From: Bill Stewart <stewarts@ix.netcom.com>
Date: Mon, 28 Aug 95 14:33:05 PDT
To: cypherpunks@toad.com
Subject: Re: SSL trouble
Message-ID: <199508282130.OAA17500@ix5.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


Several people have suggested random keyspace selection instead of servers.
The problem is that there's a high probability that the search will fail to
find the correct key.  If you split the keyspace into n segments, and randomly
select k of them, the probability of failure p = ((n-1)/n)**k ; for k=n, 
p approaches 1/e (.367) as n becomes large, and 10 is close enough to large
that you don't gain much by having independent groups that agree not to overlap
in their own 10% of the keyspace.  For k=2n (random-searching the space
about twice),
the probability of failure is still e**-2, about 13%.  You need a
coordinated search.

>Why not just have the brutes pick a slave at random?  
>Of course, you need to give them a complete list of slaves to choose from. 

That more or less works, assuming you can distribute the list of slaves
along with the code; you still hit the slaves' DNS servers unless you
also distribute IP addresses for the slaves to use (which is probably fine
as long as people get the addresses beforehand.)
You could get fancy and have a DNS server hand out slave addresses round-robin
for a dummy address slave.cracker.org.

The main failure mode seems to have been misconfigured clients grabbing the
single-threaded server for a long time; it may be worth using a multi-threaded
server, or alternatively a single-threaded server that has a fast timeout 
for how long it will talk to a client.

I gather there was some protection in the code against dishonest clients,
but a malicious attack would be to falsely ACK large portions of
the search space (especially the portion containing the real answer,
if the attacker knows it).  
#---
#                                Thanks;  Bill
# Bill Stewart, Freelance Information Architect, stewarts@ix.netcom.com
# Phone +1-510-247-0664 Pager/Voicemail 1-408-787-1281
#---