Subject: Re: [Asrg] Re: Body signature subgroup and the forwarding project

From: Bruce Lilly <[email-masked]>
Date: Thu, 4 Aug 2005 18:06:06 -0400

>  Date: 2005-07-27 23:12
>  From: John Levine <[email-masked]>

> >The root of the problem is various types of modifications made by
> >SMTP and other transport, and is a well-known problem ...
> 
> >The issue isn't so much MTAs as network and gateway issues. ...
> 
> Yes, we all know that.  My goal is to collect some real life data on
> what the modifications likely to happen to messages in transit
> actually are, to see if there are canonicalizations that will help a
> message to survive the transit MTAs that people really use.

You are likely to miss quite a bit, depending on your test messages and
the particular sites and MTAs involved.  Some examples:
o sendmail is known to rewrap message header fields at 990 octets.
  Obviously, if your test messages have no lines that long, you won't
  see any change. N.B. legal line length is <= 998 octets (the sendmail
  issue has been reported).  To see the effects of this issue, your test
  messages will need at least one line with length between 991 and 998
  octets.
o some ISP home-grown MTAs wrap at lower lengths; there is a recent set
  of messages in the comp.mail.mime and comp.mail.sendmail newsgroups
  about that.  Unless you send through such a site, using a test message
  with fairly long lines, you won't see that.
o some MTAs, due to network issues, downgrade 8bit message content to
  7bit by applying a transfer encoding.  You won't see that unless your
  test message has 8bit content *and* is sent through a system that
  downgrades to 7bit. [as of this time, the DKIM drafts recommend
  applying a transfer encoding before DKIM processing, which might not
  be practical in some mail configurations, and is likely to result in
  verification failures]

> > Simply passing messages through plain vanilla MTAs in a pure SMTP
> > context won't reveal much about the problems that can be encountered
> > in the actual global mail system.
> 
> You appear to completely misunderstand the goals of systems like DKIM.
[...]

I understand quite well; the idea is similar in many ways to something
that I suggested on the ietf-822 list and here a couple of years ago.

The DKIM base draft states that verifiers may be (among others) MUAs.
Obviously any MUAs behind MS Exchange servers, Lotus Notes systems, etc.
would be affected by transformations made by such parts of the mail
in which they reside.

> The point of DKIM is the sign messages in a way that will survive
> SMTP message transit and be verifiable without a PKI,

Well there *is* a PKI, based on DNS for key exchange.  A verifiable
signature within the context of Internet mail (no prior arrangement
between sender and recipient(s)) necessarily requires some sort of PKI.

> not to be a 
> bullet-proof signature for the ages.  Please see the DKIM drafts if
> you want to contribute here.

I've seen the drafts (such as they are); there is no mention in them nor
in the I-D announcements at this time regarding solicitation of review
comments here or anywhere else.

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From: John Levine <[email-masked]>
Date: 5 Aug 2005 00:15:03 -0000

>the particular sites and MTAs involved.

Well, yeah.  That's why I want to get as diverse a set of forwarders
as I can.  Wasn't that obvious?  Once I get it set up, the plan is to
let people register and then send test messsages which will be
available with the forwarded versions on a web site or something.

> [ various screwups ]

Yes, I'm familiar with all of those.  The question, as I think I've
said at least three times now, is to look and see how different body
canonicalization schemes fare in practice.  We all know that there are
worst cases that break any signature, but our goal is not to reinvent
S/MIME.

>would be affected by transformations made by such parts of the mail

You've confirmed that you definitely don't understand the goals of
DKIM.

We know that Exchange and some other MTAs smash messages beyond
recognition.  But some MTAs don't.  We think that there are an
interesting number of MUAs behind non-smashing MTAs, but we need more
data.  The goal is NOT, repeat NOT, to come up with something that
will survive the most hostile MTAs.  So you can stop telling us about
them, we know about them, they're not the target.  If that were the
goal, the answer would be S/MIME.

R's,
John