gpg doesn't understand crypt so no surprise there.

I'm not a Red Hat person so I don't know the precise history, but previously, crypt used to be missing because of legal/licensing reasons from some Linux distros. Still, maybe you can find a ported crypt; have you tried rpmsearch?

I believe Perl ships with the crypt function so perhaps you could use that.

Perhaps it would be simpler to install something on HPUX which you already have on Linux, such as maybe gpg?