Installing the IDEA Module in GnuPG

(THIS INFORMATION COMES FROM http://www.spywarewarrior.com)

The "official" Win32 builds of GnuPG do not include support for the IDEA algorithm, which is patented.* This can prove to be a problem when exchanging messages and files with PGP users (esp. PGP 2.6.x users) or importing old PGP keys. This web page explains how to install the IDEA module for GnuPG so that GnuPG can use the IDEA algorithm.

The IDEA module has been distributed as both a source code file (IDEA.C) and a compiled .DLL (IDEA.DLL). Unless you plan to compile the source code yourself, you want the IDEA.DLL, which is placed somewhere in the GnuPG installation directory and loaded with the load-extension option.

Please note that the IDEA module, its location within the GnuPG installation directory, and the method for loading that module with the load-extension option have changed over successive versions of GnuPG:

• GnuPG 1.2.0 and later

Please consult the appropriate directions for your version of GnuPG. All of the versions of the IDEA module discussed on this page are available for download from this site -- see the Downloadssection below.

* The IDEA is patented in many countries, including the U.S., and may be used for non-commercial
   purposes. For commercial uses you'll need to buy a license from MediaCrypt. For the "official"
   explanation of why the IDEA algorithm is not supported in GnuPG by default see HERE.

---

GnuPG 1.2.0 and later

There were big changes in GnuPG 1.2.0 with respect to the IDEA module. There's a new IDEA.DLL as well as a change from previous versions in the way the IDEA.DLL is loaded.

From the GnuPG 1.2.0 Announcement: 

* All modules are now linked statically; the --load-extension
  option is in general not useful anymore.  The only exception is
  to specify the deprecated IDEA cipher plugin.

* The IDEA plugin has changed.  Previous versions of the IDEA
  plugin will no longer work with GnuPG.  However, the current
  version of the plugin will work with earlier GnuPG versions.

Files

For "official" versions of GnuPG  (from gnupg.org), a new set of IDEA files is available from ftp.gnupg.dk (idea.c.gz & ideadll.zip):

File	Size (kb)	Notes
idea.c	13815/13727	new source (gnupg.org)
idea.dll	8192	new Win32 DLL (gnupg.org)

The Nullify builds use Disastry's IDEA module (ideadll.zip), the same as previous Nullify builds:

File	Size (kb)	Notes
idea.dll	45056	Disastry's IDEA module

See the Downloads section below for links to these files.

To Install

1. Place the IDEA.DLL in \LIB sub-dir of the GnuPG installation dir. 

2. Add the following line to the GPG.CONF file (in previous GnuPG
    versions it was in the OPTIONS file):

load-extension Lib\idea

    You can also specify a full path to the IDEA.DLL file:

load-extension C:\GnuPG\Lib\idea.dll

    This GPG.CONF file option line works for both the "official" and  
    Nullify GnuPG builds.