From: Bill Stewart <stewarts@ix.netcom.com>
Date: Fri, 5 Jan 1996 06:29:13 +0800
To: cypherpunks@toad.com
Subject: Re: 2047 bit keys in PGP
Message-ID: <199601040811.AAA27453@ix12.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


>> Are you sure it's a bug in the DOS version? When I did a pgp -kg in my 
>> UNIX shell (US version 2.6.2) I also entered 2048 bits and it too 
>> created a 2047 bit key instead.

There are some versions that can only do 2047; there are others that have
some sort of bug dealing with keys over some number like 2032, so if you're
doing a new key around that size, 2000 might be safer.

Looking at the distribution of keysizes on the MIT key server was interesting;
there are a lot of unique or lightly-used values besides the popular 
384, 510, 512, 768, 1024, 2047, 2048....  Something to think about if you're
concerned about traffic analysis and anonymity.

>> Why is there a limit to the size of the key anyway? It's too bad PGP 
>> doesn't support any size key (within reason).

By the time the Bad Guys can factor 2047-bit keys cheaply, you'll have
more serious problems to worry about, which may fundamentally change
your assumptions about cost-effective cracking and the amount of
security you need to provide.  Remember that 1024 is currently way
beyond crackable, so interesting theoretical things will have to
happen before even that much is at risk.

Also, there are a _lot_ of things around that tend to get 128-bit
MD5 calculations in them - don't get overoptimistic about anything
that pretends to be stronger than that.  (IDEA's 128-bit keys are
about as tough as 3000+bit RSA keys, and anything limited to 128 bits
is going to be in that general ballpark.)  PGP's random number stuff
_is_ stronger than that, but that's still a fundamental limit.

As a separate issue, programs do their calculations in data structures which
have _some_ sizes to them.  They could be assigned dynamically, but
large-and-static isn't that bad, and PGP was originally for DOS anyway;
if that's the least evil thing you find in the code, be happy :-)
#--
#				Thanks;  Bill
# Bill Stewart, stewarts@ix.netcom.com, Pager/Voicemail 1-408-787-1281
#
# "The price of liberty is eternal vigilance" used to mean us watching
# the government, not the other way around....