From: Eric Hughes <hughes@soda.berkeley.edu>
Date: Fri, 4 Jun 93 06:54:47 PDT
To: cypherpunks@toad.com
Subject: CryptoStacker, long term vision
In-Reply-To: <Pine.3.03.9306032254.B28432-d100000@rtfm>
Message-ID: <9306041351.AA05555@soda.berkeley.edu>
MIME-Version: 1.0
Content-Type: text/plain


>I also think that it would be a lot more valuable if it were distributed
>with code so that any user who wanted could inspect it for trapdoors.

The nature of crypto software is that is valueless unless you trust
it.  You don't have to trust a word processor, because you can see
immediately that what you typed on the screen comes out the printer.
For security software, however, breaches are invisible, or more
precisely visible only after the damage has been done.

This is the reason that I disregard DISKREET from Norton.  There's no
source, and largish companies are notorious for pushing compromised
software.  Norton's unlikely to ship source, so unless someone
decompiles it, I'm not biting.

>Umm, perhaps this is overly optimistic, but I was hoping that I would be
>able to use an algorithm that did 1 byte in / 1 byte out encryption so
>that I wouldn't have to deal with sector remapping.  

You need to do a bit of research into what a block device driver
actually does.  It deals only with blocks of characters, not with
individual ones or arbitrary length strings.  The block interface at
the driver level is different than the file access at the API level.
Don't confuse the two levels.  DOS already does the buffering required
to turn a block device into a file system.  You don't need to
replicate it.

As a result, the cipher you choose needs to be a block cipher.  DES
works on blocks of 8 bytes at a time.  A typical sector is 512 bytes.
So you are going to have 32 DES (or iterated DES) operations per
sector.

>  I was thinking that I would even basically leave
>the FAT and such intact, or at least only slightly modified.

Again, at the driver level, you don't know that a FAT even exists.
Ray Duncan's book _Advanced MSDOS Programming_ contains a good chapter
on device drivers.  You should be able to find code for a skeleton
block device driver on the net; check the msdos programming groups for
more info.

>I'll keep everyone updated no problem.  How else am I going to get
>suggestions and help?

I would also suggest that you find programming partners.  If for no
other reason than to do code review, someone else ought to be
involved.  You wouldn't want to make the group too large, but three or
four is not overlarge.  The archive at soda is available for group
work, if desired.

Eric