Table of Contents

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 Creative Commons license

This document is © 2012, Robert J. Hansen <>. 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.

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.

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:

[Do we have any other editors we need to add? — rjh]

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 assurances 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?


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.

3.6.1 … for Microsoft Windows?

A convenient Windows installer is available from GPG4WIN.

3.6.2 … for Mac OS X?

The GPGtools project has everything needed to get started.

3.6.3 … 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.

  • … 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 Slackware?


  • … for Gentoo?


3.6.4 … for FreeBSD?


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.

The best way is to visit the g10 Code donation page.

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!

4.2.1 The GnuPG-Users mailing list

visit the GnuPG-Users webpage
see above
List moderator
Werner Koch <>
Supports PGP/MIME?
Languages supported

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.

4.2.2 The Enigmail mailing list

Visit the Enigmail mailing list page
See above
List moderator(s)
Supports PGP/MIME
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.

4.2.3 PGP-Basics

visit the PGP-Basics webpage
see above
List moderator
Mike Daigle <>
Supports PGP/MIME
Languages supported

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.

4.2.4 PGPNET

visit the PGPNET page
see above
List moderator(s)
Supports PGP/MIME?
Languages supported

PGPNET exists to provide people with the opportunity to practice sending and receiving encrypted, signed, and encrypted-and-signed traffic in a group environment.

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.

4.3.1 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.

  • … Enigmail?

    Enigmail, a plugin for Mozilla Thunderbird that adds strong GnuPG support, can be found at

4.3.2 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.

  • … 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.

5 What email clients support GnuPG on…

Many email clients offer strong GnuPG integration.

5.1 … Microsoft Windows?

  • Thunderbird
    Yes, via Enigmail
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

    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
    No (natively supported)
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

    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.

5.2 … Mac OS X?


  • Thunderbird
    Yes, via Enigmail
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

    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
    Yes, via EasyPG
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed
  • Mutt
    No (natively supported)
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

    Make sure to put set crypt_use_gpgme in your ~/.muttrc file.

  • Apple Mail
    Yes, via GPGtools
    Supports GnuPG versions
    Supports PGP/MIME
    Actively developed

    PGP/MIME support in Apple Mail+GPGtools is not absent, just temporarily broken.

5.3 … Linux or FreeBSD?

  • Thunderbird
    Yes, via Enigmail
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

    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
    Yes, via EasyPG
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed
  • Mutt
    No (natively supported)
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

    Make sure to put set crypt_use_gpgme in your ~/.muttrc file.

  • Kontact
    No (natively supported)
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

    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
    No (natively supported)
    Supports GnuPG versions
    1.4, 2.0
    Supports PGP/MIME
    Actively developed

6 Is GnuPG available as a ‘portable app’?

For Windows users, check

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.

7.5 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.

7.6 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.

7.7 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.

7.8 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.

7.9 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.

7.10 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.

7.11 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.

7.12 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.

7.13 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.

7.14 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.

7.15 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.

7.16 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.

7.17 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.

7.18 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

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.

7.19 What does ‘trust’ mean?

‘Trust’ refers to how thoroughly a certificate has been validated. The terms are used somewhat interchangeably.

7.20 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]

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.

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.

8.5.1 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.

8.5.2 How do I generate a revocation certificate?

gpg --armor --output revoke.asc --gen-revoke [your key ID]

Copy revoke.asc to a safe place.

8.5.3 How do I send my certificate to the keyserver network?

gpg --keyserver --send-key [your certificate ID]

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
Windows Vista
Windows 7
Windows 8

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.


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.


Since 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 here.


Whenever I sign or encrypt a document, use certificate 0xD6B98E10

local-user 0xD6B98E10
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.


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

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?

Lorem ipsum common_options

8.11 What are the most commonly used commands?

Lorem ipsum common_commands

8.12 How do I use another person’s certificate?

Lorem ipsum using_certificates

8.12.1 How do I search the keyserver for someone’s certificate?

gpg --keyserver --search [email address, name, key ID, etc.]

8.12.2 How do I retrieve a certificate if I already know its fingerprint?

gpg --keyserver --recv-key [fingerprint]

8.12.3 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’.

8.12.4 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”)
  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’?

gpg --armor --output signed_file.asc --local-user [your key ID] --clearsign

8.19 I’m a programmer and I need a GnuPG library. Is there one?

Check out GPGME (GnuPG Made Easy).

8.20 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.

9.2 Why is GnuPG warning me about using insecure memory?

GnuPG tries to lock memory so that no other process can see it. 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?

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.

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.

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 RSA-20 key requires you to try each prime number between two and one thousand: there 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.

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?


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.

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.’

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.

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.

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.

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 Institutes 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.

Date: 2012-10-24 03:19:11 EDT

Author: Robert J. Hansen

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