encrypted file system or folder

[Tomoki Taniguchi]

I don't know if such a solution exists,
but I am looking for an encrypted filesystem/folder
solution which meet the following requirements
or comes close to meeting them.

1) accessible by both windows and linux
2) folder based rather than a full filesystem.
3) encryption is based on a "key" file
4) access to the encrypted folder is transparent once opened with key file

the following are features that i would like to have, but image pretty
hard to implement
1) can setup read-only encrypted folders
2) can differentiate access between read-only and read-write based on
different key files.
3) or an ACL based encrypted folder.  differentiating access levels
based on user name, id , or user specific key file.

TIA,
Tomoki Taniguchi