From: Bill Stewart <stewarts@ix.netcom.com>
Date: Fri, 22 Aug 1997 15:21:49 +0800
To: Irwan Hadi <phoenix@cutey.com>
Subject: Re: PGP5i supports RSA keys?
In-Reply-To: <19970817123643.13713.qmail@hotmail.com>
Message-ID: <3.0.2.32.19970821225601.0077e370@popd.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain



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>>4096 bit keys are going to be rather hard to break in this lifetime. :-) 
>but to generate and signing a message with 4096 bits key , will take 
>much times, than with 2048 or 1024

Diffie-Hellman keys have different standards for how long they
need to be, since they're based on discrete logs rather than factoring,
but the necessary lengths are similar.  However, generating
Diffie-Hellman keys is much faster, once you've settled on a modulus,
since you don't need to search for probable primes, you just
need to pick a random number that's relatively prime to the 
modulus-1 (trivial, if you're using a Sophie-Germain prime modulus)
and maybe to the generator (also trivial.)  So go for 4096,
or a least use a much longer key than the 192 bits Sun once used :-).

There's no particular reason _not_ to support 4096-bit RSA keys 
(since using dynamic-sized data structures makes programs more reliable 
and less susceptible to attacks like overly-long input data), 
but there's also really no need for keys longer than 2048 bits unless
some radical algorithmic breakthrough happens.  (Computer hardware 
breakthroughs aren't relevant; the exponential behaviour of the
algorithms mean that a few extra bits makes any device that fits
on the planet still too small.)  1024 bits is probably enough,
but maybe not, depending on how long you need to keep something secret
and how much technology improves doing your lifetime.  

Techniques for breaking into your computer and stealing the private key
will probably improve far faster than cracking algorithms,
especially as 
1) Nanotech makes it much easier to recover old data off disks and
send nanobots to collect and return it
2) The Singularity makes people smart enough to find all the security
bugs in Win2001
3) The Fingertip Escrow Act requires recording of all keystrokes and
other potentially illicit finger activities
4) Telepathy-input Yellow Sticky Notes make recording your passphrase
so much more convenient.
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#			Thanks;  Bill
# Bill Stewart, +1-415-442-2215 stewarts@ix.netcom.com
# You can get PGP outside the US at ftp.ox.ac.uk/pub/crypto/pgp
#   (If this is a mailing list or news, please Cc: me on replies.  Thanks.)