Subject: Re: Calculations of ISN for new TCP sessions

From: Vernon Schryver <[email-masked]>
Date: Mon, 18 Sep 2000 20:48:13 -0600 (MDT)

> From: stanislav shalunov <[email-masked]>

> ...
> Have you looked at the Yarrow paper I gave a link to?

That's good advise.  It says that Yarrow-160 uses 3-DES instead of Blowfish
as you said, or even Twofish as one might expect given the key scheduling
costs of Blowfish, the application (crypto-PRNG's), and the common source
of Blowfish, Twofish, and Yarrow.

> ...
> I guess the short summary should be that cryptographers know how to
> make good PRNGs, and they even give them away into public domain.
> On the other hands, non-cryptographers--predictably--have less thorough
> understanding of producing cryptographically secure PRNGs.

No, the short summary is something about people who have read a little
about things outside their strict expertise, and then want to talk about
them in yet other contexts where they not well versed.  (I admit to knowing
no more about crypto stuff than the very little I've read and a few
commercial implementations of some bog standard encryption stuff.)

As Bruce Schneier of Blowfish, Yarrow, and Twofish fame has been saying
a lot lately, designing and implementing a wonderful cipher is the
relatively easy part of the battle.  You should look at the whole picture,
from what you're trying to protect to how much you should spend protecting
it.  The old analogies of armoured car deliveries to park benches and
super-duper vault doors on cardboard boxes are relevant.

This particular context concerns TCP initial sequence numbers.
Cryptographic PRNG's, /dev/random, entropy pools, DES, PGP, SHA and so
forth are merely tools for the main issue, bad guys guessing ISN's.

The ISN issue once mattered, but today it is a minor threat blown out of
proportion by "security audit" software vendors, "computer security"
consultants, trade rag consultant authors, and others.  What bad things
happen if you use completely predictable ISN's?  You've fixed your systems
to not use IP addresses as authenticators by turning off /etc/hosts.equiv
and .rhosts, right?  So what if a bad guy predicts all of your ISN's?

Vernon Schryver    [email-masked]

---

From: stanislav shalunov <[email-masked]>
Date: 19 Sep 2000 12:32:06 -0400

Vernon Schryver <[email-masked]> writes:

> What bad things happen if you use completely predictable ISN's?
> You've fixed your systems to not use IP addresses as authenticators
> by turning off /etc/hosts.equiv and .rhosts, right?  So what if a
> bad guy predicts all of your ISN's?

The bad guy could, for example, relay spam through your
PIPELINE-enabled ESMTP server--in an almost untraceable manner.

Or they could abuse some CGI scripts, and you would not know who they
are.  Or they could try exploits on your TCP services without an easy
way to learn who they were.

Given the fact that good PRNGs are cheap (just some 50 cycles or so
per ISN), there's no excuse to not use them.

-- 
Stanislav Shalunov <[email-masked]>	Internet Engineer, Internet2

"Computer Science is no more about computers than astronomy is about
telescopes."					-- E. W. Dijkstra