Beginner's Guide to GnuPG

by Nat Queen

[Note. This is a slightly modified version of an article that originally appeared in the August 2001 issue of Archive magazine. It is written specifically for users of RISC OS computers, but much of the information applies equally to other versions of GnuPG.]

This year marks the tenth anniversary of a momentous event in the history of computing - freeware PGP (Pretty Good Privacy), created by Philip Zimmermann, became available to the world, thereby providing strong public-key cryptography for the masses.

PGP soon became the worldwide standard program for secure email over the internet. Versions are now available for all major operating systems. GnuPG (GNU Privacy Guard), developed more recently, is an alternative to PGP, which is generally compatible with all modern PGP versions.

Although GnuPG can do all the same things as PGP (and even more!), it is not so well known. However, for reasons to be explained below, it is now becoming important for RISC OS users.

Readers who are not already familiar with PGP may find it helpful to consult my previous articles "PGP for secure e-mail" and "Beginner's guide to PGP". An understanding of the basic concepts of public-key cryptography is essential for the proper use of GnuPG, as it is for PGP, and these concepts will not be repeated here.

The latest RISC OS version of PGP is 2.6.3ia, which was ported by Gareth McCaughan from the corresponding DOS version in 1997. Sadly, it is becoming seriously outdated. Windows users now have PGP 7, and versions 5 or 6 are available for most other operating systems. These later versions use a greater variety of algorithms for encryption and digital signatures, some of which are now more widely used. As a result, it has become difficult for RISC OS users to communicate securely with users of the more modern versions of PGP for other operating systems.

GnuPG to the rescue

GnuPG is an open-source program, originally written for Linux or Unix systems and released under the GNU General Public Licence (GPL). This means that its source code is freely available and that the program may be used freely by anyone. GnuPG is fully compliant with the OpenPGP standard of the Internet Engineering Task Force (IETF).

What is the relevance of all this to RISC OS users? A RISC OS port of GnuPG was created recently by Stefan Bellon (see, thus bringing RISC OS users up to date with the latest OpenPGP standards and enabling them to intercommunicate securely with users of all versions of PGP. In fact, GnuPG now provides all the same basic functionality as the latest Windows versions of PGP.

The command line

The downside is that GnuPG, like the ageing RISC OS version of PGP, can be used only from the command line. New users may be bewildered by the huge number of possible commands of GnuPG, but fortunately only a few basic commands are needed for most practical purposes. Nevertheless, until someone writes a convenient front end for GnuPG, it will be necessary to learn to use at least some of these commands in order to take advantage of this powerful program.

The reason why the RISC OS port of GnuPG is a command-line program is that it was created so as to mimic exactly the commands and functions of the original Unix-like version. This will make it easier to update the program if any enhancements are made to the original version. Indeed, such updates have already been released.

In an article such as this, it is impossible to explain all the commands fully, or even to mention all the things that GnuPG can do. My objective here is to give only enough detail to enable a beginner to make use of the main features of GnuPG for secure e-mail. I hope that this article will also be helpful to experienced PGP users, who will need to learn a somewhat different set of commands for GnuPG.

Installing GnuPG

The installation procedure is simple. Download the archive "gnupg/zip" from Stefan Bellon's URL given above. This archive contains two applications, !GnuPG and !GnuPGUser, as well as some documentation.

Copy the entire contents of the archive to any convenient directory. It is important, however, that the two applications reside in a filing system that supports long filenames. If you do not have RISC OS 4 with an E+ formatted medium, you can use the raFS filing system (available from or any other filing system that allows long filenames.

!GnuPG is the application that does all the work. !GnuPGUser will contain all the data referring to you as an individual user, including your public and secret keyrings and certain user-defined options. Both of these applications must be "seen" by the filer before you can use GnuPG. If you intend to use GnuPG frequently, it is best to ensure that the filer "sees" them during startup of your computer by placing appropriate references to them in your boot sequence. Some advice about how to do this is contained in the !Help file inside !GnuPG.

Your installation is not yet complete. GnuPG requires a good source of pseudo-random numbers. For this purpose, the RISC OS version of GnuPG makes use of a module called CryptRandom by Theo Markettos. If you do not already have a copy of this module, download the CryptRandom binary from Theo's website at A copy of the module CryptRandom (with filename "CryptRand") should be placed inside the !GnuPG directory.

Compatibility with the old PGP

There are certain difficulties in using GnuPG to communicate with people who are still using PGP 2.x. First and foremost, GnuPG does not support the IDEA cipher, which is an essential component of PGP 2.x. This is a deliberate omission. The IDEA cipher is patented by the Swiss company Ascom-Tech AG, and a licence is required to use it for commercial purposes, at least in some countries (though it may be used freely for non-commercial purposes). Such a restriction is incompatible with the terms of the GPL, and therefore IDEA is not implemented in any official version of GnuPG.

Stefan Bellon provides a simple way of overcoming this difficulty and maintaining compatibility with the old PGP 2.x. On his website there are two additional archives, "gnupgidea/zip" and "gnupgpart/zip". These are an IDEA archive and a partially linked GnuPG archive. Using them, it is possible to build your own IDEA-compliant version of GnuPG. To do this, follow the simple instructions in the !ReadMe file contained in the IDEA archive.

Note that if you add IDEA functionality to your copy of GnuPG, you should not use the IDEA algorithm for commercial purposes or distribute the enlarged program again under the GPL.

Even after IDEA is added to GnuPG, there are still some complications in using it to communicate with users of PGP 2.x. A detailed explanation of how to use GnuPG as a nearly complete replacement for PGP 2.x can be found in a document "pgp2x.html" contained inside the GnuPG distribution. However, if you need to exchange messages with PGP 2.x users, you may find it simplest, at least initially, to use the old PGP 2.6.3ia for this purpose, especially if you have already been using PGP for this purpose.

Importing and generating keys

Public and secret keys are required in order to use GnuPG for secure email. These keys are stored in two files called public and secret keyrings. If you are already a user of any version of PGP, you can import your existing keyrings into GnuPG. To import a public keyring, use the command

gpg --import <keyfile>

where <keyfile> denotes the filename of the keyring file. If this keyring file is not in your currently selected directory, you will need to specify its full pathname. The same command can be used to import a public key sent to you by another user of PGP or GnuPG.

When importing keys, GnuPG behaves differently from PGP in at least two respects. First, in its default mode of operation GnuPG will not import keys which are not self-signed. Second, contrary to what an experienced PGP user might expect, the simple command "gpg <keyfile>" does not offer the option of adding any keys to your keyring; it is essential to use the "import" command as above.

A somewhat more complex command is required to import a secret keyring:

gpg --allow-secret-key-import --import <keyfile>

Note the use of double dashes in GnuPG commands, in contrast to PGP, which uses only single dashes. However, certain GnuPG commands have standard single-letter abbreviations, which are preceded by only a single dash. For example, you can obtain a list of all the main commands by typing "gpg --help" or, in abbreviated form, "gpg -h".

If you have not previously used any version of PGP, you will need to generate new keys. To do this, enter the command

gpg --gen-key

GnuPG will then offer a choice of several options. It is recommended that you select the default option ("DSA and ElGamal") for the key types, unless you have a special reason for some other choice. To ensure high security for the foreseeable future, you should choose a reasonably large key size. GnuPG will also ask for your name and e-mail address, which together form a user ID to identify your key.

Even if you already have an RSA key pair from previous use of PGP, it is still highly desirable to generate new keys as described above, since RSA keys are deprecated in GnuPG and in some later versions of PGP.

Listing your keys

You can view basic details of any particular key in your public keyring by typing the command

gpg --list-keys <name>

where <name> is the user ID of the key, or (more conveniently) any substring of it which specifies it uniquely. As an example, for one of my key pairs you would get the following output:

pub 1024D/6B71EC75 2000-08-29 Nat M. Queen <>
sub 4097g/34100D21 2000-08-29

Here the upper line describes a 1024-bit DSA signing key with key ID 6B71EC75. The rest of the line gives the date on which the key was created and its user ID. The lower line describes a 4097-bit ElGamal encryption key with key ID 34100D21. Note that in general different keys are used for signing and for encryption, and the encryption key is usually larger. The signing and encryption keys are generated together by the single command described above.

To view details of all the keys in your public keyring, use the same command as above, but without the parameter <name>.

If you have more than one secret key, you can set one of them as your default key. This is the key that GnuPG will use to create digital signatures. To specify a default key, uncomment the "default-key" line in the GnuPG options file "!GnuPGUser.options" (by deleting the initial "#" character on that line) and add the appropriate key ID at the end of the line.

Receiving encrypted messages

If other users of PGP or GnuPG are to send you encrypted messages, they must have your public key in their keyrings. If you have just generated a new key, you can extract it into a file by means of the command

gpg --export -o <keyfile> <name>

where <keyfile> is the filename of the output file into which you want the public key to be extracted, and <name> specifies the key as described above. You can then send the keyfile to other users who want to communicate with you.

If an exported keyfile is to be included as part of an e-mail message, you should ensure that it is "ASCII-armoured". This will automatically be the case if the "armor" line in the GnuPG options file is uncommented, which is in fact the default configuration. (The RISC OS port of GnuPG retains the American spelling "armor" in its options file, as in the original version, although both spellings are actually accepted.) This setting will also ensure that all messages encrypted by GnuPG will be ASCII-armoured.

Suppose now that you receive an encrypted file from someone. You can decrypt it by means of the simple command "gpg <file>", where <file> is the filename. GnuPG will check that you have the secret key required to decrypt the file and, if so, it will ask for your passphrase for accessing that key. If the file that you decrypt contains a digital signature of the sender, GnuPG will report this fact and tell you whether the signature is "good". A good signature confirms that the file is identical to the one originally signed by the sender.

GnuPG will be able to check a signature only if the signatory's public key is in your keyring. If the required public key cannot be found in your keyring, GnuPG will automatically search a keyserver for it if you are online, or if your internet setup allows auto-dial-in. In that case, if the key is found, it will be downloaded and imported. Otherwise, if GnuPG is unable to connect to the internet, it will simply report that it is requesting the key from a keyserver, and the process will appear to hang. You can stop this by pressing <Escape>.

Sending encrypted messages

Before you can encrypt a message for someone else, that person's public key must be in your public keyring. If you receive someone's public key, you can add it to your public keyring by means of the "import" command described earlier.

To encrypt a file for someone, use the command

gpg -e -r <name> <file>

where <name> specifies the user ID of the intended recipient's key, and <file> is the filename. It is also possible to encrypt a file simultaneously for any number of multiple recipients by means of a command of the form

gpg -e -r <name1> -r <name2> ... <file>

To add a digital signature to an encrypted message, simply replace the "-e" by "-se" in the encryption command. In this case, GnuPG will ask for your passphrase for accessing your secret key.

If you want to be able to decrypt all your encrypted messages yourself, you should uncomment the "encrypt-to" line in the GnuPG options file and insert your key ID at the end of that line. Users of PGP will recognise that this serves the same function as its "encrypt-to-self" option, i.e. it encrypts all messages automatically with your own public key in addition to that of the specified recipients.

The commands described above should be sufficient to enable you to send and receive encrypted e-mail and to have "Pretty Good Privacy". In a future article I shall describe some of the other things you can do with GnuPG.

Finally, I thank Stefan Bellon for kindly suggesting some improvements to a draft of this article. Any remaining inaccuracies are, of course, entirely my own.

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