Note that the IDEA patent has expired and IDEA is now directly supported since GnuPG 1.4.13 respective 2.x with Libgcrypt 1.6. The use of IDEA is not suggested due to its short block length and it is only provided to allow decryption of archived data.

The section below is thus only of historic interest.

Why not IDEA

IDEA is a patented algorithm and therefore the use of IDEA in countries where this patent is valid can not be allowed due to restrictions in the GNU General Public License. Those restrictions are there to help protecting the freedom of software.

For more information on the nonsense of software patents and the general problem with this, please see www.noepatents.org.

If you still want to use IDEA and you are legally allowed to do so, you can download the idea.c.gz file and its signature , read the instructions at the top of the file, compile it and use it as a plugin for GnuPG. But please do us all a favor and use it only for decrypting. CAST5 and AES are good and patent unencumbered algorithms which don't bear any interoperability problems.

Windows user might be interested in the precompiled DLL and its signature.