From: wcs@anchor.ho.att.com (bill.stewart@pleasantonca.ncr.com +1-510-484-6204)
Date: Tue, 27 Dec 94 16:17:16 PST
To: cypherpunks@toad.com
Subject: Re: Making sure a program gets to the receiver intact
Message-ID: <9412280015.AA22592@anchor.ho.att.com>
MIME-Version: 1.0
Content-Type: text/plain


Eric writes:

>    From: an169306@anon.penet.fi
>    How can I insure a program, once put on FTP sites stays untampered with?
> 
> The best solution is not digital signatures but rather digital
> timestamping.  The question is not persistence of authorship but
> rather persistence through time.  
> [Discussion of the implications of getting your keys hacked, over time]

Some good points, but on the whole I'll disagree.  Either way, the solution 
pretty much comes down to "eternal vigilance"....

The interesting technique that digital timestamping provides is that it
lets you show that the version you claim you posted to the ftp site
got there before the [different] version that's there now.
To use that technique, either you need to broadcast the details of the
digital timestamping in an unhackable public fashion, or else someone
who wants to validate the archived data needs to check with you
to be sure that they have a good checksum matching your timestamp.

An ftp server *could* timestamp each incoming document, keeping the
master timestamp data in an un-hackable location, and post the current 
timestamps for the current time period [e.g. day] in the (hackable) archive,
and then register the day's timestamp file with a notary service
so you can be sure that the file hasn't been compromised later.

On the other hand, without signatures, it's not too hard for a Bad Guy
to store bogus files on the server and get them timestamped too -
the user needs a good way to check for previous editions of the 
document in the timestamp file.  With digital signatures,
at least a given file has some internal consistency.

>    The holes:
>    1:  Someone hacking the keyservers, substituting a key for all the people
>        who signed, and modifing the archive to show that.
That's why keyservers are inherently non-trustable; the trust comes from
the Web of Trust connections you have, though a keyserver run by a 
widely-trusted person carrying only keys signed by him/her/it is stronger.

>    2:  Someone breaking into my apt, sticking a keyboard monitor on, getting
>        my passphrase and key.
Yup.  That's a problem with signatures.

		Bill