Looking for a r/w filesystem contained in one file

[jwc]

Hello all,

I'd like to build an encrypted filesystem in one file, which can grow and is writable. I already have some experiences with dmcrypted filesystems, but as far as I understood them they only work on fixed-size filesystems (on block devices or in files). I am looking for a way to create one small encrypted file, which can be mounted and grows according to the data written to it.
Of course I already looked around but found nothing suitable with dmcrypt, loopback-aes, fuse and truecrypt, as they all want fixed-size filesystems.
If anyone around here has an idea, please give me a hint! I'd be very grateful for that.
Thanks in advance,

Joost

PS: I assume just an unenrcypted writable and growing fs in one file would work, too, as I could feed it through dmcrypt