portably encrypting a file system's partition, directory and/or file

Hi *,

I have firewire and USB devices with partitions/logical drives, whole
directories and/or files I would like to encrypt. The thing is that I
need to be able to just plug in the thing on any x86 machine running a
commercial OS that would just take it (and AFAIK the only filesystem
that even a MAC would seamlessly 'mount' is vfat/FAT32)

is there anyway to do that?

I have read quite a bit about it and I still don't find exactly what I
need.

Also, why exactly does encryption belong in the kernel? I think once
you make it a kernel-depending functionality 'portability' to other OS
goes out the window

Are there libraries out there (of course, preferably OSS ones) which
you could compile for different OS and have access to pluggable
devices?

I crossposted this message becuase, to me, these portability and security
issues naturally reach out into different subject areas

Also, google was a little 'temperamental' when I was trying to post this
message

thanx
otf