One of the first things I did when I got my new Mac was install Mozilla Thunderbird, the invaluable EnigMail extension, which is a very easy-to-use frontend to GNU Privacy Guard (GPG), and of course GPG itself.
All went very smoothly, and to check whether the installation was fine I tried to opened an encrypted message which I had received some days ago. Unfortunetly GPG couldn’t decrypt the message. A quick look at EnigMail’s console window told me that the message was encrypted using IDEA, and that the version of GPG I had installed was lacking support of that encryption algorithm.