encrypted image file

[Richard]

okay, on the Mac, I could make a encrypted image file with AES 256,
and enter a password, set the size 2 - 4GB it would create a image file, 
and would NOT MOUNT it,
until I enter the password correctly, the you could add folder and 
files.. and whatever
to the image.

--
NOW, is there anything like that for ubuntu.
I work on, allot of clients projects...so once I am done, I would 
unmount the image file.

Thanks-
Rich