Subject: [Asrg] 3b. SMTP Verification - Reputation Systems and their Problems

From: Yakov Shafranovich <[email-masked]>
Date: Wed, 03 Mar 2004 14:37:23 -0500

In the current world the closest approximation we have to reputation 
systems that are planned with LMAP are blacklists and Senderbase. Given 
that current blacklists have numerous problems, why would any proposed 
reputation or accrediation systems of the future be any different? How 
are we planning on avoiding the same problems we have today in the 
future, if we want to deploy such systems?

Yakov

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From: Mark Foster <[email-masked]>
Date: Wed, 3 Mar 2004 12:23:20 -0800

> future, if we want to deploy such systems?

I've been doing some research into the use of STARTTLS and certificate
verification. The possibility exists for overlaying a
PKI trust model onto the email infrastructure... not just S/MIME and
PGP/GPG, but securing the message relay transmissions (MTA to MTA).

By enabling STARTTLS on any given email server (most of them support
STARTTLS), the server must have an x509 certificate, which
is either self-signed or signed by a public (or private) certificate
authority. By also configuring the mail servers to cough up the same
certificate when relaying (outbound), something approaching a whitelist
trust model can be achieved. Also, some mail servers automatically step-up
to TLS when they encounter the STARTTLS in transmission. So if the admins
are willing and able to turn on STARTTLS, and folks start using certificate
verification in their acceptance criteria, this could take off.

Here are some of the problems with the idea. 
1. Most if not all of the MTAs do not support certificate 
revocation lists (CRLs) or OSCP (although Exim will very soon).
2. The existing CAs do not sell these "types" of certificates (usually
just web server, s/mime for email and code signing).
3. The existing CAs do not fit easily into the role currently played
by the DNSBL folks. They would need to revoke certificates upon receiving
complaints from the community at large about abuse.

I don't think any of those problems are insurmountable. I haven't seen
too many folks touting this as a possible solution going forward,
either.

http://mark.foster.cc/articles/secure-email.html

I'd be interested to know what others think.
-- 
Some days it's just not worth chewing through the restraints...
Mark D. Foster, CISSP <[email-masked]>  http://mark.foster.cc/

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From: "Alan DeKok" <[email-masked]>
Date: Wed, 03 Mar 2004 16:03:03 -0500

> PGP/GPG, but securing the message relay transmissions (MTA to MTA).

  A PKI trust model is very different than overlaying encryption on
top of SMTP.

> authority.

  Which are both trivial to get, and mean nothing.

> By also configuring the mail servers to cough up the same
> certificate when relaying (outbound), something approaching a
> whitelist trust model can be achieved.

  How?  You don't say that.  That's the important piece, and the only
one worth talking about.

> complaints from the community at large about abuse.

  Then you're using certificates for something they were never
intended to be used for, and giving CA's additional responsibility and
authority.

> too many folks touting this as a possible solution going forward,

  That's because it's not very practical.

  This whole scheme is equivalent to publishing the web of trust in
any other format, and doesn't require STARTTLS or certificates for it
to work.

  Alan DeKok.

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From: Yakov Shafranovich <[email-masked]>
Date: Wed, 03 Mar 2004 16:10:58 -0500

> PGP/GPG, but securing the message relay transmissions (MTA to MTA).

The problem with the PKI model is key distribution. If we follow the 
current SSL scheme that might work BUT then you are making Verisign and 
other CAs the gatekeepers for the Internet - and can have repercussions. 
For example, in the SSL market according to 
(http://www.securityspace.com/s_survey/sdata/200402/certca.html), 
Verisign and Thawte have 30% and 20% each. HOWEVER, according to 
(http://www.thawte.com/html/CORPORATE/today.html), Thawte is owned by 
Verisign, which implies that 50% of the SSL market is controlled by one 
company. Do you really want a few companies to choose who gets the right 
to send email? Of course, a distributed system like DomainKeys can help 
with some of these problems, but the main problems that blacklists have 
are still there.

Also, SSL certificates today only *idenfity* the site, not provide its 
reputation. Changing CAs into reputation systems, will involve a whole 
new set of challenges. These challenges are mostly present today in 
blacklists which is exactly what worries me - I don't see yet how any of 
these problems can be avoided in any reputation systems.

> complaints from the community at large about abuse.

#1 and #2 can be solved (#2 especially since CAs will jump at it). 
HOWEVER, #3 is what worries me. What does "abuse" mean, how do we know 
that any of these CAs will be any more responsible than SPEWS is today? 
What about lawsuits? Dealing with different standards of spam and 
different legal standards?

Yakov

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From: Eugene Crosser <[email-masked]>
Date: Thu, 04 Mar 2004 11:43:36 +0300

> these problems can be avoided in any reputation systems.

I would say, authentication and reputation can be done by completely
unrelated parties.  Reputation lists could use servers' certificates the
same way they use their IP addresses now.

Otherwise, I agree that there is too much contraversy and "political"
uncertainty surrounding x509.  Primarily due to its strictly
hierarchical model, not well suitable for today's world.

Eugene

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From: "Alan DeKok" <[email-masked]>
Date: Mon, 08 Mar 2004 13:18:07 -0500

Yakov Shafranovich <[email-masked]> wrote:
> reputation or accrediation systems of the future be any different?

  If you base the accreditiation system on something other than
personal prejudices (which is what most content filters and blacklists
end up being), then the accreditation becomes a little more useful.

> How are we planning on avoiding the same problems we have today in
> the future, if we want to deploy such systems?

  Keep the accreditation focused on simple data which can be trivially
verified in the network.  The only reason to have a centralized
accrediation service is to simplify record keeping.  e.g. You *could*
find web pages by rooting around the net yourself, but it's a lot
easier just to use a search engine.

  The #1 accreditation that I see is something to solve the MTA/MUA
imbalance.  STMP should be an MTA to MTA protocol.  SUBMIT has been
around for years now.  People should use it.  The lack of deployment
indicates that the IETF standards process has created a protocol for
which there's no demand.

  Simply dropping the number of SMTP originators by 3 orders of
magnitude would make the spam problem hugely easier to deal with.

  Alan DeKok.

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From: Yakov Shafranovich <[email-masked]>
Date: Wed, 10 Mar 2004 07:52:13 -0500

> easier just to use a search engine.

What about the draft that was suggested in the following message:

https://www1.ietf.org/mail-archive/working-groups/asrg/current/msg09253.html

Yakov

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From: "Alan DeKok" <[email-masked]>
Date: Thu, 11 Mar 2004 14:31:16 -0500

Yakov Shafranovich <[email-masked]> WROTE:
> > verified in the network.
...
> https://www1.ietf.org/mail-archive/working-groups/asrg/current/msg09253.html

  It's a good start.  Now if only I had the time to follow up.

  Alan DeKok.