From: rarachel@prism.poly.edu (Arsen Ray Arachelian)
Date: Sat, 12 Feb 94 20:01:09 PST
To: mg5n+@andrew.cmu.edu (Matthew J Ghio)
Subject: Re: Strategies for getting encryption in widespread use QUICKLY
In-Reply-To: <MhL5tSG00VAx02i0hU@andrew.cmu.edu>
Message-ID: <9402130341.AA00829@prism.poly.edu>
MIME-Version: 1.0
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> probably be solved by packaging the data into manageable packets with an
> error-correction protocol and then encrypting those.  Error-correction
> would be vital, as line noise can wreak havok on encrypted data. 
> (Although you could reduce the impact of the errors by weakening the
> cipher somewhat.)  I do have several ideas as to how to do this...

Look at the code for Layers and Multisession(??) they both have sources
for the unix end.  If you base your code on that it should be fairly
stable as we know the above products work.  (Beware of Layers though,
on the Unix end, it supposedly leaves opened ports after you log out
which could be used to hack in, or look like they were used to hack in.
I got my account pulled one day for running it, because some hackers were
breaking in and leaving open ports as part of their footprints....)

 
> What do you think?  Anyone want to take this on as a project?
> If I'm going to do it, it would be for MS-DOS systems, tho if there's
> enuff interest, it might be possible to develop multi-platform support
> concurrently.
> Let me know...

If you're going to do this, try to write some VERY generic code. Don't
assume the size of a char, int, long, etc.  Instead, use stuff like:

#define CHAR char
#define INT int

etc.  That way, if an int happens to be 32 bits on some weirdo machine,
it can be redefined on that machine by simply modifying the headers...
(better yet, use typedef, not #define.)

Another thing, don't use any dos i/o functions to actually send the
packets across, instead, do everything you can without calling on DOS
or whatever C language library/compiler calls directly.  Call the DOS
functions to read/write to the serial port from one single place so
that they can be easily modified.

You really don't have to do too much for the packets and stuff...
Basicallly each packet is composed of a header, data, and some
sort of check.  There are CRC codes out there already.  Pull the
32 bit version out of some Zmodem program and use that...

Your header should contain some packet id#, the size of the packet
(no need to send 1024 bytes if the user just pressed one key :-),
the CRC for the packet, and the packet itself...  A good thing might
also be to include a couple of special bytes at the begining and end
of the packet so that the receiver can try to resynch itself if it
gets hit by line noise...  Say, 0x55,0x55,0xAA,0xAA, packetid#,
packetsize, packetcrc, packet, 0x00,0x00,0xff,0xff for example.
(while this is longer, the other side can look for 2 zeroes, followed
by two 255's, followed by possibly some time before it sends a
request to resend the packet.

Or if you want to make things simpler, do as Xmodem (yuck!) send
a small tiny header, the packet, and wait for an ACK from the other
side before proceeding to send the next packet.  That ACK would be
after/before a packet that the other side sends of course... :-)

Better yet, maybe you can just hack some source code for SLIP or
UUCP or something...