RSA - IDEA Public Key Imports and Encrypts Fine with GNUPG...Why?

[Ryan Lovelady]

I have a public RSA key that uses the IDEA cipher that was created using PGP.  GNUPG will allow me to import this key and encrypt files successfully...but I don't understand how it can do this if it has no support for IDEA.  I do not have the IDEA plug-in.  It doesn't make sense to me that it would be able to natively encrypt using the RSA/IDEA key.

If this were for commercial usage, is it ok to encrypt a file with GNUPG using an RSA public key with IDEA cipher?  Or is it illegal to encrypt files this way?

I have tried to decrypt using GNUPG, but it will not decrypt unless the IDEA plug-in has been installed.  PGP will decrypt the file correctly though.