPEMD-160 as hashes.
When the United States’ National Institute of Standards and Technology
([NIST]) revised the specification to support 2048- and 3072-bit keys,
they also required longer hashes be used. DSA-2048 required a 224-bit
hash (SHA-224, or a longer hash cut down to 224 bits), and DSA-3072
required a 256-bit hash (SHA-256, or a longer hash cut down to 256
bits). They also now allowed for stronger hashes to be used for
DSA-1024: if they were more than 160 bits, they would simply be cut
down.
So, depending on how you have GnuPG configured, GnuPG might be forced
to use SHA-1 and/or RIPEMD-160 with DSA-1024; GnuPG might be able to
use any of the longer SHAs with DSA-1024; GnuPG might use SHA-224,
-256, -384 or -512 for DSA-2048; GnuPG might use SHA-256, SHA-384 or
SHA-512 for DSA-3072.
[NIST] http://www.nist.gov