Secure backup on remote untrusted server over slow line?

From: Preben Bohn (preben_bohn@yahoo.com)
Date: 06/12/02 

Date: Wed, 12 Jun 2002 16:31:58 +0200
From: Preben Bohn <preben_bohn@yahoo.com>

Hi all, I hope the subject says it all... :-)

If not, here's my problem, I really hope someone can help me:

I want to backup my linux server (~10GB data) on a remote server over a
slow line (~200 kbps). I only need a snapshot at say 1 days interval.
The changes to the server data are relatively small so an incremental
scheme is the way to go (and with 10GB over 200 kbps it is the only way
:)).

The problem is that the remote server is "public" available, so I need
to encrypt the data somehow. Does anyone have any good ideas to how I
can accomplish this?

My own ideas:
1) Make a secure filesystem on my own server and rsync the filesystem
file to the remote server. The problem with this approach is that
according to http://rsync.samba.org/fom-serve/cache/60.html rsync
requires at least 3*(the filesize)? free space on the remote server to
do this, and I havn't got that much... Also it seems like a waste of
processing power to encrypt the entire filesystem, when I only need to
encrypt what I send to (and store on) the remote server... This leads me
to option 2:

2) Make a program myself that reads /dev/hdXX in blocks, encrypt the
block, compare a checksum of this block to the corresponding checksum on
the remote server's block, and transfer the local block if they are
different. I'll probably face some problems with the disk changing while
reading, and such...?

Any ideas would be highly appreciated...

Best regards

Preben

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