From: Matt Blaze <mab@crypto.com>
Date: Fri, 10 Feb 95 10:36:41 PST
To: lethin@ai.mit.edu (Rich Lethin)
Subject: Re: Global Filesystem Guild
In-Reply-To: <9502101722.AA04330@grape-nuts>
Message-ID: <199502101839.NAA24397@crypto.com>
MIME-Version: 1.0
Content-Type: text/plain



>
>To reduce the vulnerability of a single user/BBS to having their files
>confiscated/stolen, could a distributed WAN filesystem be implemented with
>k-redundancy, e.g. the files wouldn't disappear until k servers (located in
>various unknown or inaccessible places) failed?  The price of storage would
>vary with the level of redundancy desired.  For security the servers would
>only store cyphertext, etc.  Local cacheing to reduce network load.
>

You've brought up three separate issues - fault-tolerance,  secrecy and
secret sharing.  They are probably best dealt with separately, for the
purposes of understanding them as well as implementing them.

There are replicating file systems that provide the kind of fault
tolerance that you seem to be looking for, although I'm not aware
of any "production grade" systems that can replicate read-write
files in any reasonable way.  Done right, such a system would
probably involve guarantees, implemented partially at the server
side, that a file has been replicated in some number of places
prior to any write operation returning.  You could kludge this at
the client side alone by just modifying your file system interface
to make an approprate number of replicas each time something is
written.  There are a whole bunch of issues surrounding failure
semantics - how do you handle writes when one of the replica servers
is unavailable, and so on.

Secrecy, on the other hand, is best thought of as a client-side
issue, and can be handled with existing client file system encryption
tools that run at the client file system interface level.  For
example, on Unix clients, CFS can store its underlying data on most
any remote file system, and requires no changes or special attention
from the server side.  I have no idea whether the various PC
encrypting file systems (SFS, etc) that have since come along
separate their functions from the low-level storage in a way that
makes it possible to use them with remote file servers, but there's
probably no reason you couldn't build such a system if one didn't
already exist.  The main issue in practice is key management.
Depending on the application, you might want to back up keys under
some secret sharing scheme that allows keys to be recovered by some
subset of key holders.  This is especially important for commercial
applications.

-matt